Pseudogenes

Pseudogenes are defective copies of functional genes. These may be partial or complete duplicates derived from polypeptide-encoding genes or RNA genes. The DNA sequence of a pseudogene is characteristically very similar to its functional counterpart, but contains variant mutations that render the gene inactive. The functional polypeptide-encoding gene contains an open reading frame, a long stretch of nucleotides that are transcribed and subsequently translated into a series of amino acids uninterrupted by stop codons. In contrast, pseudogenes derived from polypeptide sequences generally are punctuated with stop codons, effectively rendering them incapable of producing a functional protein.

Pseudogenes may also contain frameshift mutations, yielding a change in the reading frame. Additionally, there may be mutations that inactivate regulatory elements or intron-splicing sites. In either case, the duplicated Figure 1. Generation of pseudogenes. (A) Direct duplication of a DNA sequence including regulatory elements and introns yields a nonprocessed pseudogene. (B) Duplication via an RNA intermediate, processed with a 3′ polyadenylated tail, 5′ cap (7-methylguanosine), and intron removal, yields a retropseudogene. The arrows indicate direct repeat sequences resulting from the integration. The A-T refers to a stretch of adenines and complement thymines in the newly derived pseudogene. gene may be rendered nonfunctional. Genes and pseudogenes derived from the duplication of an ancestral gene are said to be paralogous.