6.5: Homologous Recombination at the DNA level

1. Independent assortment

2. crossing over (recombination)

The most important mechanism of generating genetic diversity is the production of new combinations of already existing alleles [2]

Tetrads

Some fungi generate asci carrying all products of individual meiosis:

Physical breakage and reunion of DNA molecules

The melson and Weigle experiment proved that genetic recombination happens through:

Spo11 protein

Meiosis specific endonuclease that cleaves phosphodiester bond on both strands of DNA

Exonuclease

enzyme degrades the 5’ end on each side of double helix break to form 3’ overhang strands

1. double strand break formation

2. DNA resection

3. First strand invasion and D-loop formation

4. Two fates, either CO or NCO

5. formation of a double Holliday Junction

6. Branch migration

7. resolution of the double Holliday Junction intermediate

Steps of double strand break repair model [7]

Dmc1 protein

protein catalyses base pairing between the invading strand and its complementary strand.

Replication Protein A

D-loop is stablilzed by this protien

The noncomplementary strand is pushed out forming a D-loop

How is heteroduplex formed?

Holliday junction

interlocked regions of two non-sister chromatids in recombination intermediates

Resolvase

enzyme that breaks the two strands at each Holliday Junction

Ligase

rejoins the ends after two strands at each holiday junction broke

nonsider chromatid is used as a template to fill in the hap

A ligase rejoins both ends. Result is no crossover, but a heteroduplex region remains in one chromatid

Describe noncrossover pathway:

Induction of a repair mechanism that corrects the mismatch

In heteroduplexes, mismatched bases occur leading to:

Gene conversion

Process by one allele in a heterozygote is physically changed into the other, provides evidence for heteroduplex formation during recombination events.

recombinases

recombination is catalyzed by enzyme, catalyzes breakage and joining of DNA strands

two specific, short DNA target sites

crossing over occurs between:

Integration

Site specific recombination can lead to this_____ of genetic material into the genome

Excision

Can allow phage DNA to be removed from bacterial genome

Small guide RNA and a nuclease (Cas9) allow to induce double strand breaks at a specific location of the genome

Describe how CRISPR Cas9 works