Chromosome biology lecture 4

Question 1

Genetic recombination

Answer 1

• Process where DNA is broken + rejoined into new segments
• Mechanisms of recomb. conserved
• 4 types: homologous recomb, site-specific recom, transposition + illegitimate recombination

Question 2

Genomic alteration associated w/ HR

Answer 2

1. Gene conversion (both sister chromatids genetically identical so not issue, but 2 homologs can carry different alleles → gene conversion)
2. Cross over (exchange of chromosomal segments, often btw DNA repeats)

Question 3

Role of HR

Answer 3

1. DNA repair (uses undamaged sister chromatid as a template, loss of heterozyg)
2. DNA replication (collapsed + broken replication forks, RDR)
3. Meiosis (programmed ds breaks by spoII, can lead to crossover)
4. Mating type switching in yeast (a or alpha type, haploid switch mating type through recombination at MAT locus, HO endonuclease)
5. Antigenic variation by African trypanosome (VSG, different copies produced)

Question 4

RDR in fission yeast

Answer 4

• Collapsed fork undergoes reversal
• 4 way DNA junction structure
• Exonucleases resect, ss DNA bound by RPA, Rad52, Rad51, D loop structure, MCM2-7
• D loop is unstable

Question 5

Meiosis vs Mitosis

Answer 5

• In 2 phase, homologs are replicated → 2 pairs of sister chromatid
• In meiosis, diploid chromosome → haploid
• Programmed ds also made, reciprocal exchange of DNA occurs, tension sensing mechanism

Question 6

Holliday model

Answer 6

• 1 strand of DNA nicked in both molecules, nicked strand re-anneals to paired DNA, 4 way junction, further unwinding + re-annealing, HJ moves

Question 7

Neuospora crassa lifecycle

Answer 7

• 2 mating types
• Meiosis → 8 spores in same ascus
• Recombination = 2:2:2:2 or 2:4:2
• Ratio of 6:2 or 2:6 also well documented (non-reciprocal exchange of info btw chromatids)
• 4:4 ascus (2 of paired products segregate as post-meiotic division)
• 5:3 or 3:5 spore ratio ascus = asymmetric exchange of a ss from 1 chromosome to another

Question 8

Meselson + Radding model

Answer 8

• Endonuc. makes a ss cut in 1 chromatid
• DNA synthesis → displaced nicked strand → invades 2nd chromatid → loop excised → free ends ligated to form HJ
• Junction can migrate to 2 heteroduplexes
• Resolved by opposite strands cleaving + rejoining along horizontal or vertical line
• Asymmetric duplex made

Question 9

1983 ds break model

Answer 9

• Hotspot for meiotic recomb. correlates w/ site of ds break
• Recombination initiated from ds break
• 2 ends resected to expose 3’ ended DNA region
• 1 ss end invades homologous DNA → D loop (x degraded)
• DNA synthesis extends from D loop
• Resolution
• GOOD EVIDENCE

Question 10

Other DSB pathways

Answer 10

1. SSA (ds break flanked by directly repeated sequences, compl sequences anneal, 3’ flaps cut away, nicks sealed
2. BIR (2nd end of ds break x anneal to D loop, DNA synthesis continues until end of chromosome)
3. SDSA (extended D loop unwound, released strand anneals to other end of break)