Fri Spr W4 - Genome Evolution - Transposons and Dynamic Genomes (Lec5) Understand and describe transposons Transposon - known as transposable element or 'jumping gene' - Is a discrete DNA sequence element that codes for a protein transposase that aids movement of the element throughout the genome transposon gene mRNA Transposon can alter gene functions via generation of insertional mutants transposon mutagenesis transposon ge ne truncated mRNA Human genome doesn't contain any actively transportable parts however 3% of genome is made up of transposons -> proof for active transposons in human evolutionary history! This proof further supported by: - repeat sequences, related to transposable elements, don't move but are still present in human genome - Sequences vary in size but lots are present Integrated transposable elements flanked by short repeat sequences -> make it easy to detect transposable elements during genome seq projects Chromosomal DNA Transposon =CTGG= -CTGG Direct repeats Figure 1215 Genomes 320 Garland Sclemes 2007) DNA transposons use either replicative or conservative transposition Retro-elements transpose replicatively via RNA intermediate DNA transposons that move conservatively move to new area of genome Replicative Conservative Figure 17-16 Genomes 3 (0 Garland Science 2007) Describe the experimental evidence that demonstrates that transposons are present in maize genome (McClintock's work)
Maize are good model for studying transposons: - Male + female gametes produced at different sites -> high level of control - Can collect large number of progeny (loads kernels on each cob) - Progeny can be stored for long amount time (decades once dried) In maize somatic cells transposition causes variegated pigmentation in maize kernels (aka it causes kernels to have different colours/ patterns/ patches) 1) If we hypothesise that wild type kernel = purple 2) Transposon is inserted into gene that codes for protein that produces purple pigment in biochemical pathway 3) When transposon is inserted into gene = mutant phenotype -> (yellow corn) 4) When transposon is transposed out of gene -> results in patches of cells w/ purple pigmentation 5) Meaning that mendelian rules are not followed since phenotype not generated by segregated phenotypes! But dependent on frequency that transposon jumps out of gene - thus is an unstable phenotype! Ac element IR IR 1 Transposase gene Ds element IR 1 IR Deleted transposase gene Transposase Transposition Figure 9-18 Genomes 3 10 Garland Science 2007) 1 Transposition Barbara Mcclintock hypothesised that DNA elements in maize = Ac and Ds Full length Ac element contains functional transposase gene Ds element doesn't code for functional transposase cos has internal deletion The transposase enzyme recognises 11bp inverted repeats flanking the Ac and Ds transposons -> It then catalyses transposition of Ac and Ds elements Maize has 10 different types of Ds elements - these are of different size, diff internal deletions (194bp - several kb) Recombination Transposon 1 Transposon 2
B.Mc concluded that: - Ac can move independently (prod transposase) - Ds can't move alone (requires As to move) - Recombination between pairs of transposons can result in deletion of genome segments