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Expression of a gene can be regulated at later stages in gene expression (see chapter 11). For example, the mRNA may be degraded by RNases as soon as it is made, before it can be translated. In translational regulation, translation of the mRNA can be regulated to determine how much of the protein product is made. Posttranslational regulation results in regulation of the activity of a protein product of a gene by degradation by proteases or modifications, such as phosphorylation or methylation, depending on the conditions in which the cell finds itself. In addition, the product of a pathway may inhibit the activity of an enzyme in the pathway by a process called feedback inhibition. In general, a type of regulation of gene expression that operates after the mRNA for a gene has been made is called posttranscriptional regulation. Specific examples of posttranscriptional regulation are also discussed in subsequent chapters.

Figure 2.44 Relationship between gene structure in DNA and the coding sequence in mRNA. Each of the different reading frames in the two strands of DNA may contain open reading frames (ORFs), but generally, only one ORF in each region (reading frame 6 in the diagram) is translated to yield a polypeptide product.