pseudogenes

Pseudogenes are segments of DNA that resemble functional genes but are typically nonfunctional due to disabling mutations or lack of regulatory elements. They arise from once-functional genes and are present in the genomes of many organisms, including humans. While they do not produce functional proteins under normal conditions, pseudogenes can be transcribed in some cases and may influence gene regulation.

Most pseudogenes originate through gene duplication followed by mutation of one copy, or through retrotransposition, where an mRNA transcript is reverse-transcribed and inserted back into the genome. Based on origin, pseudogenes are commonly classified as processed (retrogenes lacking introns and promoters, often with a poly-A tail), nonprocessed or duplicated pseudogenes (contain introns and may retain promoter regions but host disabling mutations), and unitary pseudogenes (a single-copy gene that has accumulated inactivating changes without duplication).

Characteristic features include sequence similarity to a parent gene, missing or disrupted coding potential due to frameshifts or premature stop codons, and absence of a functional promoter or essential regulatory elements. Processed pseudogenes typically lack introns and resemble reverse-transcribed mRNA, while nonprocessed pseudogenes resemble their parent genomic structure with introns. The degree of conservation and the presence of regulatory motifs vary among pseudogenes.

Although once dismissed as genomic fossils, some pseudogenes are transcribed and may play regulatory roles, such as acting as competing endogenous RNAs that sequester microRNAs or affecting chromatin states. In practice, pseudogenes are important in genome annotation and evolutionary studies as markers of gene family dynamics and molecular evolution.