Lecture Power Point to accompany Molecular Biology Fourth
Lecture Power. Point to accompany Molecular Biology Fourth Edition Robert F. Weaver Chapter 23 Transposition Copyright © The Mc. Graw-Hill Companies, Inc. Permission required for reproduction or display.
23. 1 Bacterial Transposons • A transposable element moves from one DNA address to another • Originally discovered in maize, transposons have been found in all kinds of organisms – Bacteria – Plants – Humans 23 -2
Discovery of Bacterial Transposons • • • Phage coat is made of protein Always has the same volume DNA is much denser than protein More DNA in phage, denser phage Extra DNAs that can inactivate a gene by inserting into it were the first transposons discovered in bacteria • These transposons are called insertion sequences (ISs) 23 -3
Insertion Sequences • Insertion sequences are the simplest type of bacterial transposon • They contain only the elements necessary for their own transposition – Short inverted repeats at their ends – At least 2 genes coding for an enzyme, transposase that carries out transposition • Transposition involves: – Duplication of a short sequence in the target DNA – One copy of this sequence flanks the insertion sequence on each side after transposition 23 -4
Generating Host DNA Direct Repeats 23 -5
Complex Transposons • The term “selfish DNA” implies that insertion sequences and other transposons replicate at the expense of their hosts, providing no value in return • Some transposons do carry genes that are valuable to their hosts, antibiotic resistance is among most familiar 23 -6
Antibiotic Resistance and Transposons • Donor plasmid has Kanr, harboring transposon Tn 3 with Ampr • Target plasmid has Tetr • After transposition, Tn 3 has replicated and there is a copy in target plasmid • Target plasmid now confers both Ampr, Tetr 23 -7
Transposition Mechanisms • Transposons are sometimes called “jumping genes”, DNA doesn’t always leave one place for another • When it does, nonreplicative transposition – “Cut and paste” – Both strands of original DNA move together from 1 place to another without replicating • Transposition frequently involves DNA replication – – 1 copy remains at original site New copy inserts at the new site Replicative transposition “Copy and paste” 23 -8
Replicative Transposition of Tn 3 • In first step, 2 plasmids fuse, phage replication, forms a cointegrate – coupled through pair of Tn 3 copies • Next is resolution of cointegrate, breaks down into 2 independent plasmids, catalyzed by resolvase gene product 23 -9
Nonreplicative Transposition • Starts with same 2 first steps as in replicative transposition • New nicks occur at arrow marks • Nicks liberate donor plasmid minus the transposon • Filling gaps and sealing nicks completes target plasmid and its new transposon 23 -10
23. 2 Eukaryotic Transposons • Transposons have powerful selective forces on their side • Transposons carry genes that are an advantage to their hosts – Their host can multiply at the expense of completing organisms – Can multiply the transposons along with rest of their DNA • If transposons do not have host advantage, can replicate themselves within their hosts 23 -11
Examples of Transposable Elements • Variegation in the color of maize kernels is caused by multiple reversions of an unstable mutation in the C locus, responsible for kernel color • Mutation and its reversion result from Ds (dissociation) element – Transposes into the C gene – Mutates it – Transposes out again, revert to wild type 23 -12
Ds and Ac of Maize • Ds cannot transpose on its own • Must have help from an autonomous transposon, Ac (for activator) – Ac supplies transposase – Ds is an Ac element with most of its middle removed – Ds needs • A pair of inverted terminal repeats • Adjacent short sequences that Ac transposase can recognize 23 -13
Transposable Elements in Maize 23 -14
Extra Reading 23 -15
Detailed Tn 3 Transposition 23 -16
Structures of Ac and Ds 23 -17
P Elements • The P-M system of hybrid dysgenesis in Drosophila is caused by conjunction of 2 factors: – Transposable element (P) contributed by the male – M cytoplasm contributed by the female allows transposition of the P element • Hybrid offspring of P males and M females suffer multiple transpositions of P element • Damaging chromosomal mutations are caused that render the hybrids sterile • P elements have practical value as mutagenic and transforming agents in genetic experiments with Drosophila 23 -18
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