molecular evolution 2 recombination & transposition
TRANSCRIPT
Molecular Evolution 2Molecular Evolution 2
Recombination & TranspositionRecombination & Transposition
RecombinationRecombination larger scale chromosome larger scale chromosome
rearrangementsrearrangements
Recombination is an integral part of Recombination is an integral part of evolution which allows favourable & evolution which allows favourable & unfavourable mutations to be unfavourable mutations to be separated by shuffling the genes separated by shuffling the genes
OutlineOutline
RecombinationRecombination Homologous recombinationHomologous recombination Non-homologous recombinationNon-homologous recombination
Site-specific recombinationSite-specific recombinationTranspositionTransposition
DNA transpositionDNA transpositionRNA trnaspositionRNA trnasposition
RecombinationRecombination
Homologous recombinationHomologous recombination exchange between homologous DNA exchange between homologous DNA
sequences; accomplished by a set of sequences; accomplished by a set of enzymesenzymes
function:function: meiosis I of eukaryotic cell meiosis I of eukaryotic cell division, double-strand break repair, division, double-strand break repair, telomere maintenancetelomere maintenance
replicationreplication is an integral part of the reaction, is an integral part of the reaction, allowing reformation of functional replication allowing reformation of functional replication forks after any fork blocking eventforks after any fork blocking event
Homologous Homologous recombinationrecombination
OutlineOutline
Non-homologous recombinationNon-homologous recombinationstrand exchange between DNA sequences strand exchange between DNA sequences
with very little homologywith very little homology
Site-specific recombinationSite-specific recombination
TranspositionTransposition
Site-specific recombinationSite-specific recombination
accomplished by accomplished by specific recombinasesspecific recombinases that catalyse the breaking and that catalyse the breaking and rejoining of DNA segmentsrejoining of DNA segments
function:function: controlling gene expression, controlling gene expression, increase genetic diversityincrease genetic diversity
replicationreplication is NOT part of the reaction is NOT part of the reaction
Site-specific recombination (SSR)Site-specific recombination (SSR)
Important distinguishing FeatureImportant distinguishing Feature
Conservative SSR process involvesConservative SSR process involvesProtein-DNA covalent intermediatesProtein-DNA covalent intermediates
Site-specific recombinationSite-specific recombination
Fig1 from Site-specific recombination by Anca Segall [www.els.net]
Site-specific recombination (SSR)Site-specific recombination (SSR)
2 structurally unrelated families2 structurally unrelated families
Tyrosine recombinases (Tyrosine recombinases (integrase integrase family)family)
Serine recombinases (resolvase-DNA Serine recombinases (resolvase-DNA invertase recombinases)invertase recombinases)
Site-specific recombination by David J Sherratt [www.els.net]
Recombination core sites of S and Y recombinases
Site-specific recombinationSite-specific recombination
FIG 1: Site-specific recombination by David J Sherratt [www.els.net]
Serine recombinasetyrosine recombinaseSite-specific recombinationSite-specific recombination
FIG 3: Site-specific recombination by David J Sherratt [www.els.net]
TranspositionTransposition
Discrete sequences (transposable elements Discrete sequences (transposable elements
or TEs) in the genome that have the ability or TEs) in the genome that have the ability
to translocate or copy itself across to other to translocate or copy itself across to other
parts of the genome parts of the genome without any without any
requirement for sequence homologyrequirement for sequence homology
Transposable elements move from Transposable elements move from place to place in the genomeplace to place in the genome
1930s Marcus Rhoades and 1950s Barbara 1930s Marcus Rhoades and 1950s Barbara McClintock – transposable elements in cornMcClintock – transposable elements in corn
1983 McClintock received Nobel Prize1983 McClintock received Nobel Prize
Found in all organismsFound in all organisms Most 50 – 10,000 bpMost 50 – 10,000 bp May be present hundreds of times in a genomeMay be present hundreds of times in a genome
TEs can generate mutations in adjacent genesTEs can generate mutations in adjacent genes
TE in MaizeTE in Maize
Fig 15.19 Genes VII by Fig 15.19 Genes VII by B. LewinB. Lewin
Transposition can occur via Transposition can occur via
RNA intermediateRNA intermediate
Class I TEsClass I TEs - - transpose via a RNA transpose via a RNA intermediateintermediate RetroposonsRetroposons retrotransposonsretrotransposons
DNA intermediateDNA intermediate - - transpose via a DNA transpose via a DNA intermediateintermediate Class II TEsClass II TEs - - catalysed by the enzyme catalysed by the enzyme
transposasetransposase
DNA intermediateDNA intermediateClass II TEsClass II TEs
IS elements and transposons IS elements and transposons
bounded by bounded by terminal inverted repeats terminal inverted repeats (TIR)(TIR)
Prokaryotic IS elements (e.g. IS10, Ac/Ds, Prokaryotic IS elements (e.g. IS10, Ac/Ds, mariner) encode only transposase mariner) encode only transposase sequences sequences
eukaryotic transposons encode additional eukaryotic transposons encode additional genes such as antibiotic resistance genes such as antibiotic resistance genesgenes
DNA intermediateDNA intermediate
Transposons encode transposase enzymes that Transposons encode transposase enzymes that catalyze events of transpositioncatalyze events of transposition
Fig. 13.24 a
RNA intermediateRNA intermediateClass I TEsClass I TEs – –
transpose via a RNA transpose via a RNA intermediate intermediate
RetroposonsRetroposons are are structurally similar structurally similar to mRNAto mRNA
retrotransposonsretrotransposons are structurally are structurally similar to similar to retroviruses and retroviruses and are bound by long are bound by long terminal repeats terminal repeats (LTR)(LTR)
Class I TEs encode a reverse transciptase-like Class I TEs encode a reverse transciptase-like enzymeenzyme
RetroposonRetroposon
Poly-A tail at 3’ end Poly-A tail at 3’ end of RNA-like DNA of RNA-like DNA strandstrand
retrotransposonretrotransposon Long terminal Long terminal repeat (LTRs) repeat (LTRs) oriented in same oriented in same direction on direction on either end of either end of element element
Fig. 13.23 a
Fig. 13.23 b
Transposons are now classified Transposons are now classified into 5 familiesinto 5 families
On the basis of their transposase proteinsOn the basis of their transposase proteins
1) DDE-transposases1) DDE-transposases2) RT/En transposases2) RT/En transposases
(reverse transcriptase/endonuclease) (reverse transcriptase/endonuclease) 3) Tyrosine (Y) transposases 3) Tyrosine (Y) transposases 4) Serine (S) transposases4) Serine (S) transposases5) 5) Rolling circle (RC) or Y2 transposasesRolling circle (RC) or Y2 transposases
Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77))
DDE-transposasesDDE-transposases
Fig1 from Fig1 from Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77))
RT/En transposases (reverse transcriptase/endonucleaseRT/En transposases (reverse transcriptase/endonuclease
Fig1 from Fig1 from Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77))
Tyrosine (Y) transposasesTyrosine (Y) transposases
Fig1 from Fig1 from Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77))
Serine (S) transposasesSerine (S) transposases
Fig1 from Fig1 from Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77))
Rolling circle (RC) or Y2 transposasesRolling circle (RC) or Y2 transposases
Fig1 from Fig1 from Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77Nature Rev Mol. Cell Biol (Nov2003) 4(11):865-77))
Common mechanism of Common mechanism of transpositiontransposition
Transposase requiredTransposase required Regulation of transposase expression Regulation of transposase expression
controls transpositioncontrols transposition
Catalytic domain of transposase involved in Catalytic domain of transposase involved in transphosphorylation step that initiates transphosphorylation step that initiates DNA cleavage & strand transfer. DNA cleavage & strand transfer.
Common mechanism of Common mechanism of transpositiontransposition
2 sequential steps2 sequential steps
Site specific cleavage of DNA at the end of TESite specific cleavage of DNA at the end of TE
Complex of transposase-element ends brought Complex of transposase-element ends brought to DNA target where strand transfer is to DNA target where strand transfer is carried out by covalent joining of 3’end of carried out by covalent joining of 3’end of TE to target DNA TE to target DNA
Common mechanism of transpositionCommon mechanism of transposition Fig 15.14
Fig 15.10