22 terms

Homologous Recombination

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What are two uses for homologous recombination?
1. Fixing large errors in DNA.
2. Biotechnology
Why is homologous recombination necessary?
1. Genetic diversity
2. Separate favorable/unfavorable mutations
3. Repair lesions at replication forks that are damaged and not fixed by the polymerase and restarting replication.
4. Some viruses use homologous recombination to become resistant to antiviral drugs.
Name three functions of homologous recombination
1. Assists in DNA repair
2. Links sister chromosomes to properly segregate them between self and daughter cells.
3. Source of DNA exchange and therefore genetic diversity.
What kinds of sequences does homologous recombination occur between?
Exactly corresponding sequences.
Is there a net gain or loss of base pairs when homologous recombination event?
No
What are the three classes of recombination and briefly describe them?
1. Homologous/general recombination; exchange between DNAs that share an extended region of similar sequence
2. Site-specific recombination; exchange only at a particular sequence an an integrase that recognizes the sequence
3. DNA transposition; jumping genes that are short DNAs that can move from one chromosome to another. Also, viruses use viral integrase to insert genes into human chromosomes.
What are Holliday junctions?
DNA tetrad where two strands of DNA are at 90 degree angles that will undergo homologous recombination.
Describe generalized recombination.
Homologous chromosomes and sequences align. Crossing over may or may not occur.
Describe site specific recombination.
Certain sequences match up and allows a DNA plasmid to insert itself into the chromosome in a reversible process.
What portion of the cell cycle does homologous recombination frequently occur in?
Meiosis (first meiotic prophase); diploid germ cells divide into haploid cells by producing four homologous chromosomes and divided twice. Hopefully this installs genetic diversity. crossing over is mediated by synaptonemal complexes.
What is the evidence of the locations where crossing over has occurred?
Chiasmata.
Describe double strand break repair (DSBR).
1. A double stranded break occurs in one of the DNA duplexes.
2. Spo11 generates 3' single stranded ends that can invade the donor duplex by chewing back the 5' ends.
3. Strand invasion occurs generating a D-loop.
4. Strand exchange results in heteroduplex DNA.
5. DNA synthesis extends the broken strand and DNA ligase ligates them back together.
6. This where crossing over can occur during meiosis because of the Holliday junctio
What is the role of topoisomerase? What are the two types and what makes them different?
Regulate unwinding of DNA by breaking phosphodiester backbone and unwinding DNA.
Type 1 breaks 1 strand.
Type 2 breaks 2 strands.
What is a recombinant joint and what does it allow?
Where one strand of DNA crosses from one strand to the other. Allows flexibility to the recombination apparatus to move up and down the strand.
Describe the risks associated with crossing over during DSBR.
Risk of information loss and other potential errors.
Does synthesis-dependent strand-annealing (SDSA) ever have crossing over?
NEVER. Gives it high fidelity.
Describe SDSA.
1. A double stranded break occurs.
2. 5' end resection occurs.
3. Strand invasion leads to D-loop.
4. Missing DNA is filled in by DNA polymerase
5. Invading strand displaces itself and is ligated back to itself.
6. DNA polymerase fills in the missing strand.
What is a direct repeat?
A segment of the chromosome where the same DNA sequence is repeated due to multiple insertion events over time.
What is a problem associated with DNA repair in a direct repeat?
Strand invasion to the wrong repeat resulting in large losses of information.
When is single strand annealing (SSA) initiated?
Two direct repeats on the same strand incur a double stranded break.
Does not require a separate strand of identical or similar DNA material to fix because the direct repeat on the same strand can be used to fix the problem.
Does SSA result in deletion of base pairs?
Yes; it is unavoidable.
Describe how SSA occurs.
1. Double stranded break occurs.
2. 5' ends are resected.
3. RPA binds to the 3' overhands to prevent them from reannealing to each other.
4. Rad52 binds to, and anneals the two complementary repeats together.
5. Floppy leftover ends must be chewed back.
This results in a reduction from 2 direct repeats to 1 direct repeat with loss of the middle information.
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