Defining the Gene Help
Defining the Gene
Mendel's work suggested that a gene was a discrete "factor" that controlled a given phenotype (e.g., a gene for tall vs. short pea vine growth). Although the physical nature of the gene was not understood until the middle of the 20th century, the work of geneticists established it as the basic biological unit of heredity. Later work showed that genes were composed of DNA, not protein. One of the earliest concepts of gene action to explain human metabolic disorders proposed that each such gene was responsible for a specific enzymatic reaction; hence, the "one gene–one enzyme hypothesis" was born. Then it was discovered that some enzymes consist of more than one kind of polypeptide chain. For example, the bacterial enzyme tryptophan synthetase is a tetramer of two achains and two β-chains; each of the two types of chains is specified by a different genetic locus. This type of protein is known as a heterotetramer, since it is made up of different polypeptide chains. A tetramer composed of four of the same polypeptide chains would be a homotetramer. The homo- and hetero- prefixes can be applied to protein dimers, octomers, etc. The paradigmthen became "one gene–one polypeptide chain." It is nowknown that many genes give rise to more than one different polypeptide chain by a process called alternative splicing (see Example 13.8).
EXAMPLE 13.8 Immunoglobulins of class IgM have μ-type heavy chains that are produced in two varieties as a consequence of differences in intron/exon excision/splicing events. When processed in one way, the m-chains are longer, ending with a group of hydrophobic amino acids at their carboxyl ends. This ''water-fearing'' tail tends to lodge in the lipid membrane and extends into the cytoplasm. The amino terminus extends outside the cell where it participates with an L chain to form an antigen-combining site (see Example 13.21). Thus, it becomes a cell receptor for a specific antigen. An alternative splicing step removes from the primary transcript the sequence responsible for the hydrophobic tail. This shorter version of the μ-chain readily passes out of the cell and becomes part of the secretory antibody population found in the blood and other body fluids.
It is also known that many genes may contribute to a single character or trait (polygenic traits or traits exhibiting continuous variation) and that each gene may have multiple phenotypic effects (pleio-tropy). In fact, most traits are determined by the action of many genes in concert with their environment. The terms structural gene or protein-coding gene are often used for genes whose code specifies the sequence of a polypeptide chain. Because tRNA and rRNAmolecules are also encoded byDNA sequences, it can be argued that the definition of a gene should be extended to a "transcription unit" or a region of DNA between the sites of initiation and termination of transcription. However, some transcription units may contain more than one structural or RNA gene. Some genes are overlapping in the sense that the DNA sequence of one gene begins (usually in a different reading frame) in the DNA sequence of another. In summary, a gene has the following characteristics: (1) the physical unit of heredity, (2) a sequence of DNA that occupies a particular locus on a chromosome, and (3) codes for a functional product, such as a protein or RNA molecule.
There are other important genetic elements in te genome of an organism. One major class of such elements is the "control region." Control regions, such as promoters and transcription termination signals, are not transcribed or translated so they do not specify proteins or functional RNA molecules. However, they contain sequence information that is involved in controlling when a gene is transcribed.
The term cistron was originally equated with a region of DNA that specifies a complete polypeptide chain; thus, a cistron is equivalent to a structural gene. However, the term "cistron" was originally given to a functional genetic unit defined by the complementation test (cis-trans test). This test is used to determine whether two different mutations lie in the same or different genes.
EXAMPLE 3.7 Two point mutants (m1 and m2) in the same gene are functionally allelic.
Thus, two mutations are considered to be functionally allelic (in the same gene, or cistron) if they complement in the cis position, but not in the trans position. However, if a heterozygote contains two mutations in either the cis or the trans position in different functional units (i.e., different genes, or cistrons), complementation can produce a normal phenotype because for each mutant functional unit on one DNA molecule there is a corresponding normal functional unit on the other DNA molecule. Thus, two mutants are functionally nonallelic if they complement in either the cis or the trans position.
EXAMPLE 3.8 Two point mutants in different genes are functionally nonallelic.
Many genes encode enzymes that are important for catalyzing biological synthesis (anabolic) and degradation (catabolic) reactions within a cell. These reactions are typically grouped together into a series of actions, called a biochemical pathway, starting with a substrate that is converted through several chemical strong>intermediates to the end product. Enzymes help carry out each step of the pathway; thus, ild-type enzyme function is required of all enzymes in a pathway in order for it to function properly. If any one enzyme is missing or mutated so that it is not functional, the pathway will not be completed.
EXAMPLE 3.9 A biochemical pathway is known that converts substrate A into product E, through three intermediate compounds B, C, and D. Enzymes 1–4 are required for each of the indicated steps.
If the gene encoding enzyme 3 contains a nonsense mutation so that a non-functional, truncated protein is produced, products D and E will not be produced and intermediate C will most likely accumulate inside the cell. This type of mutation can be ''rescued'' in one of several ways: (1) provide intermediate D to the cell, (2) provide product E to the cell, (3) provide a wild-type copy of the enzyme 3 gene to the cell.
In bacteria and other microorganisms, it is relatively simple to isolate a number of mutant strains that are defective in the synthesis of a particular end product, such as an amino acid. Presumably, each mutant contains a mutation in one of the enzyme genes required for the synthesis of the end product. By analysis of the nutrient requirements of these mutants, the order of the pathway can be established.
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