Crossing over between 2 homologous chromosomes involves a series of steps: o Pairing of chromosomes o Single strand breaks by endonuclease o Strand exchange by RecA-like enzymes o Sealing the breaks by ligase o Resolution of single strand bridges at Holliday junction o The recombination intermediate is a Chi structure o When genetic markers are closely linked, nonrecipricol recombination (or gene conversion) often occurs, yielding 3:1 ratios of the segregation alleles o Gene conversion results from DNA repair synthesis that occurs during recombination Chapter 14: Transposable genetic elements Transposable Elements: o A.k.a. Transposons – are found in the genomes of many kinds of organisms (~80% genome are transposons) o 3 main types: o Cut + Paste : (initial elements that Barbara discovered) an element is excised from one genomic position + inserted into another by a special enzyme, the transposase , which is usually encoded by the transposon itself (double stranded breaks) o Replicative (copy + paste) : an element is replicated + one copy is inserted at a new site; the other copy remains at the original site o Retrotransposition : an element is first transcribed into RNA, which is reverse transcribed into DNA, followed by insertion of the DNA into a new site on the same or different chromosome Transposable Elements in Bacteria o Bacterial transposons move within + between chromosomes and plasmids o Three types: o (1) Insertion sequences (IS): simplest transposons, are compactly organized and contain only genes whose translation products are involved in transposition – “cut-and-paste transposons” of prokaryotes Inverted terminal repeats are found at both ends of the IS elements IS elements can mediate recombination between different DNA molecules Insertion of an IS element causes target site duplication
Conjugative R plasmids o Can move transposons that contain genes for antibiotic resistance among cells of the same of different bacterial species through conjugation o These plasmids have two components: o The resistance transfer factor (RTF) contains genes required for conjugative transfer between bacterial genes o The R-determinant contains the genes for antibiotic resistance Composite transposons o Are cut-and-paste transposons like IS elements – denoted by symbol ‘Tn’ o Consist of 2 IS elements flanking a region containing 1 or more genes for antibiotic resistance Cut-and-Paste Transposons in Eukaryotes Ac + Ds controller elements in maize (Barbara McClintock) o Aleurone color is affected by the Ci allele (which inhibits aleurone color) o Mosaics with pigmented patches were caused by loss of the Ci allele due to transposition of the Ds (dissociator) element which is located where chromosome breakage occurs o The Ac stimulates chromosome breakage at the site of Ds (Ds cannot) o Ac element : activator, encodes transposase responsible for excision and transposition, leading to mutation, and chromosome breakage o Ds element : dissociator, related to AC elements but contain deletions
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