Transposons · A transposon or a transposable element is a discrete DNA sequence element that can move from one location in the genome to another - often under the control of a transposase encoded by the element itself · Illustrated below is a transposon initially sat upstream of a gene, that then jumps into the gene, and has a catastrophic effect, causing it to no longer produce a full-length mRNA. It produces a truncated mRNA that can't be translated into a protein & has generated a insertion mutation into that gene · Natural transposons can be exploited to generate insertional mutants . There have been evidence of transposons in every organism looked at by scientists. In most organisms they move from one position to the next. . However, in humans, there is no evidence of recent transpositions transposon gene mRNA transposon mutagenesis transposon ge ne truncated mRNA · 44% of the Human genome consist of genome wide repeats o Transposons could be responsible for this o 2.8% of the human genome consists of DNA transposons (300,000 copies from a few bp to 3kb). This is similar to the amount of DNA that encodes our genes Human genome 3200 Mb Genes and gene-related sequences 1200 Mb Intergenic DNA 2000 Mb Exons 48 Mb Related sequences 1152 Mb Genome-wide repeats 1400 Mb Other intergenic regions 600 Mb Pseudogenes Gene fragments Introns, UTRs Microsatellites 90 Mb LINES 640 Mb LTR elements 250 Mb Various 510 Mb SINEs 420 Mb DNA transposons 90 Mb . Transposable elements are flanked by short direct repeat sequences o Direct repeats allow transposons to be identified in genome sequencing projects o In the diagram below, this is CTGG. You would use a computer program to search for these repeats if you are looking for a transposon o You could also look for transposons by looking at the effects they have as they jump from one place to the next
Chromosomal DNA Transposon =CTGG= =CTGG= Direct repeats . DNA transposons use either replicative or conservative transposition o DNA transposons transposing conservatively move to a new position within the genome o Retro-elements transpose replicatively via an RNA intermediate Replicative Conservative Studying transposons . Maize is a model system for studying transposons o Variegated pigmentation in maize kernels caused by transposition in somatic cells o Maize is useful for looking at the phenotypes of transposons (e.g. spots/flecks on the image of corn below are tiny sectors that represent transposition events w/in the genome of the cells of the kernels) · Mcclintock's work o Barbara McClintock won the Nobel prize for physiology or medicine in fifl83 for discovering transposons ("mobile genetic elements") in maize plants, bc of how wide the implications of her discovery were and how fundamental it was. . The Ac - Ds transposons in maize Mcclintock looked at the activity of what she believed to be transposons. She looked at 2 classes of transposons, which she called Ac (acceptor) and Ds (disassociator) o A full-length Ac element contains a