Читать книгу Cell Biology - Stephen R. Bolsover - Страница 78

The order of the codons in DNA and the amino acid sequence of a protein are colinear. The start signal for protein synthesis is the codon AUG, specifying the incorporation of methionine. Because the genetic code is read in blocks of three, there are three potential reading frames in any mRNA. Figure 3.10 shows that only one of these results in the synthesis of the correct protein. When we look at a sequence of bases, it is not obvious which of the reading frames should be used to code for the protein. As we shall see later (page 89), the ribosome scans along the mRNA until it encounters an AUG. This both defines the first amino acid of the protein and the reading frame used from that point on. A mutation that inserts or deletes a nucleotide will change the normal reading frame and is called a frameshift mutation (Figure 3.11).

The codons UAA, UAG, and UGA are stop signals for protein synthesis. A base change that causes an amino acid codon to become a stop codon is known as a nonsense mutation (Figure 3.11). If, for example, the codon for tryptophan UGG changes to UGA, then a premature stop signal will have been introduced into the messenger RNA template. A shortened protein, usually without function, is produced.