The backbone of the DNA chain is composed of alternating sugar and phosphate groups. Protruding from each sugar group is one of four nitrogenous bases: adenine (A), guanine (G), thymine (T), or cytosine (C). Structurally, each DNA molecule consists of two entwined chains, linked together by bonds between the bases of one chain and those of the other. An A is always bonded to a T, and every G is paired with a C; the result is that the sequence of bases in the two strands is complementary. The arrangement of the bases determines the genetic code of an organism.
This code directs the synthesis of proteins at the cellular level. It is written in units called codons, each of which specifies a particular amino acid. (Proteins are composed of amino acids. ) A codon consists of a sequence of three bases–for example, GAG or TCA. The four bases can be assembled into 64 possible codons. Because proteins are built from only 20 amino acids, most amino acids are specified by more than one codon.
Mutations occur when one base is substituted for another or when one or more bases are inserted or deleted from a gene. Substitution mutations affect only one codon, and in most cases the effect is minimal. One reason is the redundancy of the genetic code mentioned above. Because many codons mean the same thing, the altered codon might still specify the same amino acid. Furthermore, even if a mutation causes a wrong amino acid to be inserted into a protein, the change might be harmless. Most proteins consist of scores of amino acids; a change in one of these may have little or no effect on the biological properties of the protein.
Also, almost all higher organisms have two sets of genes–one inherited from each parent. In such organisms, a mutated gene may be recessive and have its effect canceled by a dominant gene. This is not to say that substitution mutations never produce serious consequences. The protein hemoglobin, an important component of red blood cells, is made up of hundreds of amino acids. The incorporation of one wrong amino acid–the product of a single substitution mutation–results in hemoglobin that forms an abnormal sickle shape. If a person inherits this mutation from both parents, the disease sickle-cell anemia results.
Whereas only one codon is affected by a substitution mutation, base insertions and deletions alter the reading frame of the entire gene, thus changing every codon from the site of the mutation to the end of the gene. For example, assume that the end of a gene reads TAG GGC ATA ACG ATT. The insertion of an extra A in the first codon will alter the entire sequence as follows: TAA GGG CAT AAC GAT T. Because it affects many codons, a base insertion or deletion is more likely to have significant results. Even these mutations, however, may be masked by the presence of a dominant normal gene. Mutations in humans and in other animals that reproduce sexually can be divided into two types: somatic and germinal.
Somatic mutations occur in body cells (as opposed to sex cells). Such mutations can produce a localized change–e. g. , the streak of white (albino) sometimes found in the hair of an otherwise normal individual.
All the cells descendant from the mutant body cell will carry the mutation, but it cannot be passed on to offspring. Germinal mutations, however, affect the sex cells (eggs or sperm) and can be transmitted to the individual’s offspring. When germinal mutations alter an organism, the effect is usually harmful. Many genetic diseases are the result of such mutations. Harmful genes eventually may be eliminated from a population if they impair the carriers’ ability to reproduce at the same rate as their fellows.
A mutation will rarely produce a beneficial change. When this does occur, the percentage of organisms with this gene will increase until the mutated gene becomes the norm in the population. In this way, beneficial mutations serve as the raw material of evolution.