Mutations
MutationsA mutation is an alteration in the genetic material of a cell that is transmitted to the cell's offspring. Mutations may be spontaneous (the result of accidents in the replication of genetic material) or induced by external factors (e.g., electromagnetic radiation and certain chemicals).
Mutations take place in the genes, which are found in the long, chainlike molecules of deoxyribonucleic acid (DNA). 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,...
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