retrotransposonit

Retrotransposons are genetic elements that propagate within genomes through an RNA intermediate and a process called reverse transcription. They use a copy-and-paste mechanism, meaning their insertion increases their copy number and can alter genome structure and function. Most retrotransposons are transcribed by RNA polymerase II and encode proteins such as reverse transcriptase and, in some lineages, integrase.

Retrotransposons are classified into two major groups: long terminal repeat (LTR) retrotransposons and non-LTR retrotransposons. LTR retrotransposons, including Ty-like elements in yeast and copia and gypsy elements in plants and animals, resemble endogenous retroviruses and typically encode gag, protease, reverse transcriptase, and RNase H. Non-LTR retrotransposons include LINEs (long interspersed nuclear elements) and SINEs (short interspersed nuclear elements); LINEs are autonomous, while SINEs are non-autonomous and rely on LINE-encoded proteins, such as LINE-1 in humans and Alu elements.

Retrotransposons can shape genomes by insertional mutagenesis, creating or disrupting genes, altering gene expression, and providing regulatory sequences such as promoters and enhancers. They also drive genome expansion and promote recombination. To prevent harmful activity, hosts employ epigenetic silencing, RNA interference pathways, and DNA methylation that suppress transcription and retrotransposition.

Active retrotransposons contribute to genetic variation and evolution. In some species, mobile elements remain active in the germline and, in some contexts, in somatic tissues, where they can be linked to diseases, including cancers, though this activity is typically tightly controlled.