In more advanced animals, the senses are constantly at work, making the animal aware of stimuli—such as light, or sound, or the presence of a chemical substance in the external environment—and monitoring information about the organism’s internal environment. All bilaterally symmetric animals have a sensory system, and the development of any species’ sensory system has been driven by natural selection; thus, sensory systems differ among species according to the demands of their environments. The shark, unlike most fish predators, is electrosensitive—that is, sensitive to electrical fields produced by other animals in its environment. While it is helpful to this underwater predator, electrosensitivity is a sense not found in most land animals.
6.1 The Genome
The continuity of life from one cell to another has its foundation in the reproduction of cells by way of the cell cycle. The cell cycle is an orderly sequence of events in the life of a cell from the division of a single parent cell to produce two new daughter cells, to the subsequent division of those daughter cells. The mechanisms involved in the cell cycle are highly conserved across eukaryotes. Organisms as diverse as protists, plants, and animals employ similar steps.
Before discussing the steps a cell undertakes to replicate, a deeper understanding of the structure and function of a cell’s genetic information is necessary. A cell’s complete complement of DNA is called its genome . In prokaryotes, the genome is composed of a single, double-stranded DNA molecule in the form of a loop or circle. The region in the cell containing this genetic material is called a nucleoid. Some prokaryotes also have smaller loops of DNA called plasmids that are not essential for normal growth.
In eukaryotes, the genome comprises several double-stranded, linear DNA molecules (Figure 6.2) bound with proteins to form complexes called chromosomes. Each species of eukaryote has a characteristic number of chromosomes in the nuclei of its cells. Human body cells (somatic cells) have 46 chromosomes. A somatic cell contains two matched sets of chromosomes, a configuration known as diploid . The letter n is used to represent a single set of chromosomes therefore a diploid organism is designated 2n. Human cells that contain one set of 23 chromosomes are called gametes , or sex cells these eggs and sperm are designated n, or haploid .
The matched pairs of chromosomes in a diploid organism are called homologous chromosomes . Homologous chromosomes are the same length and have specific nucleotide segments called genes in exactly the same location, or locus . Genes, the functional units of chromosomes, determine specific characteristics by coding for specific proteins. Traits are the different forms of a characteristic. For example, the shape of earlobes is a characteristic with traits of free or attached.
Each copy of the homologous pair of chromosomes originates from a different parent therefore, the copies of each of the genes themselves may not be identical. The variation of individuals within a species is caused by the specific combination of the genes inherited from both parents. For example, there are three possible gene sequences on the human chromosome that codes for blood type: sequence A, sequence B, and sequence O. Because all diploid human cells have two copies of the chromosome that determines blood type, the blood type (the trait) is determined by which two versions of the marker gene are inherited. It is possible to have two copies of the same gene sequence, one on each homologous chromosome (for example, AA, BB, or OO), or two different sequences, such as AB.
Minor variations in traits such as those for blood type, eye color, and height contribute to the natural variation found within a species. The sex chromosomes, X and Y, are the single exception to the rule of homologous chromosomes other than a small amount of homology that is necessary to reliably produce gametes, the genes found on the X and Y chromosomes are not the same.
Virtually every task performed by living organisms requires energy. Energy is needed to perform heavy labor and exercise. Humans also use a great deal of energy while thinking and even during sleep. In fact, the living cells of every organism constantly use energy. Nutrients and other molecules are imported, metabolized (broken down), synthesized into new molecules, modified if needed, transported around the cell, and, in some cases, distributed to the entire organism. For example, the large proteins that make up muscles are actively built from smaller molecules. Complex carbohydrates are broken down into simple sugars that the cell uses for energy. Just as energy is required to both build and demolish a building, energy is required for both the synthesis and breakdown of molecules. Additionally, signaling molecules such as hormones and neurotransmitters are actively transported between cells. Pathogenic bacteria and viruses are ingested and broken down by cells. Cells must also export waste and toxins to stay healthy. Many cells swim or move surrounding materials via the beating motion of cellular appendages such as cilia and flagella.
All of the cellular processes listed above require a steady supply of energy. From where, and in what form, does this energy come? How do living cells obtain energy and how do they use it? This chapter will discuss different forms of energy and the physical laws that govern energy transfer.
How enzymes lower the activation energy required to begin a chemical reaction in the body will also be discussed in this chapter. Enzymes are crucial for life without them the chemical reactions required to survive would not happen fast enough for an organism to survive. For example, in an individual who lacks one of the enzymes needed to break down a type of carbohydrate known as a mucopolysaccharide, waste products accumulate in the cells and cause progressive brain damage. This deadly genetic disease is called Sanfilippo Syndrome type B or Mucopolysaccharidosis III. Previously incurable, scientists have now discovered a way to replace the missing enzyme in the brain of mice. Read more about the scientists’ research here.
Metabolism encompasses a wide range of cellular activities, including the need for energy and the elimination of wastes from cells, tissues, and organs.
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Support, Movement, and Protection
Some functions of the skeletal system are more readily observable than others. When you move you can feel how your bones support you, facilitate your movement, and protect the soft organs of your body. Just as the steel beams of a building provide a scaffold to support its weight, the bones and cartilages of your skeletal system compose the scaffold that supports the rest of your body. Without the skeletal system, you would be a limp mass of organs, muscle, and skin. Bones facilitate movement by serving as points of attachment for your muscles. Bones also protect internal organs from injury by covering or surrounding them. For example, your ribs protect your lungs and heart, the bones of your vertebral column (spine) protect your spinal cord, and the bones of your cranium (skull) protect your brain (see Figure 6.1.1).
In 1996 Dolly the sheep became the first successfully cloned mammal. There was no addition of DNA from a father, no genetic “mixing,” just an identical copy of DNA from a mother. This accomplishment means many things, including the fact that we currently have the technology to mass-produce hundreds of copies of what seem to be the best and healthiest livestock.
But the scientific community seems to have little interest in setting up giant cloning labs. Instead, sex is viewed as desirable over asexual reproduction, or cloning. But why risk making a potentially inferior child when an ideal parent is available? With current genetic technology, humans can rewrite the genetic code of sheep. We have the capabilities to impart disease resistance, increase meat production and soon, we may be able to eliminate genetic disease. Why don’t we just make a “super sheep” and copy it thousands of times? What does sex do for individuals, particularly to their genetic code, that’s too important to replace with a really cool cloning lab?
Figure 6.1 Dolly the Sheep
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Chapter 10: Introduction to Biotechnology
Figure 10.1 (a) A thermal cycler, such as the one shown here, is a basic tool used to study DNA in a process called the polymerase chain reaction (PCR). The polymerase enzyme most often used with PCR comes from a strain of bacteria that lives in (b) the hot springs of Yellowstone National Park. (credit a: modification of work by Magnus Manske credit b: modification of work by Jon Sullivan)
The latter half of the twentieth century began with the discovery of the structure of DNA, then progressed to the development of the basic tools used to study and manipulate DNA. These advances, as well as advances in our understanding of and ability to manipulate cells, have led some to refer to the twenty-first century as the biotechnology century. The rate of discovery and of the development of new applications in medicine, agriculture, and energy is expected to accelerate, bringing huge benefits to humankind and perhaps also significant risks. Many of these developments are expected to raise significant ethical and social questions that human societies have not yet had to consider.
Primary and Secondary Detection Reagents
Both enzyme and macrofluorophore labels can be coupled directly to target-specific affinity reagents (primary detection) or to more generic affinity reagents that form stable complexes with unlabeled primary reagents, usually on the basis of immunorecognition (secondary detection). As indicated schematically in Figure 6.1.1, secondary detection inherently provides some degree of signal amplification, although sometimes at the expense of additional background due to nonspecific binding. These basic concepts of primary and secondary detection apply not only to the signal amplification techniques addressed in the current chapter but also to the dye-labeled affinity reagents described in Antibodies, Avidins and Lectins—Chapter 7.
Primary Detection Reagents
Any easily detectable molecule that binds directly to a specific target is a primary detection reagent. Such reagents are detected by fluorescence, chemiluminescence, absorption or electron diffraction conferred by stably attached labels. The conjugation and crosslinking chemistries used to create these stable attachments are discussed in detail in Fluorophores and Their Amine-Reactive Derivatives—Chapter 1, Thiol-Reactive Probes—Chapter 2 and Crosslinking and Photoactivatable Reagents—Chapter 5. In addition to our fluorophore-labeled anti-dye antibodies (Anti-Dye and Anti-Hapten Antibodies—Section 7.4) and monoclonal antibodies (www.invitrogen.com/handbook/antibodies), many of the Molecular Probes site-selective products can be considered primary detection reagents. These include our fluorescent lectins (Lectins and Other Carbohydrate-Binding Proteins—Section 7.7), nucleic acid stains (Nucleic Acid Detection and Analysis—Chapter 8), protein and glycoprotein stains (Protein Detection on Gels, Blots and Arrays—Section 9.3, Detecting Protein Modifications—Section 9.4), phallotoxins (Probes for Actin—Section 11.1), membrane probes (Probes for Lipids and Membranes—Chapter 13), annexin V conjugates for detecting apoptotic cells (Assays for Apoptosis—Section 15.5) and various drug and toxin analogs (Probes for Neurotransmitter Receptors—Section 16.2, Probes for Ion Channels and Carriers—Section 16.3). These primary detection reagents can typically be detected by fluorescence microscopy, fluorometry or flow cytometry methods.
Secondary Detection Reagents
Although many biomolecules, such as antibodies and lectins, bind selectively to a biological target, they usually need to be chemically modified before they can be detected. Often the biomolecule is conjugated to a fluorescent or chromophoric dye or to a heavy atom complex such as colloidal gold. However, the researcher may wish to avoid the time and expense required for these conjugations, choosing instead to use a more generic secondary detection reagent. Typically, secondary detection reagents recognize a particular class of molecules. For example, labeled goat anti–mouse IgG antibodies can be used to localize a tremendous variety of target-specific mouse monoclonal antibodies. Our extensive secondary antibody offering (Secondary Immunoreagents—Section 7.2) provides a wide selection of labels including our superior Alexa Fluor dye series, phycobiliproteins, Alexa Fluor dye–phycobiliprotein tandem fluorophores, Qdot nanocrystals, biotin and enzyme labels (HRP and alkaline phosphatase). We also offer many options in terms of immunoreactivity, an essential consideration in avoiding confounding cross-reactivity when performing simultaneous secondary immunodetection of two or more targets. Our labeled secondary antibody portfolio contains antibodies against IgG and IgM from several mammalian species, including various isotypes of mouse IgG, as well as antibodies against avian (chicken) IgY. Our Zenon antibody labeling technology (Zenon Technology: Versatile Reagents for Immunolabeling—Section 7.3) uses conjugates of an Fc-specific anti-IgG Fab fragment for the rapid and quantitative labeling of the corresponding mouse, rabbit, goat or human antibody.
6.1: Introduction - Biology
Figure 1. Experimenting with thousands of garden peas, Mendel uncovered the fundamentals of genetics. (credit: modification of work by Jerry Kirkhart)
Genetics is the study of heredity. Johann Gregor Mendel set the framework for genetics long before chromosomes or genes had been identified, at a time when meiosis was not well understood. Mendel selected a simple biological system and conducted methodical, quantitative analyses using large sample sizes. Because of Mendel’s work, the fundamental principles of heredity were revealed. We now know that genes, carried on chromosomes, are the basic functional units of heredity with the capability to be replicated, expressed, or mutated. Today, the postulates put forth by Mendel form the basis of classical, or Mendelian, genetics. Not all genes are transmitted from parents to offspring according to Mendelian genetics, but Mendel’s experiments serve as an excellent starting point for thinking about inheritance.
6.1 Chapter Objectives
In this chapter, we’ll discuss, broadly, two types of cell division—mitosis and meiosis. Specifically, we’ll emphasize meiosis, and how meiosis is responsible for generating much of the diversity that we associate with sexual reproduction. Our primary goal is for you to understand how meiosis generates genetic diversity, and how the cells that result—sperm and eggs—combine to create unique individuals in the next generation.
By the end of your reading and our in-class discussions, you should be able to:
- Define the following terms
- Homologous pair
- Chromatid pair
- Crossing over
- Independent assortment
- Prophase, metaphase, anaphase, telophase, cytokinesis
- Meiosis I and Meiosis II
- Punnett square
- Sex-linked inheritance
- Monoallelic inheritance
- Compare the processes of mitosis and meiosis
- Describe the progeny cells from both mitosis and meiosis, especially relative to the original cell
- Describe how meiosis generates genetic diversity via independent assortment and crossing over
- Use Punnett squares to predict inheritance probabilities for single-gene traits, including sex-linked traits
- Differentiate between the inheritance patterns of recessive and dominant alleles
Watch the video: Four Spheres Part 1 Geo and Bio: Crash Course Kids # (January 2022).