Genes: Definition and Structure

The earliest notion of heredity was that inherited characteristics were transmitted in parental fluids that intermixed in the offspring. Mendel demonstrated that hereditary ‘factors’ (later dubbed ‘genes’) behaved as particles remaining intact from one generation to another. Chemically, genes are deoxyribonucleic acid (DNA), strings of nucleotides (adenine, A; thymine, T; cytosine, C and guanine, G) whose order determines the order of the 20 amino acids in proteins. Proteins in turn determine the chemistry of the cell by catalysing reactions. And proteins constitute much of the cell's structure. Protein production comprises two steps: (1) Transcription, copying the nucleotide sequence of the gene into an message ribonucleic acid (mRNA) and (2) Translation, using the message in the mRNA to direct the synthesis of proteins. We now know the genetic code and understand the machinery that makes proteins. We also know how gene expression is controlled so that genetically identical cells of an individual can differentiate to serve diverse functions. Key Concepts: Individuals of a species vary considerably in appearance and function. These differences are to a large extent heritable. The observable appearance and physiology of an organism (i.e. its phenotype) result from the combined influence of its genes and its environment. The factors (genes) responsible for inherited properties of organisms are passed from one generation to the next as if they were particles. Although the phenotypes of offspring may be intermediate between the phenotypes of their parents, the genes do not ‘blend’ within the offspring but maintain their chemical and functional integrity. Genes are made of nucleic acids, linear molecules consisting of a string of four nucleotides, which, in deoxyribonucleic acid (DNA), are adenine (A), thymine (T), guanine (G) and cytosine (C). The expression of genes requires two steps. The first is transcription of the DNA sequence of the gene into a messenger RNA (mRNA) molecule. The second is the translation of the mRNA into protein. There are specific sequences of bases that mark the start and stop points for both transcription and translation. Most genes contain the recipe for making a specific polypeptide (protein) product. The proteins are linear molecules consisting of a string of amino acids. The genetic code, the ‘language’ in which the information in the gene is written, is made up of triplets of nucleotides. Of the 64 possible triplets of A, T, C and G, 61 encode one of the 20 amino acids found in proteins and 3 signify ‘stop’, marking the end of the protein. The catalysts that control the chemistry of living things are (almost always) proteins. A given enzyme may be the polypeptide product of a single gene or a complex of polypeptides encoded by one or several genes. A typical gene may be a stretch of 1000 or more bases. Changes in the base composition (mutations) at any point in the gene can cause the gene product to be abnormal or inactive. Thus, mutations may occur at many different places within a given gene. A functional test is used to determine whether two independently isolated, phenotypically similar recessive mutations lie in same gene. If a hybrid containing the two genes has the mutant phenotype, the mutations are said to be in the same gene (or ‘cistron’). Determination of the base sequence of whole genomes is a powerful method for identifying mutations that affect specific functions and for finding sequences involved in the regulation of gene expression. Regions of the DNA that serve vital functions tend to be conserved over long evolutionary distances. Keywords: allele; complementation; gene; genetic code; genotype; one gene–one polypeptide; phenotype; transcription; translation