Transposition (CMB part 21)

Transposable elements, also known as transposons, are DNA segments capable of moving from one location to another within the genome. Some transposable elements replicate during the process, leaving one copy in the original position and inserting a new copy at a different site, while others move without replication, vacating the original site entirely. Bacterial transposons can be categorized as follows: (1) insertion sequences, such as IS1, which consist solely of the genes required for transposition and are flanked by inverted terminal repeats; and (2) transposons like Tn3, which resemble insertion sequences but include at least one additional gene, often conferring antibiotic resistance.Eukaryotic transposons exhibit diverse replication strategies. DNA transposons, such as Ds and Ac in maize or the P elements in Drosophila, function similarly to bacterial DNA transposons like Tn3.The immunoglobulin genes in mammals undergo rearrangement through a mechanism analogous to transposition. Vertebrate immune systems generate immense diversity in immunoglobulin production by assembling genes from two or three components selected from a heterogeneous pool. This process, called V(D)J recombination, relies on recombination signal sequences (RSSs) that include a heptamer and a nonamer separated by either 12-bp or 23-bp spacers. Recombination occurs exclusively between a 12 signal and a 23 signal, ensuring the incorporation of only one of each type of coding region into the assembled gene. Key players in human V(D)J recombination are RAG1 and RAG2, which create single-strand nicks in DNA adjacent to a 12 or 23 signal. This triggers a transesterification reaction where the newly formed 3'-hydroxyl group attacks the opposite strand, leading to a break and forming a hairpin at the end of the coding segment.