These mechanisms correct any mistakes or damage in DNA before they become established in the cell and are passed on to progeny cells. These repair mechanisms will be discussed shortly. ■ exponents, Appendix I, p. A-1

Mutations are stable, so that the progeny of streptomycin-sensitive cells that mutate to streptomycin resistance will remain streptomycin-resistant. On rare occasions, however, the nucleotide will change back to its original state, resulting in the streptomycin-resistant cells becoming streptomycin-sensitive. The change in a cell's genotype and phenotype to its original state through a change in the mutated gene is termed reversion. It occurs spontaneously at low frequencies that can be increased by mutagens.

Base Substitution

The most common type of mutation results from a mistake during DNA synthesis, when an incorrect base is incorporated into the DNA, an event called base substitution (figure 8.1). If only one base pair is changed, the mutation is called a point mutation. This mistake in incorporation then is passed on to the cell's progeny (offspring), resulting in an incorrect amino acid being incorporated into the protein coded by the gene (see figure 8.1). A mutation which results in the substitution of a different amino acid is termed a missense mutation. Frequently, the incorporation of an incorrect amino acid results in the synthesis of a protein that still functions partially. For example, a mutation in a gene of tryptophan biosynthesis may result in a strain that grows slowly in the absence of tryptophan. Such a mutation is termed leaky. A mutation that changes a codon that normally encodes an amino acid to a stop codon is called a nonsense mutation. Any mutation that totally inactivates the gene resulting in a strain that is unable to grow at all unless tryptophan is added is termed a null or knockout mutation.

Removal or Addition of Nucleotides

The deletion (removal) or addition of one or several nucleotides also changes the nucleotide sequence. Because translation of a gene begins at a specific codon and proceeds one codon at a time, the deletion or addition of a nucleotide shifts the codons of the DNA when it is transcribed into mRNA (figure 8.2). This type of mutation is termed a frameshift mutation. If three nucleotides are added or removed, however, then a single amino acid usually will be added to or subtracted from the protein encoded by the DNA. ■ codon, pp. 169,175 ■ translation, p. 175

A frameshift affects all amino acids incorporated beyond the original site at which the addition or deletion occurred. Frequently, one of the many new codons generated by the frameshift will be a stop codon, as in figure 8.2, so that the protein synthesized will be incomplete and non-functional. Frameshift mutations change many more codons than are changed by the substitution of a single base, and so they commonly result in a knockout mutation. ■ stop codon, p. 177 ■ operon, p. 183 ■ downstream genes, p. 174

Transposable Elements (Jumping Genes)

Transposable elements, also called transposons or jumping genes, are special segments of DNA that can move sponta-

DNA undergoing replication; a cytosine is incorporated opposite adenine by mistake

8.2 Spontaneous Mutations

DNA undergoing replication; a cytosine is incorporated opposite adenine by mistake

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