retrotransposoner

Retrotransposoner, or retrotransposons, are genetic elements that propagate within the genome by copying themselves through an RNA intermediate and reverse transcription. They are a major subclass of transposable elements, classified as class I transposable elements. The life cycle typically starts with transcription of the element by RNA polymerase II, followed by translation of encoded proteins such as reverse transcriptase and, in long terminal repeat (LTR) retrotransposons, an integrase. The RNA-protein complex then reverse-transcribes the element into DNA and inserts it at a new genomic location, often via target-primed reverse transcription for non-LTR LINE elements or through integrase-mediated insertion for LTR elements.

Two broad groups exist: long terminal repeat (LTR) retrotransposons and non-LTR retrotransposons. LTR retrotransposons contain long terminal repeats at both ends and are related to retroviruses; they are found in plants, fungi, and animals. Non-LTR retrotransposons include LINEs (long interspersed elements) and SINEs (short interspersed elements); they do not rely on LTRs and use reverse transcription with endonuclease activity to insert themselves.

Impact on genomes is substantial. Retrotransposons contribute to genome size and structure and can cause insertional mutagenesis, alter gene expression, and promote recombination. They also provide regulatory sequences and, in some cases, exapted or domesticated functions. In human genomes, retrotransposons account for a large fraction of sequence content, with notable families such as LINE-1 and Alu elements.

Regulation and evolution are intertwined. Host genomes suppress retrotransposon activity through DNA methylation, histone modifications, and RNA interference pathways (e.g., piRNA). Over evolutionary time, retrotransposons have shaped genome architecture, contributed to gene and regulatory innovation, and given rise to domesticated genes and regulatory networks.