Genetics, Cells, and Mendel's Law Help (page 2)
Genetics is that branch of biology concerned with heredity and variation. The hereditary units that are transmitted from one generation to the next (i.e., inherited) are called genes. Genes reside in the long molecules of deoxyribonucleic acid (DNA) that exist within all cells. DNA, in conjunction with a protein matrix, forms nucleoprotein and becomes organized into structures called chromosomes that are found in the nucleus or nuclear region of cells. A gene contains coded information for the production of proteins. DNA is normally a stable molecule with the capacity for self-replication. On rare occasions a change may occur in some part of DNA code. This change is called a mutation. Mutations may occur spontaneously or be induced by exposure to mutagenic chemicals or radiation. The resulting mutation has varying effects on gene function depending on where within the gene code the mutation has occurred. Rarely, a mutation will result in an alteration of the code that leads to the production of a defective protein. The net result of a mutation is sometimes seen as a change in the physical appearance of the individual or a change in some other measurable attribute of the organism called a character or trait. Through the process of mutation a gene may be changed into two or more alternative forms called alleles.
Example 1.1 Healthy people have a gene that specifies the normal protein structure of the red blood cell pigment called hemoglobin. Some anemic individuals have an altered form of this gene, i.e., an allele, which makes a defective hemoglobin protein that is unable to carry the normal amount of oxygen to the body cells when oxygen is scarce.
Each gene occupies a specific position on a chromosome, called the gene locus (loci, plural). All allelic forms of a gene therefore are found at corresponding positions on genetically similar (homologous) chromosomes. The word "locus" is sometimes used interchangeably for "gene." All the genes on a chromosome are said to be linked to one another and to belong to the same linkage group. Wherever the chromosome goes it carries all of the genes associated with it. Linked genes are not transmitted independently of one another, but genes on different chromosomes (in different linkage groups) are transmitted independently of one another. In most organisms, DNA is the molecule that carries the genetic information. However, in some viruses, ribonucleic acid (RNA) carries the genes.
The smallest unit of life is the cell. Each living thing is composed of one or more cells. The most primitive cells alive today are the bacteria. Bacteria are generally single-celled organisms that characteristically lack a nucleus. Bacteria therefore belong to a group of organisms called prokaryotes (literally, "before a nucleus" had evolved).
However, most other life forms (including algae, fungi, plants, and animals) are characterized by the presence of a nucleus and are referred to as eukaryotes (literally, "truly nucleated"). The nucleus is a membrane-bound compartment that isolates the genetic material from the rest of the cell (cytoplasm). Most of this book deals with the genetics of eukaryotes.
Cells are delimited by a plasma membrane and contain all the necessary chemicals and structures for the life of a particular type of cell. The cells of a multi-cellular organism are often differentiated to perform specific functions. For example, a neuron is specialized to conduct nerve impulses, a muscle cell contracts, a red blood cell carries oxygen, and so on. Thus, there is no such thing as a typical cell type. Figure 1-1 is a complete diagram of an animal cell showing common subcellular structures.
Subcellular structures that are surrounded by a membrane are called organelles. Most organelles and other cell structures are too small to be seen with the light microscope, but they can be studied with the electron microscope. Mitochondria and chloroplasts also contain their own separate chromosomes with a small number of genes related to their functions in cellular metabolism. Mitochondrial chromosomes contain nearly 40 genes and chloroplast chromosomes contain a few dozen to several hundred genes, depending on the plant species. The characteristics of organelles and other parts of eukaryotic cells are outlined in Table 1.1.
Table 1.1 Characteristics of Eukaryotic Cellular Structures
|Cell Structures||Physical Characteristics||Function(s)|
|Extracellular Structures||A cell wall surrounding the plasma membrane; composed primarily of cellose in plants, peptidoglycan in bacteria, chitin in fungi. Animal cells are not surrounded by cell walls. Some bacteria produce extracellular capsules composed of polysaccarides or glycoproteins||Gives strength and rigidity to the cell|
|Plasma membrane||Phospholipid bilayer that also contains proteins and sterols (in animal cells)||Regulates molecular extracellular substances (e.g., nutrients, water) enter the cell and waste substances or secretions exit the cell; passage of substances may require expenditure of energy (active transport) or may be passive (diffusion)|
|Nucleus||Surrounded by a double membrane (the nuclear membrane) that controls the movement of materials between the nucleus and cytoplasm; the membrane contains pores that communicate with the ER. Contains chromatin, which is the nuceoprotein component of the chromosomes. Only the DNA portion of chromosomes contains the hereditary material.||Master control of cellular functions via its genetic material (DNA)|
|Nucleolus||Site(s) on chromatin where ribosomal RNA (rRNA) is synthesized; disappears from view in light microscope during cellular replication.||Involved in DNA replication and gene expression.|
|Cytoplasm||Contains multiple structures (see below) and enzymatic systems (e.g., glycolysis and protein synthesis)||Involved in providing energy to the cell; execution of the genetic instruction from the nucleus.|
|Ribosome||Consists of three or four ribosomal RNA molecules and over 50 different proteins||Site of proten synthesis|
|Endoplasmic reticulum (ER)||Internal membrane system; rough endoplasmic reticulum (RER) is studded with ribosomes; smooth endoplasmic reticulum (SER) is free of ribosomes||RER is responsible for modification of polypeptide chains into mature proteins (e.g., by glycosylation) and SER is the site of lipid synthesis|
|Mitochondria||Surrounded by a double phospholipid bilayer membrane; contains enzymes required for ATP production. Contains a separate set of genes on a circular chromosome that are involved in cellular respiration||fatty acids; ATP is the main source of energy to power biochemical reactions|
|Plastid||Plant structure surrounded by a double membrane; contains pigments such as chlorophyll and carotinoids. Example, chloroplasts in plants. Chloroplasts contain a separate set of genes on a cirular chromosome, many of which are involved in photosynthesis.||Storage and synthesis of food (e.g., starch); photosynthesis occurs in chloroplasts|
|Golgi body (apparatus)||Organelle composed of flattened, saclike cisternae is close proximity and communication to the ER, Less well-developed cisternae sometimes called dictyosomes in some fungi, protozoans, and plants||Site where sugars, phosphate, sulfate, or fatty acids are added to certain proteins; as membranes bud from the Golgi system they are marked for shipment in transport vesicles to arrive at specific sites (e.g., plasma membrane for secretion outside the cell, lysosome)|
|Lysosome||Membrane-bound sac of digestive system enzymes in all eukaryotic cells||Aids in intracelleular digestion of bacteria and other foreign particles; may cause cell destruction if ruptured|
|Vacuole||Membrane-bound storage organelle||Storage of water and metabolic products (e.g., amino acids, sugars); plant cells often have a large central vacuole that (when filled with fluid to create turgor pressure) makes the cell turgid|
|Centrioles||Composed of microtubules; capable of being replicated after each cell division; rarely present in plants||Form poles of the spindle apparatus during cell divisions|
|Cytoskeleton||Consists of microtubules of the protein tubulin (as in the spindle fibers responsible for chromosomal movements during nuclear division or in flagella and cilia), microfilaments of actin and mysosin (as occurs in muscle cells), and intermediate filaments (each with a distinct protein such as keratin)||Contributes to shape, division, and motility of the cell and the ability to move and arrange its components|
|Cytosol||The fluid portion of the cytoplasm exclusive of the formed elements listed above; contains water, minerals, ions, sugars, amino acids, and other nutrients||Components are involved in building macromolecular biopolymers (nuclei acids, proteins, lipids, and large cabohydrates such as starch and cellulose) and other aspects of cellular metabolism.|
Mendel's Laws of Inheritance
When Gregor Mendel published the results of his genetic studies on the garden pea in 1866 the foundation of modern genetics was laid. He proposed some basic genetic principles. One of these is known as the principle of segregation. In his model any one parent contains two copies of a unit of inheritance (now called a gene) for each trait; however, only one of these two genes (an allele) is transmitted through a gamete to the offspring. For example, a plant that contains two allelic forms of a gene for seed shape, one for round and one for wrinkled seeds, will transmit only one of these two alleles through a gamete to its offspring. Mendel knew nothing of DNA, chromosomes or meiosis, as they had not yet been discovered. We now know that the physical basis for segregation is in anaphase I, where homologous chromosomes (each containing a different allele of the gene for seed shape, in this case) separate from each other. If the gene for round seed is on one chromosome and its allelic form for wrinkled seed is on the homologous chromosome, then both alleles will not normally be found in the same gamete.
The second important principle that Mendel's work helped to establish is the principle of independent assortment. This law states that the segregation (or separation) of one gene pair occurs independently of any other gene pair. We now know that this is true only for unlinked genes on nonhomologous chromosomes. For example, on one homologous pair of chromosomes are the alleles for seed shape (round vs. wrinkled) and on another pair of homologues are alleles for green and yellow seed color. The segregation of the alleles for seed shape occurs independently of the segregation of the alleles for seed color because each pair of homologues behaves as an independent unit during meiosis. Furthermore, because the orientation of bivalents on the first meiotic metaphase plate is essentially random, four combinations of factors could be found in the meiotic products: (1) round-yellow, (2) wrinkled-green, (3) round-green, (4) wrinkled-yellow.
Genetic Model Systems
Genetic model systems are organisms that can be easily grown and manipulated in laboratory settings to explore various genetic and developmental effects. There are viral, bacterial (Escherichia coli), yeast (Saccharomyces cerevisiae, Schizosaccharomyces pombe), plant (Arabidopsis thaliana), and animal model systems. The soil dwelling nematode (Caenorhhabditis elegans), the freshwater zebrafish (Danio rerio), the fruit fly (Drosophila melanogaster), and the mouse (Mus musculus) are common animal systems used to study genetics.
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