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Molecular Evolution 2 Recombination & Transposition.

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Presentation on theme: "Molecular Evolution 2 Recombination & Transposition."— Presentation transcript:

1 Molecular Evolution 2 Recombination & Transposition

2 Recombination larger scale chromosome rearrangements larger scale chromosome rearrangements Recombination is an integral part of evolution which allows favourable & unfavourable mutations to be separated by shuffling the genes Recombination is an integral part of evolution which allows favourable & unfavourable mutations to be separated by shuffling the genes

3 Outline Recombination Recombination Homologous recombination Homologous recombination Non-homologous recombination Non-homologous recombination Site-specific recombination Site-specific recombination Transposition Transposition DNA transposition DNA transposition RNA trnasposition RNA trnasposition

4 Recombination Homologous recombination exchange between homologous DNA sequences; accomplished by a set of enzymes exchange between homologous DNA sequences; accomplished by a set of enzymes function: meiosis I of eukaryotic cell division, double-strand break repair, telomere maintenance function: meiosis I of eukaryotic cell division, double-strand break repair, telomere maintenance replication is an integral part of the reaction, allowing reformation of functional replication forks after any fork blocking event replication is an integral part of the reaction, allowing reformation of functional replication forks after any fork blocking event

5 Homologous recombination

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8 Outline Non-homologous recombination strand exchange between DNA sequences with very little homology Site-specific recombination Site-specific recombination Transposition Transposition

9 Site-specific recombination accomplished by specific recombinases that catalyse the breaking and rejoining of DNA segments function: controlling gene expression, increase genetic diversity function: controlling gene expression, increase genetic diversity replication is NOT part of the reaction replication is NOT part of the reaction

10 Site-specific recombination (SSR) Important distinguishing Feature Conservative SSR process involves Protein-DNA covalent intermediates

11 Site-specific recombination Fig1 from Site-specific recombination by Anca Segall [www.els.net]

12 Site-specific recombination (SSR) 2 structurally unrelated families Tyrosine recombinases ( integrase family) Serine recombinases (resolvase-DNA invertase recombinases) Site-specific recombination by David J Sherratt [www.els.net]

13 Recombination core sites of S and Y recombinases Site-specific recombination FIG 1: Site-specific recombination by David J Sherratt [www.els.net]

14 Serine recombinasetyrosine recombinase Site-specific recombination FIG 3: Site-specific recombination by David J Sherratt [www.els.net]

15 Transposition Discrete sequences (transposable elements or TEs) in the genome that have the ability to translocate or copy itself across to other parts of the genome without any requirement for sequence homology Discrete sequences (transposable elements or TEs) in the genome that have the ability to translocate or copy itself across to other parts of the genome without any requirement for sequence homology

16 Transposable elements move from place to place in the genome 1930s Marcus Rhoades and 1950s Barbara McClintock – transposable elements in corn 1930s Marcus Rhoades and 1950s Barbara McClintock – transposable elements in corn 1983 McClintock received Nobel Prize 1983 McClintock received Nobel Prize Found in all organisms Found in all organisms Most 50 – 10,000 bp Most 50 – 10,000 bp May be present hundreds of times in a genome May be present hundreds of times in a genome

17 TEs can generate mutations in adjacent genes TE in Maize Fig Genes VII by B. Lewin

18 Transposition can occur via RNA intermediate Class I TEs - transpose via a RNA intermediate Retroposons Retroposons retrotransposons retrotransposons DNA intermediate - transpose via a DNA intermediate Class II TEs - catalysed by the enzyme transposase

19 DNA intermediate Class II TEs IS elements and transposons bounded by terminal inverted repeats (TIR) Prokaryotic IS elements (e.g. IS10, Ac/Ds, mariner) encode only transposase sequences eukaryotic transposons encode additional genes such as antibiotic resistance genes

20 DNA intermediate

21 Transposons encode transposase enzymes that catalyze events of transposition Fig a

22 RNA intermediate Class I TEs – transpose via a RNA intermediate Retroposons are structurally similar to mRNA Retroposons are structurally similar to mRNA retrotransposons are structurally similar to retroviruses and are bound by long terminal repeats (LTR) retrotransposons are structurally similar to retroviruses and are bound by long terminal repeats (LTR)

23 Class I TEs encode a reverse transciptase-like enzyme Retroposon Poly-A tail at 3 end of RNA-like DNA strand retrotransposon Long terminal repeat (LTRs) oriented in same direction on either end of element Fig a

24 Fig b

25 Transposons are now classified into 5 families On the basis of their transposase proteins 1) DDE-transposases 2) RT/En transposases (reverse transcriptase/endonuclease) (reverse transcriptase/endonuclease) 3) Tyrosine (Y) transposases 4) Serine (S) transposases 5) Rolling circle (RC) or Y2 transposases Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77)

26 DDE-transposases Fig1 from Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77)

27 RT/En transposases (reverse transcriptase/endonuclease Fig1 from Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77)

28 Tyrosine (Y) transposases Fig1 from Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77)

29 Serine (S) transposases Fig1 from Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77)

30 Rolling circle (RC) or Y2 transposases Fig1 from Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77)

31 Common mechanism of transposition Transposase required Transposase required Regulation of transposase expression controls transposition Regulation of transposase expression controls transposition Catalytic domain of transposase involved in transphosphorylation step that initiates DNA cleavage & strand transfer.

32 Common mechanism of transposition 2 sequential steps Site specific cleavage of DNA at the end of TE Site specific cleavage of DNA at the end of TE Complex of transposase-element ends brought to DNA target where strand transfer is carried out by covalent joining of 3end of TE to target DNA

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34 Common mechanism of transposition Fig Fig 15.10


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