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10.1A: The Role of the Cell Cycle - Biology


The cell cycle allows multiicellular organisms to grow and divide and single-celled organisms to reproduce.

Learning Objectives

  • Explain the role of the cell cycle in carrying out the cell’s essential functions

Key Points

  • All multicellular organisms use cell division for growth and the maintenance and repair of cells and tissues.
  • Single-celled organisms use cell division as their method of reproduction.
  • Somatic cells divide regularly; all human cells (except for the cells that produce eggs and sperm) are somatic cells.
  • Somatic cells contain two copies of each of their chromosomes (one copy from each parent).
  • The cell cycle has two major phases: interphase and the mitotic phase.
  • During interphase, the cell grows and DNA is replicated; during the mitotic phase, the replicated DNA and cytoplasmic contents are separated and the cell divides.

Key Terms

  • somatic cell: any normal body cell of an organism that is not involved in reproduction; a cell that is not on the germline
  • interphase: the stage in the life cycle of a cell where the cell grows and DNA is replicated
  • mitotic phase: replicated DNA and the cytoplasmic material are divided into two identical cells

Introduction: Cell Division and Reproduction

A human, as well as every sexually-reproducing organism, begins life as a fertilized egg or zygote. Trillions of cell divisions subsequently occur in a controlled manner to produce a complex, multicellular human. In other words, that original single cell is the ancestor of every other cell in the body. Once a being is fully grown, cell reproduction is still necessary to repair or regenerate tissues. For example, new blood and skin cells are constantly being produced. All multicellular organisms use cell division for growth and the maintenance and repair of cells and tissues. Cell division is tightly regulated because the occasional failure of regulation can have life-threatening consequences. Single-celled organisms use cell division as their method of reproduction.

While there are a few cells in the body that do not undergo cell division, most somatic cells divide regularly. A somatic cell is a general term for a body cell: all human cells, except for the cells that produce eggs and sperm (which are referred to as germ cells), are somatic cells. Somatic cells contain two copies of each of their chromosomes (one copy received from each parent). Cells in the body replace themselves over the lifetime of a person. For example, the cells lining the gastrointestinal tract must be frequently replaced when constantly “worn off” by the movement of food through the gut. But what triggers a cell to divide and how does it prepare for and complete cell division?

The cell cycle is an ordered series of events involving cell growth and cell division that produces two new daughter cells. Cells on the path to cell division proceed through a series of precisely timed and carefully regulated stages of growth, DNA replication, and division that produces two identical (clone) cells. The cell cycle has two major phases: interphase and the mitotic phase. During interphase, the cell grows and DNA is replicated. During the mitotic phase, the replicated DNA and cytoplasmic contents are separated and the cell divides.


10.1A: The Role of the Cell Cycle - Biology

By the end of this section, you will be able to:

  • Understand how the cell cycle is controlled by mechanisms both internal and external to the cell
  • Explain how the three internal control checkpoints occur at the end of G1, at the G2/M transition, and during metaphase
  • Describe the molecules that control the cell cycle through positive and negative regulation

The length of the cell cycle is highly variable, even within the cells of a single organism. In humans, the frequency of cell turnover ranges from a few hours in early embryonic development, to an average of two to five days for epithelial cells, and to an entire human lifetime spent in G0 by specialized cells, such as cortical neurons or cardiac muscle cells. There is also variation in the time that a cell spends in each phase of the cell cycle. When fast-dividing mammalian cells are grown in culture (outside the body under optimal growing conditions), the length of the cycle is about 24 hours. In rapidly dividing human cells with a 24-hour cell cycle, the G1 phase lasts approximately nine hours, the S phase lasts 10 hours, the G2 phase lasts about four and one-half hours, and the M phase lasts approximately one-half hour. In early embryos of fruit flies, the cell cycle is completed in about eight minutes. The timing of events in the cell cycle is controlled by mechanisms that are both internal and external to the cell.


BIOLOGICAL SIGNIFICANCE AND ROLES OF THE KREB’S CYCLE

Krebs cycle is the cyclic system that comprises several enzymatically catalyzed reactions that play significant biological role in the metabolic activities of living organisms inclusive of prokaryotic and eukaryotic cells. It can also be called tricarboxylic acid (TCA) cycle or citric acid cycle. Citric acid is an important metabolic intermediate in living organism and this molecule is a tricarboxylic acid molecule that can also be found in plants especially citrus fruits. Abnormal accumulation of citric acid cycle in an organism could be due to the malfunctioning of the organism’s TCA or Kreb’s cycle. The enzymes that catalyze the TCA cycle are mainly located in the cytoplasm of both eukaryotic and prokaryotic cells. Although, these enzymes are largely located in the mitochondrion of eukaryotic cells since some of the enzymes such as succinate dehydrogenase are membrane-bound. The Kreb’s cycle is so named because of the discoverer of this important metabolic pathway who happened to be known as Hans Krebs (1900-1981).

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The TCA cycle is generally an amphibolic metabolic pathway because it can carry out both anabolic and catabolic reactions depending on the current physiological status of the cell. Some of the important intermediates of the TCA cycle include: citrate, isocitrate, succinyl CoA, succinate, fumarate, malate, α-ketoglutarate and oxaloacetate and each of the reactions responsible for the production of each of these intermediates are catalyzed by different enzymes (Figure 1). The major biological function of the TCA cycle in living systems is in the generation of energy or ATP in corporation with the oxidative phosphorylation that occurs in the mitochondrion of the cell. In the TCA cycle, acetyl CoA recovered from the breakdown of glucose, lipids or proteins are oxidized to form carbondioxide (CO2) and ATP.

Figure 1. Illustration of Kreb cycle.

The TCA cycle is mainly catalyzed by eight (8) different enzymes including: citrate synthetase (which catalyzes the condensation of acetyl CoA with oxaloacetate to produce citrate) aconitase (which catalyzes the isomerization of citrate to isocitrate) isocitrate dehydrogenase (which catalyzes the oxidation of isocitrate to α-ketoglutarate) α-ketoglutarate dehydrogenase (which catalyzes the oxidation of α-ketoglutarate to succinyl CoA) succinyl CoA synthetase (which catalyzes the conversion of succinyl CoA to succinate) succinate dehydrogenase (which catalyzes the oxidation of succinate to fumarate) fumarase (which catalyzes the hydration of fumarate to malate) and finally malate dehydrogenase (which catalyzes the oxidation of malate to oxaloacetate). The formation of oxaloacetate via the oxidation of malate by malate dehydrogenase marks the completion of the TCA cycle and the cycle continues from here again.

In the Kreb’s cycle, much more energy is produced when pyruvate is degraded aerobically to CO2 and this occurs in the TCA cycle. Acetyl CoA is the starting molecule or substrate for the TCA cycle and it is produced from the oxidation or breakdown of pyruvate (the end product of the glycolytic pathway). In eukaryotic cells, the acetyl CoA produced from the breakdown of pyruvate enters the TCA cycle in the mitochondrion while in prokaryotes the acetyl CoA enters the bacterial or prokaryotic cytosol or cytoplasm. Citric acid is of immense industrial importance because it is used to produce a wide variety of products aside their significant health benefits. Aspergillus niger is one important microbe (a fungal mould in particular) from which citric acid can be sourced from industrially. Citric acid can be used as antioxidants and they are important component of most beverages especially drinks.

FURTHER READING

Alberts B, Bray D, Lewis J, Raff M, Roberts K and Watson J.D (2002). The molecular Biology of the Cell. Fourth edition. New York, Garland, USA.

Campbell, Neil A. Brad Williamson Robin J. Heyden (2006). Biology: Exploring Life. Boston, Massachusetts: Pearson Prentice Hall.

Cooper G.M and Hausman R.E (2004). The cell: A Molecular Approach. Third edition. ASM Press.

Karp, Gerald (2009). Cell and Molecular Biology: Concepts and Experiments. John Wiley & Sons.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of microorganisms. 12 th edition. Pearson Benjamin Cummings Publishers. USA. Pp.795-796.

Nelson, David L. Cox, Michael M. (2005). Lehninger Principles of Biochemistry (4th ed.). New York: W.H. Freeman.

Verma P.S and Agarwal V.K (2011). Cytology: Cell Biology and Molecular Biology. Fourth edition. S. Chand and Company Ltd, Ram Nagar, New Delhi, India.


Cell Cycle and Cell Division for Class 10 | Biology

In this article we will discuss about:- 1. Introduction to the Cell Cycle 2. Regulation of Cell Cycle 3. Division of the Nucleus of a Cell 4. Errors in Cell Division.

Introduction to the Cell Cycle:

The higher plants and animals begin their life as a single cell called ovum which is fertilized by a male cell resulting into formation of a zygote. The growth and development of the zygote to form an embryo is affected by a series of divisions. Cells increase in number by dividing into two, after which each daughter cell grows and divides again.

In other words, cells are not dividing always instead there is a period of rest between two successive divisions which is called as an interphase. Previously interphase was called as the resting phase which is in fact a misnomer, because it is a period of intense biosynthetic activity in which the cell doubles its size and duplicates its chromosome complement. Interphase is immediately followed by the phase of division or M-phase both of these phases constitute the cell cycle.

Howard and Pelc have divided the entire cell cycle into four successive intervals, viz.:

It immediately follows the phase of division (mitosis or meiosis). It is also called as the pre-DNA synthesis phase. This phase includes the synthesis of the substrates and enzymes necessary for DNA synthesis. Therefore G1-phase is marked by the transcription of various types of RNAs and synthesis of different types of proteins.

The regulation of the duration of cell cycle occurs primarily by arresting it at specific point of G1. The cell in the arrested condition is said to be in the G0-stage. When conditions change and growth is resumed, the cell re-enters the G1-phase.

The G1-period is most variable in length. Depending upon the physiological condition of the cell, it may last days, months or years. For a cell cycle of 24 hours duration, the G-phase takes the first 10 hours.

It is the phase of DNA synthesis. Histones are also synthesized during this phase, and become associated with the newly replicated DNA. During this phase, the euchromatic regions of the genome replicate earlier than the heterochromatic regions. Moreover, in some cells, the G-C rich regions of the genome replicate earlier than A-T rich ones.

It is also called as the post-DNA synthesis phase. It is the period between the end of DNA synthesis and the start of cell division. During this phase all the metabolic activities concerning the growth of cytoplasm and its constituent organelles and macromolecules are performed. Also the factors necessary for chromosome condensation during mitosis are synthesized during this phase. During G2, a cell contains 2-times the amount of DNA present in the original diploid cell.

4. M-Phase or the Phase of Division:

During this phase the cell divides. During cell division, the nucleus divides first (karyokinesis), which is followed by the division of cytoplasm (cytokinesis).

The karyokinesis may occur in following ways:

Regulation of Cell Cycle:

For proper continuation of life processes of a eukaryotic cell, it is essential that the different phases of cell cycle are precisely regulated. Any loss of regulation may have serious consequences in the form of chromosomal abnormalities. For exerting an effective control, the cells are allowed to cross these points only if all the necessary conditions are satisfied, otherwise the cell cycle is stopped.

In a eukaryotic cell three check points are proposed-viz.-between G1 and S, between G2 and M-phase and in M-phase itself. At G1/S check-point the damage to DNA and the size of a cell are checked. If these are normal, the cell is allowed to enter S-phase.

At G2/S check point, if DNA has been replicated completely and properly, then only the cell is allowed to enter M-phase.

In M-phase, if spindle fibre is not formed properly and chromosomes are not properly attached to spindle fibre through their centromeres, the cell division stops.

There are two theories to explain the mechanism of regulation of cell cycle:

1. According to domain theory, the cell enters next phase of the cell cycle only if the previous phase is properly completed.

2. However according to clock theory, the cell oscillates between interphase and M-phase due to an inbuilt timer. In such case, inspite of inhibition of one phase of cell cycle, the cell proceeds normally to the next phase. Entry of a cell from one phase to the other is genetically controlled. It is proposed that it is controlled by two groups of enzymes-viz.-cyclin dependent kinases (CdK) and phosphorylating enzymes called cyclins.

The M-phase kinase regulates the entry of ceil into M-phase. It has 2 subunits-one has the catalytic property (catalytic subunit) and the other exerts a regulatory control (regulatory subunit or cyclin). The catalytic subunit is activated by a reversible cycle of phosphorylation and dephosphorylation at the beginning of M-phase. This process is regulated by cyclins hence they are called cyclin dependent kinases (CdK). The cyclins bind to the M-phase kinase and select the specific proteins which are to be phosphorylated. Similarly the entry of cell into S-phase is regulated by an S-phase kinase.

For their discoveries concerning the regulation of cell cycle Hartwell, Hunt, and Nurse were awarded Nobel Prize in the year 2001. They compared CdK molecules with an engine and the cyclins with a gear box which controls whether the engine will run in the idling state (neutral gear) or will drive the cell forward in the cell cycle.

Division of the Nucleus of a Cell:

This type of nuclear division is affected simply by the elongation of the nucleus followed by its complete division into two halves. The nuclear membrane remains intact throughout the duration of division. Amitosis is usually seen in bacteria, protozoans, algae and fungi.

It is the usual form of nuclear division. It takes place in somatic cells. It was first reported by W. Fleming in 1882.

Mitosis is divided into four successive stages which are called prophase, metaphase, anaphase and telophase.

Chromatin undergoes condensation and takes the form of chromosomes. The chromosomes now begin to contract longitudinally. Due to their contraction the chromosomes become thicker and smoother. The spiral coiling, characteristic of the early prophase chromosomes is lost. This change is often referred to as despiralization.

Towards the end of the prophase, nucleolus and the nuclear membrane disappear and the nucleoplasm is set free, at this stage spindle fibres make their appearance in the cytoplasm. These fibres form the nuclear spindle.

Each chromosome gets attached to a spindle fibre at centromere. Although each chromosome is divided into two chromatids, but the centromere is a single undivided structure at this stage. The chromosomes now quickly become arranged at the equator of the spindle.

During this stage the centromere divides into two. Each half centromere carries one chromatid alongwith it. Due to shortening of the spindle fibers the two chromatids separate from one another and move towards the opposite poles of the spindle, so that the chromosomes frequently assume a V, I, J or L shape. The two groups of chromosomes converge towards the poles, exhibiting a figure of two radiating clusters, which is called the diaster.

Like prophase, telophase also takes a longer time as compared to other phases. As the chromosomes reach the opposite poles, they begin to absorb water again and gradually become thin, hence more difficult to observe. At this stage nuclear membrane and nucleoli make their appearance. Thus two daughter nuclei are produced from one nucleus.

The division of the cytoplasm is known as cytokinesis.

It takes place by any of the following two methods:

(b) By cleavage of the cytoplasm.

(a) By Cell-Plate Method:

During telophase, droplets of liquid appear in the equatorial region of the spindle. Gradually these droplets unite to form the so-called cell plate. The spindle region along with equatorial plate is called as the phragmoplast. Protoplasmic membranes are then formed by the protoplasm on both the sides of the liquid layer and the cell walls are laid between the protoplasmic membranes.

(b) By Cleavage or Constriction:

This kind of division takes place in many simple plants like fungi and in animal cells. In this case cell division takes place by the formation of a ring like furrow in the plasma membrane. This cleavage deepens until it has cut entirely through the cell. The furrow results in the formation of two plasma membranes.

Importance of Mitosis:

The fundamental importance of mitosis is the equal quantitative and qualitative division of the nuclear material thus the genetic constitution of the nuclei can be maintained. It provides the opportunity for growth and development of the organism by causing an increase in the number of cells. Even the gonads and sex cells also depend on mitosis for the increase in their number after the chromosome number has been halved by meiosis.

Regulation of Mitosis:

R. Hertwig suggested that the cytoplasmic and nuclear coordination regulates mitosis. It is governed by the rate of cellular metabolism which itself is regulated by temperature, nutrients, oxygen, chemicals and other such factors. Cells produce some substances called chalones which inhibit mitosis.

Significance of Mitosis:

1. Chromosomal number in the daughter cells is maintained.

2. Mitosis contributes in growth and development of the organisms.

3. It maintains the DNA and RNA ratio in the cells.

4. It maintains proper size and shape of the cells.

5. Old and decaying cells of body are replaced by new cells produced by mitosis.

3. Meiosis or Reduction Division:

Every plant has a fixed number of chromosomes. If the egg and sperm had the same number of chromosomes as the vegetative cells, the fertilized egg of each generation would contain twice as many chromosomes as the nuclei of the preceding generation. This is prevented by the presence of two successive divisions in the life cycle in which the number of chromosomes is reduced to half that found in the vegetative nuclei. Thus, the number of chromosomes always remains constant by a special type of cell division called meiosis or reduction division. The term meiosis was coined by Farmer and Moore (1905).

The process of meiosis is fundamentally same in all the plants and animals but certain biologists recognize following three types of meiotic divisions according to their occurrence at different stages of the life cycle of the organisms:

i. Sporogenic or Intermediate Meiosis:

It occurs during sporogenesis in higher plants and results into formation of microspores and megaspores.

ii. Gametic or Terminal Meiosis:

In animals and lower plants, meiosis occurs during gametogenesis and results into formation of gametes (sperm or ova).

iii. Zygotic or Initial Meiosis:

It is seen in lower plants. Here meiosis occurs in the zygote immediately after it is formed by fertilisation.

The cells, in which meiosis takes place, are called meiocytes. The process of meiosis comprises actually the two cell divisions. First division is a reductional division in which the number of chromosomes becomes half and second division is equational division. The reductional division is called as first meiotic division or heterotypic division and the equational division is called as second meiotic division or homotypic division. These two nuclear divisions follow each other in a rapid sequence. As a result of these two divisions, four nuclei are formed but in each daughter cell the number of chromosomes remains half.

First Meiotic Division or Heterotypic Division:

The division takes place in the following stages:

This is the longest stage.

It is divided into a number of sub-stages:

During this stage, chromosomes are extremely thin and can be seen only with difficulty. Only the sex-chromosomes may stand out as compact heteropycnotic bodies.

During this stage, the meiocyte and its nucleus become larger, the chromosomes become more distinct, and their double nature (the two chromatids) is seen in many organisms. They appear as slender threads bearing a series of bead-like structures, called chromomeres.

During zygonema, the chromosomes become shorter and thicker and the homologous chromosomes start pairing or synapsis. Each pair is called a bivalent. Pairing is highly specific, being chromomere for chromomere. If pairing occurs between non-homologous chromosomes, it is called as pseudosynapsis.

During synapsis the homologous chromosomes remain separated by a space of about 0.15 to 0.20 mm which is occupied by the synaptonemal complex (SC). The SC was discovered by Moses (1956). It is composed of three parallel elements and is supposed to bring the homologous chromosomes together.

In this stage chromosomes become shorter and thicker. Each chromosome in a bivalent, at this stage has two chromatids, thus, a bivalent really consists of four chromatids in pachynema stage, and is called a tetrad. At this stage interchange of chromatid segments (i.e., crossing over) between homologous chromosomes takes place. The nucleolus still persists.

The chromosomes are further thickened and shortened. By now, the intimately paired chromosomes repel each other and begin to separate. The separation starts at centromere and proceeds towards the end. However, the separation is not complete, as the homologous chromosomes are joined together by chiasmata which represent the places of crossing-over. Due to repulsive force between the chromosomes, the chiasmata move towards the ends of chromosomes, this process is called as terminalisation.

This is the final stage of meiotic prophase-l. In this stage chromosomes become very short, thick and coiled. Bivalents are separated off and they move towards periphery of the nucleus. Nuclear membrane disappears. Nucleolus also disappears and spindles are fully formed and well-oriented.

In this stage the bivalents are arranged along the spindle fibres in an equatorial region in such a way that the centromeres of their bivalents are towards the poles.

During this stage, spindle fibres contract due to which bivalents (dyad) move towards the opposite poles of the spindle. Due to the pulling each dyad becomes like an inverted ‘V’. Dyad of a maternal chromosome is completely separated from the dyad of the paternal chromosome. Separation of homologous chromosomes at this stage is called as disjunction.

At the opposite poles, the chromosomes become uncoiled and greatly elongated. Nucleolus and nuclear membrane reappear.

As a result of cytokinesis the cytoplasm is divided into two daughter cells.

This is the interphase stage between the first meiotic and the second meiotic division. If may be of a longer duration or shorter duration.

Second Meiotic Division or Homotypic Division:

The meiotic process is completed only when two Vacuole haploid nuclei divide by a process, which is almost similar to mitosis. This second meiotic division is, sometimes, called meiotic mitosis. As a result of this process four haploid nuclei are formed.

This division takes place in the following stages:

In this stage the nucleolus and nuclear membrane disappear and chromosomes are liberated into the cytoplasm. This stage is followed by the formation of a spindle.

In this stage the chromosomes become arranged along the equatorial plane of the spindle. The centromeres of chromosomes become oriented in the centre and arms extend towards the opposite poles and then centromeres also divide into two each.

In this stage, spindle fibres contract by which a pulling force is exerted on the centromeres. As a result the two sister- chromatids go towards the opposite poles.

In this stage nuclear membrane appears around the chromatids. Nucleolus also appears again.

As result of cytokinesis, the two daughter-nuclei are separated and thus, two daughter cells are formed. Each nucleus has a haploid number of chromosomes. Thus four haploid daughter cells are formed from a single diploid parent cell.

Significance of Meiosis:

1. It helps in production of haploid gametes which maintains the chromosomal number in the offspring after fertilization.

2. It produces new generation.

3. It leads to variations in generations caused by crossing overs which occur during meiosis.

Errors in Cell Division:

During cell division if some error occurs at any stage or phase of division then that causes abnormalities in an individual. These abnormalities can be due to structural or numerical changes in the chromosomes. These changes can be in the autosomes or the sex chromosomes.

Abnormal Human Karyotype/Syndromes:

Chromosomes are the structures which carry the genetic information in the form of DNA. These are highly dynamic structures having proper regulatory organization. Chromosomal confirmation plays important role in expression of gene while the topology of the chromosome also plays important role. A slight change in the topology of chromosome can affect the action of gene.

Structural Aberrations in Chromosomes:

When the structure of a chromosome is altered it is called chromosomal aberrations.

It is of four main types:

When a portion of the chromosome is missing or deleted then it is called deleted.

Deletion is a genetic aberration in which a part of a chromosome or a sequence of DNA is missing. Deletion is the loss of genetic material. Any number of nucleotides can be deleted, from a single base to an entire piece of chromosome. It can be caused by errors in cross-over during meiosis. Deletion of a number of base pairs that is not evenly divisible by three will lead to a frame- shift mutation, causing all of the codons occurring after the deletion to be read incorrectly during translation, producing a severely altered and potentially non-functional protein.

Duplication is the opposite of a deletion. Duplications arise from an event termed unequal crossing over that occurs during meiosis between misaligned homologous chromosomes. The chance of this happening is a function of the degree of sharing of repetitive elements between two chromosomes. The product of this recombination is duplication at the site of the exchange and a reciprocal deletion.

3. Translocation:

When a portion of one chromosome is transferred to another non-homologous chromosome it is called translocation. Translocation is chromosome abnormality caused by rearrangement of parts between non-homologous chromosomes. When an even exchange of material with no genetic information extra or missing and ideally full functionality occurs it is called balanced translocation. When the exchange of chromosome material is unequal resulting in extra or missing genes it is called unbalanced translocation.

An inversion is a chromosome rearrangement in which segment of a chromosome is reversed end to end i.e., when a segment of chromosomes breaks but later rejoins after rotating by 180°, it results in inversion.

An inversion occurs when a single chromosome undergoes breakage and rearrangement within itself. Inversions are of two types, viz., Paracentric inversions do not include the centromere and both breaks occur in one arm of the chromosome. Pericentric inversions include the centromere and there is a break point in each arm.

A. Abnormalities due to Structural Changes in Chromosomes:

1. Cat Cry Syndrome (Cri-Du-Chat Syndrome):

It is due to deletion of short arm (p-arm) of the 5 th chromosome affected child has small epiglottis and larynx, therefore cries like a kitten face becomes moon like, head is small and the child shows physical and mental retardation.

Malignant tumor of eye, is due to the deletion of part of 13 th chromosome.

3. Deletion of long arm of 18 th chromosome produces symptoms like skeletal and ophthalmic abnormalities along with profound mental retardation and facial alterations.

4. Granulocytic Leukemia:

It occurs due to translocation of long arm (q-arm) of 22 nd chromosome to the 9 th chromosome, causing increased proliferation and accumulation of the granulocytes. The deleted 22 nd chromosome is called Philadelphia chromosome.

Sometimes the chromosomes of a chromosome pair are unable to separate at the first meiotic division. This leads to the formation of a gamete containing all the chromosomes of that chromosomal pair, while the other gamete does not contain any chromosome part.

This phenomenon is called non disjunction.

Non-disjunction is of two types:

(a) Primary Non-Disjunction:

C.B. Bridges during his experiments reported unexpected conditions in the inheritance of the sex linked characters. While crossing red-eyed male with white eyed female drosophila he observed red-eyed males and white-eyed females in F1 generation (contrary to the expected condition of white-eyed males and red-eyed females).

This condition was explained on the basis of non-disjunction of X-chromosome in the female drosophila. According to him the white-eyed females carry two X-chromosomes which were inherited from the mother, therefore, the colour of eyes was white. On the other hand red-eyed males could not get the X-chromosome from the mother due to non-disjunction. They received X-chromosome only from the father.

(b) Secondary Non-Disjunction:

When Bridges crossed the white-eyed females of the F1 generations (carrying XXY-chromosome due to primary non-disjunction) with red-eyed normal male the offsprings had 96% of red-eyed females and 4% of white-eyed females. At meiosis, the XXY will disjoin to form X & XY in a normal condition, but the secondary non disjunction results into XX & Y gamete. This gave rise to XXY white-eyed females and XY red-eyed males.

B. Abnormalities due to Numerical Changes in Chromosomes:

1. Due to Aneuploidy of Sex Chromosomes:

Monosomy of X-chromosome (44 + XO). Phenotypically such females are short stature, having streak gonads, multiple pigmented nervi (birth marks) and webbed neck.

(b) Klinefelter’s Syndrome:

It is the Trisomy of sex chromosomes (44 + XXY). Trisomies show gigas effect for certain genes. These are sterile males with gynaecomastia. This syndrome includes even XXXY and XXXXY – type males showing even greater degree of sterility.

(c) XXX Syndrome/Super Male (Jacob’s) Syndrome:

Although such males are said to have criminal records, but several workers have recently criticized this view.

Super female syndrome, short statured, sterile females.

2. Due to Aneuploidy of Autosomes:

Commonly it involves the chromosomes of groups D, E and G.

Trisome of 13 th chromosome. The affected children show mental retardation, defective eyes as well as cardiac disorders.

Trisomy of 18 th chromosome. The affected children show anomalies of fingers, achodroplasia, complex digital prints, heart defects, low set ears and small mouth. They die before one year of age.

(c) Down’s Syndrome/Mongolism/Mongoloid Idiocy:

It is trisomy of 21 st chromosome (due to non-disjunction during oogenesis). It is the most familiar trisomy of man. It was described by Langdon Down (1866) of England for the first time.

The symptoms of Down’s syndrome are as follows:

(a) Short broad hands with simian type palmar crease.

(c) Hyperflexibility of joints.

(e) Broad head with round face.

(f) Open mouth with large tongue.

Behaviour Genetics:

The complexity of behavioural traits of any animal develops under joint, tightly entwined effects of heredity and environment.

Behaviour genetics is thus concerned with the effects of genotype on behaviour and with the role that genetic differences play in the determination of behavioural differences in a population.

Uniparental Disomy (UPD):

The presence of two copies of a chromosome or part of a chromosome from only one parent and not from the other parent is called uniparental disomy or UPD.

In humans uniparental disomy was first discovered in 1988 in a child suffering from a disease called cystic fibrosis and short height. It was discovered by Spence and coworkers who proved that the child had received two copies of chromosome number 7 with a mutant CF gene (CF gene) from her mother but none from the father.

After this, many other disorders are also reported due to uniparental disomy. Although the frequency of occurrence of uniparental disomy is not clear, yet it has been estimated that about one out of 500 affected individuals is due to maternal uniparental disomy.

Ring Chromosomes:

A normal chromosome is linear in shape which may have two sister chromatids at some stage of cell cycle. If the topology changes from linear to circular form it may affect the sequence of events or actions.

The change from linear to circular form results in the formation of ring chromosome. It results in various abnormalities such as tumors etc.

Ring chromosomes can be formed by following ways:

(a) Ring chromosome may be formed due to breakage in the chromosome arms and fusion of the proximal ends resulting into loss of genetic material at distal ends.

(b) It may also be formed by telomeric disjunction resulting into fusion of relative chromosome ends. Here the genetic materials at distal ends are lost causing either minimum effect or they remain intact.

Cells (and their owners) are said to be polyploid if they contain more than two haploid (n) sets of chromosomes i.e., their chromosome number is some multiple of n greater than the 2n content of diploid cells. For example, triploid (3n) and tetraploid (4n) cells are polyploid.

Polyploidy in Plants:

Polyploidy is very common is plants, especially in angiosperms. From 30% to 70% of today’s angiosperms are thought to be polyploid. Species of coffee plant with 22, 44, 66 and 88 chromosomes are known. This suggests that the ancestral condition was a plant with a haploid (n) number of 11 and a diploid (2n) number of 22, from which evolved the different polyploid descendants.

The present day wheat, with its 42 chromosomes, is a hexaploid (6n) of its ancestral plant with its haploid number (n) equal to 7.

Polyploid plants not only have larger cells but the plants themselves are often larger. This has led to the deliberate creation of polyploid varieties of such plants as watermelons, marigolds and snapdragons.

Polyploidy has occurred often in the evolution of plants. The process can begin if diploid (2n) gametes are formed.

These can arise in at least two ways:

(a) The gametes may be formed by mitosis instead of meiosis.

(b) Plants, in contrast to animals, form germ cells (sperm and eggs) from somatic tissues. If the chromosome content of a precursor somatic cell has accidentally doubled (e.g., as a result of passing through S phase of the cell cycle without following up with mitosis and cytokinesis), then gametes containing 2n chromosomes are formed.

Polyploidy also occurs naturally in certain plant tissues. As the endosperm (3n) develops in corn kernels (Zea mays), its cells undergo successive rounds (as many as 5) of endoreplication producing nuclei that range as high as 96n.

Polyploidy can be of following two types:

1. Allopolyploidy:

Autopolyploids are polyploids with multiple chromosome sets derived from a single species. Autopolyploids can arise from a spontaneous, naturally occurring genome doubling, like the potato. Others might form following fusion of diploid (2n) gametes. Bananas and apples can be found as autotriploids. Autopolyploid plants typically display polysomic inheritance and are therefore often infertile and propagated clonally perfect.

2. Allopolyploidy:

Allopolyploids are polyploids with chromosomes derived from different species. Precisely it is the result of doubling of chromosome number in an F1 hybrid. Triticale is an example of an allopolyploid, having six chromosome sets, allohexaploid, four from wheat (Triticum turgidum) and two from rye (Secale cereale). Amphidiploid is another word for an allopolyploid.


How the cell cycle clock ticks

Eukaryotic cell division has been studied thoroughly and is understood in great mechanistic detail. Paradoxically, however, we lack an understanding of its core control process, in which the master regulator of the cell cycle, cyclin-dependent kinase (CDK), temporally coordinates an array of complex molecular events. The core elements of the CDK control system are conserved in eukaryotic cells, which contain multiple cyclin–CDK forms that have poorly defined and partially overlapping responsibilities in the cell cycle. However, a single CDK can drive all events of cell division in both mammalian and yeast cells, and in fission yeast a single mitotic cyclin can drive the cell cycle without major problems. But how can the same CDK induce different events when activated at different times during the cell cycle? This question, which has bewildered cell cycle researchers for decades, now has a sufficiently clear mechanistic answer. This Perspective aims to provide a synthesis of recent data to facilitate a better understanding of this central cellular control system.

CDK—the master regulator of the cell cycle—sends signals to control all major steps of cell division. CDK is activated at each stage of the cell cycle by binding of stage-specific cyclins. For example, in the late G1 phase, G1 cyclin–CDK complexes phosphorylate and inactivate transcriptional repressors to unleash the transcription of hundreds of genes required for cell cycle entry and S phase (Bertoli et al., 2013). Subsequently, DNA replication, centrosome duplication, spindle formation and other cell cycle processes are initiated by cyclin-CDK complexes that drive phosphorylation of multiple key regulators. Distinct cyclin-CDK complexes can be linked to specific processes, however, in most cases the responsibilities are shared among different cyclin–CDK complexes (Bloom and Cross, 2007). Importantly, often the later cyclin–CDK complexes can compensate for the absence of earlier complexes, the most extreme example being in fission yeast, where a single mitotic cyclin–CDK complex is sufficient to drive the cell cycle (Coudreuse and Nurse, 2010). To understand the scale of the networks formed by regulatory paths that emanate from the central CDK node, one can consider that in Saccharomyces cerevisiae, where the CDK system is best known, CDK is estimated to phosphorylate ∼500–700 proteins (Ubersax et al., 2003 Holt et al., 2009), which make up ∼10% of the proteome. Furthermore, the phosphoregulatory significance for roughly 100 of these targets has already been sufficiently well demonstrated both in vitro and in vivo (Enserink and Kolodner, 2010). Thus, each major cell cycle process is triggered and regulated by tens of different CDK phosphorylation events until cyclin accumulation culminates in metaphase. After the metaphase–anaphase transition, cyclin levels start to decline, and at least a fraction of CDK-controlled phosphorylations are reversed by phosphatases (Bremmer et al., 2012). These dephosphorylation events act as specific switches that drive mitotic exit (Bloom et al., 2011).

The ordering of cell cycle events is dependent on CDK activity (Coudreuse and Nurse, 2010 Swaffer et al., 2016). For example, if fission yeast cells arrested in G1 are directly released to mitotic cyclin–CDK activity, then S-phase and mitosis occur simultaneously (Swaffer et al., 2016). These findings have led to one of the core questions in cell cycle: how does activation of the same catalytic subunit induce different events at different times during the cell cycle?

A particular uniqueness of the CDK system among most of the eukaryotic protein kinases is that it is not a simple binary kinase-ON/kinase-OFF system, but rather, as elegantly proposed by Stern and Nurse in 1996, the accumulating CDK fires its downstream targets when reaching increasing thresholds at each subsequent cell cycle transition (Figure 1A Stern and Nurse, 1996). The range of different thresholds and the complexity of the switching order could be further increased if CDK were sequentially activated by different cyclins, which might dynamically alter the functional specificity of the net mix of CDK kinase complexes (Figure 1B). Therefore, a multitude of discovered versions of cyclins (and CDKs in higher eukaryotes) have prompted researchers in past decades to pose the question of cyclin specificity in substrate recognition: are there cyclin-specific CDK substrates, and if so, what are the specificity mechanisms that the particular CDK complexes use to recognize them within the pool of all potential CDK targets?

FIGURE 1: (A) The principle of the quantitative model of CDK function in the cell cycle (Stern and Nurse, 1996). Accumulating CDK activity triggers cell cycle stages at different activity thresholds. (B) The net profile of activated CDK complex (light blue line) is a sum of CDK complexes activated by cell cycle–specific cyclins. For example, in budding yeast, there are four major types of cyclins that drive the cell cycle, designated here by different colors. It is not yet clear how the quantitative model and the functional specificities of periodically expressed cyclins can be combined into a unified model. (C) When the increase in specificity of sequential CDK complexes is taken into account, the general activity profile of CDK (dark blue line) has a delayed response compared with the accumulation of cyclin–CDK complexes (light blue line). Patterns of linear docking motifs and cyclin-specific time windows can create a wide dynamic range of CDK thresholds in the cell cycle.

In fact, this question was often formulated without acknowledging the fact that protein kinases are generally very similar in their active site specificity. The whole kinome can roughly be divided into basophilic, acidophilic, and proline-directed kinases ,with large subfamilies of kinases having overlapping specificity (Miller and Turk, 2018). Although distant docking interactions provided by different cyclins could provide an additional level of specificity, the basal active site specificity of CDK kinase subunits, which recognize both full consensus motifs (S/TPXK/R) and minimal consensus motifs (S/TP), would apparently prevent absolute discrimination between targets. Because such extreme specificity among different CDK complexes was not conspicuous, the general mechanism of CDK function remained unexplained and presented several fundamental questions, including how different thresholds and cell cycle execution time are encoded into cyclin–CDK complexes and their targets.

The first possible solution to this problem was offered by early studies on cyclin specificity, which revealed that S-phase cyclins target the CDK complex to phosphorylate specific substrates via binding to a short linear motif—the RXL motif—on substrates (Schulman et al., 1998 Wilmes et al., 2004 Loog and Morgan, 2005). In S. cerevisiae, G1 cyclins have evolved a different linear motif for G1-specific CDK phosphorylation (Bhaduri and Pryciak, 2011 Kõivomägi et al., 2011b). In recent studies, we have also found that the G2- and M-phase specific cyclins can use specific docking sites of their own to enhance the phosphorylation of sites and fine tune the phosphorylation thresholds of different targets (unpublished data).

The second important finding was that the phosphorylation rate and specificity of the CDK complexes for a peptide containing a full consensus CDK phosphorylation site increase in the order of appearance of cyclins in the cell cycle (Figure 2 and Box 1 Loog and Morgan, 2005 Kõivomägi et al., 2011b). It is important to bear in mind that the kinetic analysis was performed using a short peptide substrate interacting only with the CDK active site and not with cyclins. A similar increasing specificity profile has been found in biochemical studies with mammalian CDKs (Jan Skotheim and Mardo Kõivomägi, personal communication). This mechanism ensures that early CDK complexes will not induce late cell cycle events and also makes it possible to keep the early substrates phosphorylated throughout the cell cycle, which is critical in the case of replication proteins, for example. The poor specificity of the early complexes can be compensated for by cyclin-specific docking interactions in G1- and S-phase specific CDK targets, which explains how, at very low CDK activity in the early stages of the cell cycle, efficient phosphorylation switches can be triggered (Box 1).

FIGURE 2: The activity of CDK complex toward a peptide containing a full consensus phosphorylation motif rises in the order of expression of cyclins in the cell cycle. The kinetic analysis was performed using a short peptide substrate that only interacts with the CDK active site and not with cyclins. Figure adapted from Kõivomägi et al., 2011b.

BOX 1: Cyclin specificity of CDK substrate phosphorylation.

Besides being activators of CDK kinase subunits, different cyclins can introduce different activation levels of cyclin–CDK complexes. Surprisingly, in both yeast and mammals, the intrinsic activity of cyclin–CDK complexes was found to increase in correlation with the appearance of the particular cyclin in the cell cycle. That is, the G1 and G1/S cyclins produce complexes with the lowest activity, which manifests in the highest KM and the lowest kcat values toward the model substrate, while the following S-, G2-, and M-phase complexes gradually lower the KM and have higher kcat values. These kinetic parameters were measured using a short model peptide containing a full consensus CDK site whose phosphorylation rate would be influenced only by the active site of CDK and not by any distant docking interaction with cyclin or Cks1. This cyclin-­specific stepwise increase creates a delay in the CDK activity profile, if one considers only the active site. However, this delay can be used efficiently to create highly specific low-KM targets for early thresholds using distant docking interactions via pockets that only the early cyclins have. For example, the G1-specific cyclins Cln1/2 in budding yeast use so-called LLPP motifs in substrates for cyclin targeting, while the S-phase cyclins use the RXL motifs to lower the KM of specific substrates. This docking-induced potentiation can reach up to 100-fold when compared with the active site model peptide. In this way, the early cyclins do not prematurely trigger the later thresholds, but can phosphorylate specific substrates with cyclin-specific docking motifs at lower net CDK thresholds at early stages of the cell cycle.

Finally, in addition to cyclin-specific docking motifs, poor specificity of phosphorylation sites can be enhanced by the phosphoadaptor Cks1, which binds phosphorylated TP sites and potentiates the phosphorylation of secondary sites (Figure 3 Kõivomägi et al., 2011a, 2013 McGrath et al., 2013). Because the majority of CDK targets have multiple phosphorylation sites clustered in disordered regions, we set out to clarify the mechanistic logic of these multisite phosphorylation processes (Kõivomägi et al., 2013 Valk et al., 2014). First, we found that only phosphorylated TP sites, but not SP sites, bind to Cks1. Second, the effect of both Cks1-mediated docking and cyclin-specific docking on the phosphorylation rate is highly dependent on the relative positioning of docking sites and phosphorylation sites (Figure 3). Therefore, the docking interactions direct CDK to phosphorylate specific sites, which leads to an ordered multisite phosphorylation process. The net rate of multisite phosphorylation is governed by the distances between phosphorylation sites and docking motifs, the distribution of TP and SP sites, consensus motif elements around the phosphorylation sites, and other parameters (Kõivomägi et al., 2013). These multisite phosphorylation network parameters, or the CDK multisite phosphorylation code, could control the thresholds via the net rate of accumulation of a critical combination of phosphorylated sites required for the downstream signaling switch. For example, depending on the presence or absence, and also positioning of the Cks1-docking sites, the mechanism of multisite phosphorylation can have a high degree of processivity, or alternatively, be entirely distributive. The former means that every phosphate in the multisite cluster is added without the CDK complex dissociating between the subsequent phosphorylation events. In contrast, in the case of the distributive mechanism, the kinase complex dissociates between every pair of phosphorylation events. With respect to CDK thresholds, a more processive mechanism reaches the fully phosphorylated state—the output signal, at lower CDK activity. Taken together, the combination of cyclin-specific docking motifs and Cks1-dependent phosphorylation mechanism enables differential phosphorylation of a wide range of CDK substrates (Figures 1C and 3). Because CDK phosphorylation clusters are almost exclusively located in intrinsically disordered regions of the proteins (Holt et al., 2009), the coding can be entirely linear, using short linear motifs (SLiMs: CDK phosphorylation motifs, cyclin docking motifs, phosphodegrons, etc.), and linkers, where amino acids can be counted for necessary distances between the SLiMs.

FIGURE 3: Schematic diagram showing the main interactions between substrate proteins and the CDK complex that determine the phosphorylation rate and specificity. The CDK active site phosphorylates full consensus motifs (S/TPXK/R) and minimal consensus (S/TP) motifs. The phosphorylation rate of a site can be increased by two docking interactions: Cks1 can bind to phosphorylated TP sites and cyclins can interact with substrates via specific short linear motifs. In both cases, the effect of docking is dependent on the relative positioning of docking sites and phosphorylation sites along the disordered substrate.

In a proteomics study aimed at analyzing the phosphorylation dynamics of CDK targets in Schizosaccharomyces pombe, it was found that CDK substrates can be divided into early, middle, and late targets (Swaffer et al., 2016). Importantly, phosphorylation of late targets required higher CDK activity than for earlier targets. Surprisingly, no correlation was found between the substrate’s sensitivity to CDK activity and the phosphorylation motif being either minimal or full consensus. This finding was unexpected, because early work on CDK specificity had shown a great increase in the phosphorylation rate of peptides that contained the full-consensus motif in comparison with ones that had a minimal-consensus motif (Songyang et al., 1994). This suggests that the protein context, including the linear patterns of helper docking motifs, may be even more important than the specificity of phosphorylation sites themselves.

Discrimination between early and late CDK substrates can also be driven by the differing phosphatase activity toward these sets of targets. For example, the S. cerevisiae phosphatase PP2A Cdc55 specifically counteracts phosphorylation of threonine residues, and this leads to threonine-based CDK sites being phosphorylated at higher CDK activity and therefore later in the cell cycle than serine-based sites (Godfrey et al., 2017). Studies in S. pombe, however, have shown that while threonines are indeed dephosphorylated faster than serines, the dephosphorylation rates of early and late targets are similar (Swaffer et al., 2016). Therefore, while there is significant evidence that phosphatase activity denies early phosphorylation of some CDK targets (Ndd1, Net1 Queralt et al., 2006 Godfrey et al., 2017), it is not clear whether differential phosphatase activity leads to global ordering of CDK thresholds. In addition, phosphatase activity, in combination with cyclin-­specific docking interactions, has been shown to play a role in ordering the thresholds during mitotic exit. Because the S-phase cyclin is degraded in metaphase before the M-phase cyclin, S-CDK-specific targets were found to be dephosphorylated earlier (Jin et al., 2008).

In conclusion, the system comprising a mix of CDK activities, mediated by different cyclin–CDK complexes, with changing but also a common baseline specificity, is apparently different from a hypothetical system of binary switches triggered by orthogonal kinase activities activated at each cell cycle stage. For example, in budding yeast, the changes in specificity are caused by different cyclins, while the common baseline activity occurs because these periodically expressed cyclins activate a common Cdk1 kinase. Most importantly, if there were an exclusively specific kinase pathway evolved for each stage of the cell cycle, then it would be difficult to maintain phosphorylation of the targets that should be phosphorylated during the whole span of S and M phases. These include the sets of targets that safeguard the mechanisms that prevent relicensing, rereplication, reduplication, and the like—the core principles of the once-per-cell-cycle. Thus, due to the special character of the cell cycle, a central control system with a set of ON–OFF-style branched kinase pathways with absolute specificity would not be an optimal solution. Instead, a fine ladder of thresholds encoded into the substrates by highly cyclin-specific docking motifs, different phosphorylation site patterns, and Cks1-binding sites would create a virtually unlimited set of CDK input–output functions and different mixtures of switching orders. However, because the mitotic cyclin, like a central administrator, has a key to every threshold, the system is safe and robust to withstand anomalies in cyclin accumulation waves. Simultaneous fine tuning and robustness of the CDK system are especially important for single-cell organisms such as yeasts that depend on competitive growth. Indeed, the extreme complexity of processive multisite phosphorylation schemes has been found in budding yeast to coordinate thresholds with the highest precision between Start and G1/S, a stage spanning just 10–15 min (Kõivomägi et al., 2011a Repetto et al., 2018). In this respect, the single–mitotic cyclin system (Coudreuse and Nurse, 2010) is like a clock that shows hours, while the full cyclin-specific system ticks with minute or even second precision (Figure 1C).


Cyclin-Dependent Kinases

CDK-Activating Kinase

CDKs are only fully active following phosphorylation of a conserved threonine residue within the activation segment (Thr160 in the human CDK2 sequence). In Sc. pombe, the enzyme responsible, called CDK-activating kinase (CAK), is a CDK/cyclin complex, Mop1(Crk1)/Mcs2. This complex is functionally similar to CDK7/cyclin H of higher organisms, which is described below. In addition to their CAK activity, both enzymes can phosphorylate the CTD of RNA polymerase II and so play a role in the regulation of transcription. In Sa. cerevisiae cells, activation of Cdc28 results from phosphorylation by CIV1. CIV1 is distantly related to the CDK family and does not require a cyclin partner for activity. Another CDK/cyclin complex in Sa. cerevisiae, Kin28/Ccl1, phosphorylates the CTD of RNA polymerase II.


Clinical Relevance - Neoplasia

Neoplasia is a disease of unchecked cell division and its progression is attributed to a change in activity of cell cycle regulators. If a mutation occurs in a protein that regulates the cell cycle, e.g. p53, it can lead to rapid, uncontrolled multiplication of these cells.

When there is a defect in p53 tumour suppressor gene, it cannot detect and bind to cells with damaged DNA to either repair the damage or cause apoptosis. This leads to unchecked replication of cells in the cell cycle and an increase in mutated p53. This increases the risk of neoplasms and also brings out the cancerous properties in the mutant p53.


Biology 171

By the end of this section, you will be able to do the following:

  • Understand how the cell cycle is controlled by mechanisms that are both internal and external to the cell
  • Explain how the three internal “control checkpoints” occur at the end of G1, at the G2/M transition, and during metaphase
  • Describe the molecules that control the cell cycle through positive and negative regulation

The length of the cell cycle is highly variable, even within the cells of a single organism. In humans, the frequency of cell turnover ranges from a few hours in early embryonic development, to an average of two to five days for epithelial cells, and to an entire human lifetime spent in G0 by specialized cells, such as cortical neurons or cardiac muscle cells.

There is also variation in the time that a cell spends in each phase of the cell cycle. When rapidly dividing mammalian cells are grown in a culture (outside the body under optimal growing conditions), the length of the cell cycle is about 24 hours. In rapidly dividing human cells with a 24-hour cell cycle, the G1 phase lasts approximately nine hours, the S phase lasts 10 hours, the G2 phase lasts about four and one-half hours, and the M phase lasts approximately one-half hour. By comparison, in fertilized eggs (and early embryos) of fruit flies, the cell cycle is completed in about eight minutes. This is because the nucleus of the fertilized egg divides many times by mitosis but does not go through cytokinesis until a multinucleate “zygote” has been produced, with many nuclei located along the periphery of the cell membrane, thereby shortening the time of the cell division cycle. The timing of events in the cell cycle of both “invertebrates” and “vertebrates” is controlled by mechanisms that are both internal and external to the cell.

Regulation of the Cell Cycle by External Events

Both the initiation and inhibition of cell division are triggered by events external to the cell when it is about to begin the replication process. An event may be as simple as the death of nearby cells or as sweeping as the release of growth-promoting hormones, such as human growth hormone (HGH or hGH) . A lack of HGH can inhibit cell division, resulting in dwarfism, whereas too much HGH can result in gigantism. Crowding of cells can also inhibit cell division. In contrast, a factor that can initiate cell division is the size of the cell: As a cell grows, it becomes physiologically inefficient due to its decreasing surface-to-volume ratio. The solution to this problem is to divide.

Whatever the source of the message, the cell receives the signal, and a series of events within the cell allows it to proceed into interphase. Moving forward from this initiation point, every parameter required during each cell cycle phase must be met or the cycle cannot progress.

Regulation at Internal Checkpoints

It is essential that the daughter cells produced be exact duplicates of the parent cell. Mistakes in the duplication or distribution of the chromosomes lead to mutations that may be passed forward to every new cell produced from an abnormal cell. To prevent a compromised cell from continuing to divide, there are internal control mechanisms that operate at three main cell-cycle checkpoints : A checkpoint is one of several points in the eukaryotic cell cycle at which the progression of a cell to the next stage in the cycle can be halted until conditions are favorable. These checkpoints occur near the end of G1, at the G2/M transition, and during metaphase ((Figure)).


The G1 Checkpoint

The G1 checkpoint determines whether all conditions are favorable for cell division to proceed. The G1 checkpoint, also called the restriction point (in yeast), is a point at which the cell irreversibly commits to the cell division process. External influences, such as growth factors, play a large role in carrying the cell past the G1 checkpoint. In addition to adequate reserves and cell size, there is a check for genomic DNA damage at the G1 checkpoint. A cell that does not meet all the requirements will not be allowed to progress into the S phase. The cell can halt the cycle and attempt to remedy the problematic condition, or the cell can advance into G0 and await further signals when conditions improve.

The G2 Checkpoint

The G2 checkpoint bars entry into the mitotic phase if certain conditions are not met. As at the G1 checkpoint, cell size and protein reserves are assessed. However, the most important role of the G2 checkpoint is to ensure that all of the chromosomes have been replicated and that the replicated DNA is not damaged. If the checkpoint mechanisms detect problems with the DNA, the cell cycle is halted, and the cell attempts to either complete DNA replication or repair the damaged DNA.

The M Checkpoint

The M checkpoint occurs near the end of the metaphase stage of karyokinesis. The M checkpoint is also known as the spindle checkpoint, because it determines whether all the sister chromatids are correctly attached to the spindle microtubules. Because the separation of the sister chromatids during anaphase is an irreversible step, the cycle will not proceed until the kinetochores of each pair of sister chromatids are firmly anchored to at least two spindle fibers arising from opposite poles of the cell.

Watch what occurs at the G1, G2, and M checkpoints by visiting this website to see an animation of the cell cycle.

Regulator Molecules of the Cell Cycle

In addition to the internally controlled checkpoints, there are two groups of intracellular molecules that regulate the cell cycle. These regulatory molecules either promote progress of the cell to the next phase (positive regulation) or halt the cycle (negative regulation). Regulator molecules may act individually, or they can influence the activity or production of other regulatory proteins. Therefore, the failure of a single regulator may have almost no effect on the cell cycle, especially if more than one mechanism controls the same event. However, the effect of a deficient or non-functioning regulator can be wide-ranging and possibly fatal to the cell if multiple processes are affected.

Positive Regulation of the Cell Cycle

Two groups of proteins, called cyclins and cyclin-dependent kinases (Cdks), are termed positive regulators. They are responsible for the progress of the cell through the various checkpoints. The levels of the four cyclin proteins fluctuate throughout the cell cycle in a predictable pattern ((Figure)). Increases in the concentration of cyclin proteins are triggered by both external and internal signals. After the cell moves to the next stage of the cell cycle, the cyclins that were active in the previous stage are degraded by cytoplasmic enzymes, as shown in (Figure) below.


Cyclins regulate the cell cycle only when they are tightly bound to Cdks. To be fully active, the Cdk/cyclin complex must also be phosphorylated in specific locations to activate the complex. Like all kinases, Cdks are enzymes (kinases) that in turn phosphorylate other proteins. Phosphorylation activates the protein by changing its shape. The proteins phosphorylated by Cdks are involved in advancing the cell to the next phase. ((Figure)). The levels of Cdk proteins are relatively stable throughout the cell cycle however, the concentrations of cyclin fluctuate and determine when Cdk/cyclin complexes form. The different cyclins and Cdks bind at specific points in the cell cycle and thus regulate different checkpoints.


Because the cyclic fluctuations of cyclin levels are largely based on the timing of the cell cycle and not on specific events, regulation of the cell cycle usually occurs by either the Cdk molecules alone or the Cdk/cyclin complexes. Without a specific concentration of fully activated cyclin/Cdk complexes, the cell cycle cannot proceed through the checkpoints.

Although the cyclins are the main regulatory molecules that determine the forward momentum of the cell cycle, there are several other mechanisms that fine-tune the progress of the cycle with negative, rather than positive, effects. These mechanisms essentially block the progression of the cell cycle until problematic conditions are resolved. Molecules that prevent the full activation of Cdks are called Cdk inhibitors. Many of these inhibitor molecules directly or indirectly monitor a particular cell-cycle event. The block placed on Cdks by inhibitor molecules will not be removed until the specific event that the inhibitor monitors is completed.

Negative Regulation of the Cell Cycle

The second group of cell-cycle regulatory molecules are negative regulators, which stop the cell cycle. Remember that in positive regulation, active molecules cause the cycle to progress.

The best understood negative regulatory molecules are retinoblastoma protein (Rb) , p53 , and p21 . Retinoblastoma proteins are a group of tumor-suppressor proteins common in many cells. We should note here that the 53 and 21 designations refer to the functional molecular masses of the proteins (p) in kilodaltons (a dalton is equal to an atomic mass unit, which is equal to one proton or one neutron or 1 g/mol). Much of what is known about cell-cycle regulation comes from research conducted with cells that have lost regulatory control. All three of these regulatory proteins were discovered to be damaged or non-functional in cells that had begun to replicate uncontrollably (i.e., became cancerous). In each case, the main cause of the unchecked progress through the cell cycle was a faulty copy of the regulatory protein.

Rb, p53, and p21 act primarily at the G1 checkpoint. p53 is a multi-functional protein that has a major impact on the commitment of a cell to division because it acts when there is damaged DNA in cells that are undergoing the preparatory processes during G1. If damaged DNA is detected, p53 halts the cell cycle and then recruits specific enzymes to repair the DNA. If the DNA cannot be repaired, p53 can trigger apoptosis, or cell suicide, to prevent the duplication of damaged chromosomes. As p53 levels rise, the production of p21 is triggered. p21 enforces the halt in the cycle dictated by p53 by binding to and inhibiting the activity of the Cdk/cyclin complexes. As a cell is exposed to more stress, higher levels of p53 and p21 accumulate, making it less likely that the cell will move into the S phase.

Rb, which largely monitors cell size, exerts its regulatory influence on other positive regulator proteins. In the active, dephosphorylated state, Rb binds to proteins called transcription factors, most commonly, E2F ((Figure)). Transcription factors “turn on” specific genes, allowing the production of proteins encoded by that gene. When Rb is bound to E2F, production of proteins necessary for the G1/S transition is blocked. As the cell increases in size, Rb is slowly phosphorylated until it becomes inactivated. Rb releases E2F, which can now turn on the gene that produces the transition protein, and this particular block is removed. For the cell to move past each of the checkpoints, all positive regulators must be “turned on,” and all negative regulators must be “turned off.”


Rb and other proteins that negatively regulate the cell cycle are sometimes called tumor suppressors. Why do you think the name tumor suppressor might be appropriate for these proteins?

Section Summary

Each step of the cell cycle is monitored by internal controls called checkpoints. There are three major checkpoints in the cell cycle: one near the end of G1, a second at the G2/M transition, and the third during metaphase. Positive regulator molecules allow the cell cycle to advance to the next stage of cell division. Negative regulator molecules monitor cellular conditions and can halt the cycle until specific requirements are met.

Art Connections

(Figure) Rb and other proteins that negatively regulate the cell cycle are sometimes called tumor suppressors. Why do you think the name tumor suppressor might be appropriate for these proteins?

(Figure) Rb and other negative regulatory proteins control cell division and therefore prevent the formation of tumors. Mutations that prevent these proteins from carrying out their function can result in cancer.

Free Response

Describe the general conditions that must be met at each of the three main cell-cycle checkpoints.

The G1 checkpoint monitors adequate cell growth, the state of the genomic DNA, adequate stores of energy, and materials for S phase. At the G2 checkpoint, DNA is checked to ensure that all chromosomes were duplicated and that there are no mistakes in newly synthesized DNA. Additionally, cell size and energy reserves are evaluated. The M checkpoint confirms the correct attachment of the mitotic spindle fibers to the kinetochores.

Compare and contrast the roles of the positive cell-cycle regulators negative regulators.

Positive cell regulators such as cyclin and Cdk perform tasks that advance the cell cycle to the next stage. Negative regulators such as Rb, p53, and p21 block the progression of the cell cycle until certain events have occurred.

What steps are necessary for Cdk to become fully active?

Cdk must bind to a cyclin, and it must be phosphorylated in the correct position to become fully active.

Rb is a negative regulator that blocks the cell cycle at the G1 checkpoint until the cell achieves a requisite size. What molecular mechanism does Rb employ to halt the cell cycle?

Rb is active when it is dephosphorylated. In this state, Rb binds to E2F, which is a transcription factor required for the transcription and eventual translation of molecules required for the G1/S transition. E2F cannot transcribe certain genes when it is bound to Rb. As the cell increases in size, Rb becomes phosphorylated, inactivated, and releases E2F. E2F can then promote the transcription of the genes it controls, and the transition proteins will be produced.

Glossary


10.1A: The Role of the Cell Cycle - Biology

The nucleus is a membrane bound organelle that contains the genetic information in the form of chromatin, highly folded ribbon-like complexes of deoxyribonucleic acid (DNA) and a class of proteins called histones.

When a cell divides, chromatin fibers are very highly folded, and become visible in the light microscope as chromosomes. During interphase (between divisions), chromatin is more extended, a form used for expression genetic information.

The DNA of chromatin is wrapped around a complex of histones making what can appear in the electron microscope as "beads on a string" or nucleosomes. Changes in folding between chromatin and the mitotic chromosomes is controlled by the packing of the nucleosome complexes.

DNA or d eoxyribo n ucleic a cid is a large molecule structured from chains of repeating units of the sugar deoxyribose and phosphate linked to four different bases abbreviated A, T, G, and C. We will later show how the simple structure of DNA contains the information for specifying the proteins that allow life. The process of mitosis is designed to insure that exact copies of the DNA in chromosomes are passed on to daughter cells.


Steps in the cycle

  • DNA damage checkpoints. These sense DNA damage both before the cell enters S phase (a G1 checkpoint) as well as after S phase (a G2 checkpoint).
    • Damage to DNA before the cell enters S phase inhibits the action of Cdk2 thus stopping the progression of the cell cycle until the damage can be repaired. If the damage is so severe that it cannot be repaired, the cell self-destructs by apoptosis.
    • Damage to DNA after S phase (the G2 checkpoint), inhibits the action of Cdk1 thus preventing the cell from proceeding from G2 to mitosis.
    • detect any failure of spindle fibers to attach to kinetochores and arrest the cell in metaphase until all the kinetochores are attached correctly (M checkpoint &mdash example)
    • detect improper alignment of the spindle itself and block cytokinesis
      Link to discussion of the roles of the spindle in mitosis.
    • trigger apoptosis if the damage is irreparable.

    All the checkpoints examined require the services of a complex of proteins. Mutations in the genes encoding some of these have been associated with cancer that is, they are oncogenes. This should not be surprising since checkpoint failures allow the cell to continue dividing despite damage to its integrity.

    Examples

    The TP53 protein senses DNA damage and can halt progression of the cell cycle in G1 (by blocking the activity of Cdk2). Both copies of the TP53 gene must be mutated for this to fail so mutations in TP53 are recessive, and TP53 qualifies as a tumor suppressor gene.

    Further discussion of tumor suppressor genes and TP53.

    The p53 protein is also a key player in apoptosis, forcing "bad" cells to commit suicide. So if the cell has only mutant versions of the protein, it can live on &mdash perhaps developing into a cancer. More than half of all human cancers do, in fact, harbor p53 mutations and have no functioning p53 protein.

    A genetically-engineered adenovirus, called ONYX-015, can only replicate in human cells lacking p53. Thus it infects, replicates, and ultimately kills many types of cancer cells in vitro. Clinical trials are now proceeding to see if injections of ONYX-015 can shrink a variety of types of cancers in human patients. (You will find that the human gene is variously designated as P53, TP53 ["tumor protein 53"], and TRP53 ["transformation-related protein 53"])

    ATM ( ) gets its name from a human disease of that name [Link], whose patients &mdash among other things &mdash are at a greatly increased (

    100 fold) risk of cancer. The ATM protein is involved in

    • detecting DNA damage, especially double-strand breaks
    • interrupting (with the aid of p53) the cell cycle when damage is found
    • maintaining normal telomere length.

    ATR ( ) also gets its name from the human disease. The ATR protein halts the progression of the cell cycle until DNA replication is complete. Thus ATR provides an S/G2 checkpoint.

    MAD ( ) genes (there are two) encode proteins that bind to each kinetochore until a spindle fiber (one microtubule will do) attaches to it. If there is any failure to attach, MAD remains and blocks entry into anaphase (by inhibiting the anaphase-promoting complex).

    Mutations in MAD produce a defective protein and failure of the checkpoint. The cell finishes mitosis but produces daughter cells with too many or too few chromosomes, a condition called aneuploidy. More than 90% of human cancer cells are aneuploid.

    Infection with the human T-cell lymphotropic virus-1 (HTLV-1) leads to a cancer (ATL = "adult T-cell leukemia/lymphoma") in 3&ndash5% of those infected. HTLV-1 encodes a protein, called Tax, that binds to MAD protein causing failure of the spindle checkpoint. The leukemic cells in these patients show many chromosome abnormalities including aneuploidy.

    A kinesin that moves the kinetochore to the end of the spindle fiber also seems to be involved in the spindle checkpoint [More].

    Many times a cell will leave the cell cycle, temporarily or permanently. It exits the cycle at G1 and enters a stage designated G0 (G zero). A G0 cell is often called "quiescent", but that is probably more a reflection of the interests of the scientists studying the cell cycle than the cell itself. Many G0 cells are anything but quiescent. They are busy carrying out their functions in the organism. e.g., secretion, attacking pathogens.

    Often G0 cells are terminally differentiated: they will never reenter the cell cycle but instead will carry out their function in the organism until they die.

    For other cells, G0 can be followed by reentry into the cell cycle. Most of the lymphocytes in human blood are in G0. However, with proper stimulation, such as encountering the appropriate antigen [View], they can be stimulated to reenter the cell cycle (at G1) and proceed on to new rounds of alternating S phases and mitosis.

    G0 represents not simply the absence of signals for mitosis but an active repression of the genes needed for mitosis. Cancer cells cannot enter G0 and are destined to repeat the cell cycle indefinitely. [More]