Retroposons

Retroposons are a class of mobile genetic elements that move within the genome through a "copy-and-paste" mechanism, unlike transposons, which typically employ a "cut-and-paste" strategy. They are named for their reliance on reverse transcription, a process that synthesizes DNA from an RNA template. Retroposons were first identified in the 1970s and are now recognized as significant components of eukaryotic genomes, particularly in humans and other complex organisms.

The primary types of retroposons include long interspersed nuclear elements (LINEs), short interspersed nuclear elements (SINEs), and processed pseudogenes. LINEs are the most autonomous, encoding proteins necessary for their own transposition, such as reverse transcriptase and endonuclease. The most well-studied LINE, L1, constitutes roughly 17% of the human genome and remains active today. SINEs, such as Alu elements in humans, lack coding capacity but can hijack the machinery of LINEs for amplification. Processed pseudogenes are derived from mRNA transcripts that have been reverse-transcribed and reinserted into the genome, often lacking introns and regulatory sequences.

Retroposon insertion can disrupt genes, alter gene expression, or contribute to genomic diversity. Some insertions are neutral or beneficial, while others may cause disease or developmental abnormalities. For example, certain Alu insertions have been linked to conditions like hemophilia and α-thalassemia. Additionally, retroposons play roles in shaping genome architecture, influencing chromosome structure and gene regulation.

The study of retroposons has advanced our understanding of genome evolution, as they provide insights into horizontal gene transfer, adaptive mutations, and the origins of complex traits. Their prevalence in eukaryotic genomes underscores their historical and ongoing impact on genetic variation and species evolution.