SU 6

• Genetic recombination


  Natural process of rearranging genetic information to form new associations
• Genetic recombination


  Exchange or incorporation of 1 DNA sequence with/into another

  
• Recombination in higher organisms

  • Can occur in the DNA of somatic cells (any cell of a living organisms other than the reproductive cells)

  
• Somatic cells


  Responsible for expressing proteins of the immune system response
• Somatic recombination


  Rearranges Ig genes increasing diversity of these molecules from a fixed amount of genetic info

  
• Transposons


  Also known as insertion sequences
• Transposons can cause mutation

  If a gene or regulatory protein at the site is disrupted
• Homologues recombination


  Reaction between similar DNA sequences

  
• Homologues recombination


  • Prevalent during the production of gametes (meiosis)
• Meiosis


  A type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell
• Non-homologues recombination


  Reaction between different DNA sequences

  
• Non-homologues recombination


  • Occurs at low frequency
• Non-homologues recombination


  • Powerful force reshapes the genomes of organisms
• Transposition


  Enzymatic insertion of a transposon (movable/mobile segment of DNA) into a new location in the genome

  
• Non-homologues recombination and transportation

  Have an evolutionary role
• Homologues recombination


  To fix DNA so that information is not lost

  
• Homologues recombination


  • Repair damaged chromosome with homologous chromosome
• Recombination requirements


  • DNA has to 'break'
  • DNA has to reunite to allow for exchange of parts

  
• Denaturation/melt


  Melting temperature: 50% DNA dissociate to single strands
• Similar sequences


  More for fixing DNA so information is not lost
• Different/altered sequences


  For evolution
• Meselson and Weigle E. coli experiments

  1. Infect bacteria with respective viruses

  2. Viruses grown in 13C and 15N containing media (heavy light)

  3. Recover progeny by density centrifugation

  4. Phage particles with recombinant genotypes distributed throughout the gradient bt heavy and light

  5. Parental particles as distinct heavy and light bands in the gradient

  6. The recombinant phage had DNA in varying proportions from both parents

  
• DNA must have 'broke' and rejoined
• Some plaques formed via single virus infecting a bacterium had 2 different geneotypes

  
• Some of the phages must have had a region of heteroduplex DNA to begin with (a part of each strand is contributed by a different paren't)
• Hybridization


  Piece of DNA/RNA of known nucleotide sequence identifies fragments with complementary sequence
• Reunion of strands


  Needs to occur
• How well hybridized/re-associates/re-anneals


  • Dependent on base composition: G:C more stable, higher
  • Strand length: longer more bp
  • Reaction condition: high cation concentration favours dsDNA

  
• The holiday model of genetic recombination


  1. 2 homologous DNA duplexes are juxtaposed (sequences aligned)

  2. Chromosome pairing = synapsis

  3. Single stranded nicks in the DNA at homologous sites of the 2 paired chromosomes

  4. 2 duplexes partially unwinds (RecBCD enzymes mark site for recombination (nuclease domain) + helicase function to unwind + repair DNA)

  5. Single strand end of 1 duplex bp with nearly complementary ss region along intact strands in the other duplex and vice versa

  6. Ligation of the free ends from the different duplexes

  7. This yields cross stranded intermediate = holliday junction

  8. The junction migrates either direction (unwinding and rewinding of 2 duplexes)

  9. Results in strand exchange

  10. To resolve 2 duplexes: planar representation

  11. Another pair of nicks introduced

  12. Patch recombination heteroduplex: Nicks in strands originally nicked, 1 strand of duplex remains intact

  13. Splice recombination heteroduplex: Nicks in strands not previously nicked, 2 strands are heteroduplex

  
• Barbary McClintock


  Identified activator genes in maize → mutation of the 2nd gene
• Activator gene


  • Was an international source of mutation
  • Activator and mutated gene in same DNA
  • Activator genes moved about freely in the genome

  
• 1983 she was awarded the Nobel Prize in physiology/medicine
• Jumping genes


  Genes which moved from 1 site to another in the genome
• Mobile/transposable elements/transposons


  Segments of DNA enzymatically moved from 1 place in genome to another

  
• Transposons

  • Their location within DNA is unstable
  • Range in size: several hundred bp to >80kbp
  • Contain a gene which encodes an enzyme for movement of the transposons to different locations and insertion into chromosomes

  
• Transposition events


  Movement of transposons
• Transposons


  Also referred to as insertion sequences
• Transposons can cause mutation


  If a gene or regulatory protein at that site is disrupted
• DNA repair via


  • Homologous recombination
  • Excision repair
  • Mismatch repair
• Transposition


  • Drives evolution