Lec 5 - Ch6.5

shows that in tetrads, two of the four chromatids contain a combination of genetic material from both parental strands, indicating a crossover event between homologous chromosomes

- Genetic material is exchanged between homologous chromosomes
- creates hybrid DNA strands
- Rejoining occurs at specific points during the process

- Meselson and Weigle infected E coli with heavy and light isotopes of N
- Heavy is AB and light is ab
- Results show Ab with mostly heavy DNA which supports hypothesis

1. Double strand break formation
2. Resection
3. Strand Invasion
4. Double Holliday Junction
5. Branch Migration
6. Holliday Junction Resolution

Spo11 protein breaks phosphodiester bonds of BOTH strands of ONE chromatid

5′ ends of each broken strand are degraded by an EXONUCLEASE to create 3′ single-stranded tails

- Dmc1 helps one single-strand tail invade a non-sister chromatid and forms stable heteroduplex
- Displacement loop (D-loop) from invaded chromatid is stabilized by single-strand binding protein

- D-loop and other 3′ tail form a second heteroduplex
- DNA synthesis replaces the DNA degraded during formation of the 3′ tails
- DNA ligase reseals the DNA backbones

Heteroduplex region of both DNA molecules is lengthened

Holliday junctions are resolved by resolvase and ligase

- Invading strand is lengthened by DNA synthesis using blue strand as template
- Anticrossover helicase disentangles the invading strand from the nonsister chromatid preventing Holliday junction formation

- Two strands of DNA don’t break and rejoin at the same location
- DNA repair enzymes eliminate mismatches and gene conversion occurs.

the physical change of one allele in a heterozygote to the other