L6 - Homologous recombination

Question 1

compare homologous recombination and Non-homologous end joining

Answer 1

Homologous:
- uses a section of identical/homologous DNA to repair
- product is identical to before ds break

NHEJ:
- no need for donor DNA
- product is slightly different due to random bases added

Question 2

mechanism of homolous recombination - no enzymes

Answer 2

Aim is to have swap 1 strand of broken DNA with an undamaged strand from homologous

1. end resection - protein binds and ‘chews’ ends
   = chews 5’ end more than 3’ to create 3’ overhangs
2. Homology seach and strand invasion by 1st 3’ strand end creating D-loop
   = opens up homologous DNA and seaches for complimentary base paring
3. anneals to complimentary sequnce
4. DNA polymerase adds nucleotdies lost from chewing on invaded 3’ strand
5. 2nd strand invades and is extended
   = end with 2 dsDNA molecules but with nicks
6. Branch migration of branch/crossover point

= double holiday jucntion formed with cross shaped structure + 2 DNA helices crossed over

7. ‘Resolution’ of holiday junctions by nucleases seperates 2 DNA molecules
8. ligase heals nick caused by nucleases in resolution

Question 3

what is branch migration

Answer 3

sideways movement of crossover/branch point

DNA on one strand is unwound and TRANSFERED to the other as there the same

= cross section moves along both strands

Question 4

2 outcomes of resolution of holiday junctions

Answer 4

crossover and non-crossover

Question 5

describe the proscess that would form non-crossover products in resolution

Answer 5

Junctions are cleaved in the exact same way

= chromosones have not exchanged material = strand runs from green to green with a ‘patch’ exhanged DNA in middle

Question 6

describe the proscess that would from crossover products from resolution

Answer 6

Junctions are cleaved in different ways

chromosones have exchanged DNA
- strand starts as blue but ends as green

= known as ‘splice products’

Question 7

using homologousbvs sister chromatids in homologous recombination and the effectsv this has on when it is preffered to do this

Answer 7

Homologous chromosomes (1 from maternal + 1 from paternal) are very similar but not identical

= genetic differences between mum and dad

sister chromatids in S or G2 phase when ready for cell division ARE identical

= produces error free recombination = no mixing of alleles = cant tell difference between before and after

= cells usallu only undergo Homologous recombination in S phase when chromatids are present

Question 8

why can prokaryotes undergo homologous recombination at any point in the cell cycle but eukaryotes cannot

Answer 8

binary fission produces gentically identical cells

Question 9

why could it be harmful for genes/alleles to be switched from one homologous chromosone to another

Answer 9

DNA expression works in combination with lots of factors
- promoters

= if crossover takes place may have a functional gene but no promoteror other way round

= person can no longer produce this protein = bad