Information

15.5: Pre-mitotic Phases - Biology


G1/G0 Phase

The G1 phase is the state a cell is in immediately following cytokinesis. Most types of cells spend the majority of their cycle in G1, although there are exceptions, such as the frog oocytes mentioned earlier. The length of G1 is generally constant for a given cell type under normal conditions, but can vary greatly between different cell types. Post-mitotic cells, which have left the cell cycle and will no longer divide, are in G1 until they die, barring reactivation of the cell cycle by stress conditions. This continuous G1-like state is referred to as G0.

For those cells preparing to move from G1 into S, cyclins D and E, and cdk 2, 4, and 6 predominate, with activation of cyclin D complexes preceding activation of cyclin E complexes. Two major questions are asked by the cell: is the DNA undamaged and complete, and is the extracellular environment favorable for cell division? The cellular sensors for these conditions then link to cyclin complexes effect restriction points on cell cycle progression. The extracellular environment questions can be a tricky one, because this can include more than just assessment of nutrient availability or predatory threats; it can also be a requirement for an external trigger such as a mitogenic hormone or paracrine signal. In fact, nearly all normal animal cells require an extracellular signal to progress through the G1/S checkpoint. The cyclin E/cdk2 combination is the principal regulator of entry into S phase and DNA replication.

The active cyclin E/cdk2 complex phosphorylates the tumor suppressor protein Rb (retinoblastoma), which causes E2F to translocate to the nucleus and turn on genes needed for entry into S phase.

S phase

The mechanisms of DNA replication were discussed in Chapter 7. It is important to note that once a cell has entered S phase, it has essentially committed to going through cell division. Cells do not cope well with extra copies of chromosomes, and a cell that went through S phase without going through mitosis would likely have major malfunctions in gene regulation. For similar reasons, the cell must only undergo DNA replication once per cell division. The cyclinA/ cdk2 complex plays a key role in initiation of repli- cation by activating the pre-replicative complex. It also phosphorylates cdc6, causing it to dissociate from the ORC, and consequently the rest of the pre-RC. This prevents immediate re-use of this origin of replication, and since the phosphorylation of cdc6 allows it to be recognized by a ubiquitin ligase complex, it is tagged for proteolysis In addition to DNA replication, S phase is also the cell cycle stage in which centrosomes are duplicated in animal cells. The cyclin E/cdk2 combination licenses the duplication of centrosome, phosphorylating nucleophosmin, which then dissociates from the cen- trosome. This helps to trigger the centrosome duplication. Nucleophosmin does not reassociate with centrosomes until telophase, when it is no longer phosphorylated. Plk4 (Polo-family kinase 4) activity is necessary for centriole duplication, and appears to initiate the centriole assembly mechanism.

The ubiquitin ligase complex, SCF, is made up of three major proteins and several minor species. Skp1 (S-phase kinase-associated protein 1) can be an RNA polymerase elongation factor, but in this complex links the other two proteins together. Cul1 (Cullin 1) is an E3 type ubiquitin ligase. Finally, an F-box family protein like Rbx1 (Ring-box 1), that heterodimerizes with cullin-1 and may also recruit E2 ubiquitinating enzyme.. In addition to cdc6, it also recognizes and ubiquitinates CKIs (cyclin complex kinase inhibitors) such as p27, which is involved in processes such as DNA repair and error-checking.

G2 Phase

The G2 phase begins when DNA replication has completed. Having said that, before the cell is allowed out of G2 and on to M phase, it must pass a DNA fidelity checkpoint, ensuring that not only has replication been fully completed, but that there are no major errors. G2 is a relatively short phase (compared to G1) in most cell types, and it is spent building up energy and material stores for cell division and checking the DNA. If everything is ok, and the cyclin B/cdk1 complex has been activated, the cell proceeds to M phase.


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 produce two genetically identical cells. The cell cycle has two major phases: interphase and the mitotic phase (Figure 1). 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.

Figure 1. A cell moves through a series of phases in an orderly manner. During interphase, G1 involves cell growth and protein synthesis, the S phase involves DNA replication and the replication of the centrosome, and G2 involves further growth and protein synthesis. The mitotic phase follows interphase. Mitosis is nuclear division during which duplicated chromosomes are segregated and distributed into daughter nuclei. Usually the cell will divide after mitosis in a process called cytokinesis in which the cytoplasm is divided and two daughter cells are formed.


15.5: Pre-mitotic Phases - Biology

Group Questions Chapter 15

1. List six events in cell division, and list how they vary between Mitosis, Meiosis I, and Meiosis II

2. Diagram the plant life cycle that has alternation of generations

3. List seven Aneuploid conditions in humans, list the common name, and list the characteristics.

The Eukaryotic Cell Cycle, Mitosis, and Meiosis (p. 303-326)

Cell division: The process in which

Cytogenetics: The field of_______________ that involves the

Karyotype: A _______________ representation of the _______________ in an actively dividing cell.

Sex chromosomes: A distinctive pair of chromosomes

Autosomes: ____of the chromosomes found in the cell ______________ of eukaryotes

Diploid: Refers to cells containing

Gamete: A ________ _______ that is involved with sexual reproduction, such as a

Haploid: Refers to cells containing ________ set of chromosomes designated as

Homologue: A member of a pair of chromosomes in a

*the DNA of a homologues pair

Homology: A fundamental similarity that occurs due to descent from a common ancestor.

Cell cycle: The series of phases a eukaryotic cell progresses through from its

Phases of the cell cycle:

Interphase: The ___ ___ ______ ___ phases of the cell cycle. It is the

*G0 – The phase in which a cell may enter and

*cell _______________ typically occurs in this phase

S phase: The phase of the cell cycle during which

Sister chromatids: The

G2 phase: The _______________ gap phase of the cell cycle.

M phase: Includes the _______________ process and the _______________ process

Mitosis: In eukaryotes, the process in which ____________ division results in __________ nuclei, each of which receive

*look at times of each phase* (p. 307)

Cytokinesis: The _______________ of the _______________ to produce

Cyclins: A _______________ responsible for _______________ a cell through the phases of the cell cycle by binding to a

Cyclin-dependant kinase (cdk): _______________ responsible for advancing a cell through the

*Different _______________ increase during

*The specific_______________ binds to the_______________ to form activated

*once activated, the_______________ functions as a protein kinase to _______________ other _______________ needed in the next step

Checkpoint: One of _______________ critical _______________ points found in the cell cycle of_______________ cells.

At these checkpoints a variety of _______________ act as _______________ to determine if a cell is in the

*the _______________ degrades when the phase is over

Restriction point: A _______________ in the cell cycle in which a cell

*____– checkpoint proteins can sense if the

*___ – checkpoint proteins check the DNA for _______________ and ensure all

*_______________ checkpoint senses

Maturation promoting factor (MPF): A factor, now known to be a complex of

*It was first discovered in

15.2 – Mitotic Cell Division

Mitotic cell division: A process whereby a eukaryotic cell divides to produce

Asexual reproduction: A reproduction strategy that occurs when _______________ are produced from a ____________

parent, _______________ the _______________ of gametes from two parents. The _______________ are therefore

Reasons for mitotic cell division:

*_______________ transmission of cells of the body are

Centromere: The _______________ where the two sister chromatids are _______________ associated the centromere is

Kinetochore: A group of _______________ that _______________ to a _______________ and are

Mitotic spindle: The _______________ responsible for _______________ and _______________ the chromosomes

Centrosomes: A single _______________ often _______________ the cell _______________ of eukaryotic cells that

forms a _______________ site for the _______________ of _______________ also

Three types of spindle microtubes:

2) Polar microtubules (interpolar tubules) –

3) Kinetochore microtubules – attach to

Pole: A structure of the

Centrioles: A _______________ of _______________ within the _______________ of animal cells.

*transmission of chromosomes requires a

Interphase: The ___ ___ ______ ___ phases of the cell cycle.

*mitosis was first observed by

Prophase: The phase of mitosis during which the

Prometaphase: The phase of mitosis during which the _______________ _______________ is completely __________ and the

1) _______________move apart

2) _______________rapidly grow out from the _______________ and

Metaphase: The phase of mitosis during which the

*The_______________ _______________ pulled the

Metaphase plate: A _______________ _______________between the _______________of the _______________apparatus

Anaphase: The phase of mitosis during which the _______________ _______________ _______________from each other and

*The separated chromatids are now chromosomes and are only linked

*_______________ _______________lengthen to

*_______________ microtubules short to

Telophase: The phase of mitosis during which the chromosomes decondense and the nuclear membrane re-forms.

*two separate nuclei form with identical chromosomes

Cytokinesis: The _______________ of the _______________ to produce ______ distinct daughter cells.

*approx. ____ ___________are involved in cytokinesis in nearly all animal cells

Cleavage furrow: In _______________cells, an _______________ that _______________ like a _______________ to

Cell plate: In plant cells, a _______________that _______________ a ______ _________between the

15.3 – Meiosis and Sexual Reproduction

Sexual reproduction: A process that requires a_______________event in which_________gametes _______________to produce a cell called a

Fertilization: The _______________ of _____ __________, such as an egg cell with a sperm cell, to form a

Zygote: A _______________cell formed by the

*The _______________ grows by _______________to produce all body cells

Meiosis: The process by which_______________cells are produced _______________ a cell that was _______________

*______ _________ events happen at the beginning of meiosis that do not occur in mitosis

Bivalent: _______________ pair of sister chromatids associated with each other,

Tetrad: See bivalent.

Synapsis: The _______________of forming a

*a protein structure _______________complex _______________ homologous chromosomes

*the role of synaptonemal is ________ ______________________because some fungi lack this protein

Crossing over: The _______________ of _______________material between _______________ _____________

_______________ _______________ allows for _______________ _______________in the _______________

*the _______________ from the _______________, the

Chiasma: The _______________ at a

Meiosis I: The first division of meiosis in which the homologues are separated into different cells.

*Prophase I - homologous chromosomes_______________

*Prometaphase I – nuclear envelope completely

*Metaphase I – _______________ are aligned in a

*Anaphase I – the bivalent _____ __________, but _______ the

The pair of chromatids are pulled by the

*Telophase I – sister _______________ have reached their respective poles, and they _______________.

Meiosis II: The _______ ________of meiosis in which sister chromatids are

Life cycle: The sequence of events that characterize the steps of

*most multi-cellular organisms

Diploid-dominant species: Species in which the _______________ organism is the _______________ organism in the life cycle.

Haploid-dominant species: Species in which the _______________ organism is the _______________organism in the life cycle. Examples include

Alteration of generations: The _______________that occurs in _______________ and some _______________ in which the life cycle alternates between

*Both generations are multi-cellular

Sporophytes: The _______________generation of plants or _______________ protists that have a sporic life cycle this generation

Gametophytes: In plants and many multicellular protists, the haploid stage that

15.4 – Variation in chromosome Structure and Number

*chromosome structure and number an have major effects on organisms characteristics (look at p. 321 with chromosome numbers)

Metacentric: A chromosome in which the

Submetacentric: A chromosome in which the centromere

Acrocentric: A chromosome in which the centromere is

Telocentric: A chromosome in which the centromere is

G-banding: a result of ______ __ _______o chromosomes due to

Ways mutation can alter chromosome structure:

1) Deletion: A type of mutation in which a segment of genetic material is

2) Duplication: A type of mutation in which a section of a chromosome occurs

3) Inversion: A type of mutation that involves a

4) Translocation: 1. A type of mutation in which on segment of a chromosome

a. Simple translocation: A type of mutation in which a

b. Reciprocal translocation: A type of mutation in which

Variations in the number of chromosome sets and number of individual chromosomes

Euploid: An organism that has a chromosome number that is a

Polyploid: An organism that has

Triploid: An organism or cell that has

Tetraploid: An organism or cell that has

Aneuploidy: An alteration in the number of particular chromosomes so that the total number of chromosomes is

Trisomic: An _______________ organism that has one

Monosomic: An _______________organism that has one

Nondisjunction: An event during which the

Changes in chromosome number have important consequences:

*_______________ is generally lethal in mammals

Male bees are _______________ while female bees are _______________

Amphibians and reptiles occasionally have

30-35% of ferns and flowering plants are

*_______________among plants generally produces

Down syndrome: A human disorder caused by the inheritance of _______________ copies of


Success Rates Indicate Funding Levels

Typically higher than paylines, success rates are a better indicator than paylines of the percentage of applications we are funding for each activity code, especially for R01s.

The reason: many applications are funded beyond the R01 payline, for example, through NIAID's requests for applications, selective pay, and R56-Bridge awards.

A success rate is roughly the number of applications funded by an institute divided by the number of peer reviewed applications referred to it (excluding resubmissions that occur in the same fiscal year—each application is counted only once).

To find success rate data, go to Success Rates on NIH's RePORT Web site. For NIAID information by fiscal year, see Research Project Success Rates for NIAID.