Mutation is a change in the DNA base sequence of the wildtype organism. The latter is a strain whose properties are similar to the organism isolated from nature. The change in a nucleotide or nucleotide sequence may lead to an altered phenotype, or it may not lead to any observable change in which case it is called a silent mutation. Any change in phenotype results from altering the ability of the protein coded by the gene to function. The substitution of even one amino acid for another in a critical location in the protein such as in the catalytic site may cause the protein to be non-functional, thereby changing the properties of the cell. For example, if any gene of the tryptophan operon is altered so that the enzyme for which it codes no longer functions, the cells will grow only if this amino acid is in its environment. A mutant that requires a growth factor is called an auxotroph (auxo means "increase" as an increase in requirements). Cells that grow in the absence of any added growth factors are termed prototrophs. ■ enzymes, pp. 131,138 ■ protein structure, p. 27 ■ operon, p. 183 By convention, the phenotype of a cell is designated by a three-letter abbreviation. A cell that requires tryptophan for growth is designated Trp:, the superscript indicating the requirement for growth. The first letter is in caps. Each growth factor has its own three-letter designation. If a cell grows without the addition of tryptophan, then it is Trp+. However, it is cumbersome to indicate all of the growth factors that are not required, so only the growth factors that are required are indicated. Likewise, only if a cell is resistant to an antimicrobial agent such as streptomycin is this indicated, such as StrR. Otherwise, the cells are assumed to be sensitive, StrS, and this is not indicated in the description of the phenotype of the cell.
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