Lecture 3 Unit 4 Strobel - Homologous Recombination Flashcards
(24 cards)
When do double strand breaks occur?
when both strands of a DNA double helix are severed
What are the 2 severe consequences to the genome when double strand breaks occur?
- major chromosomal rearrangements
- deletion of larger chromosome segments
What are some things that cause double strand breaks?
- DNA replication blockage/collisions
- ionizing radiation
- chemical agents
Name the 2 strategies for repairing double strand breaks.
Homology-directed repair and non-homologous end-joining
What does Homology-directed Repair do?
repairs the broken DNA ends using homologous DNA sequences as a template
* this is high fidelity
* this needs a sister chromatid
What does Non-homologous end-joining do?
joins the ends of a double-strand break back together WITHOUT USING A TEMPLATE
* fidelity is variable
* can be used at any time, but may result in imprecise double-strand break repair
What occurs during the 1st step of non-homologous end-joining?
Step 1: end-binding proteins assemble onto the DNA ends at the double strand break
What occurs during step 2 of non-homologous end-joining?
Step 2: a protein crossbridge is assembled between the end-binding proteins
* the protein crossbridge keeps the DNA ends from drifting apart
What occurs during step 3 of non-homologous end-joining?
Step 3: the DNA ends are processed in preparation for repair
* for successful repair, the DNA must have 3’ OH and 5’ phosphate ends
What occurs during step 4 of non-homologous end-joining?
Step 4: the processed DNA ends are ligated together (the ends come together)
What occurs during the 1st step in the general mechanism of Homologous Recombination Repair?
Step 1: End processing: the 5’ ends at the double-strand break are digested in the 5’ to 3’ direction
* this yields 3’ overhang that are complementary to one strand
What occurs during Step 2 in the general mechanism of Homologous Recombination Repair?
Step 2: Strand exchange; the 3’ overhangs base pair with the complementary region in the DNA that will be used as a repair template
* the overhangs will be used as primers
What occurs during Step 3 in the general mechanism of Homologous Recombination Repair?
Step 3: DNA synthesis; the 3’ overhangs are extended by DNA polymerase and once DNA synthesis is complete, they are ligated to seal the double strand break
* this results in a complex DNA structure called a double-Holliday junction
What occurs during Step 4 in the general mechanism of Homologous Recombination Repair?
Step 4: Resolution: the two DNA duplexes are disentangled so that the double strand break is repaired
What does homologous recombination enable the exchange of?
it enables the exchange of genetic material between chromosomes
* this is called sister chromatid exchange
Why does sister chromatid exchange NOT result in new alleles during mitosis?
because the sister chromatids are genetically identical
When does gene conversion occur?
when one allele is converted to the allele on the homologous chromosome
Name the model that was proposed to explain the phenomenon of gene conversion.
the Holliday Model
* proposed by Robin Holliday
What is the first step proposed in the Holliday Model?
Nicks are introduced at identical sites in one strand of two homologous chromatids
What is the second step proposed in the Holliday Model?
the strands invade the helix of the opposite chromatid by base pairing with the complementary strand
What is the third step proposed in the Holliday Model?
the invading strands are linked to the opposite chromatid to yield a Holliday Junction
What is the fourth step proposed in the Holliday Model?
The Holliday junctions can migrate laterally, so that strands from each homologous chromatid are swapped
How does homologous recombination lead to gene conversion?
- DNA mismatch repair of heteroduplexes
- DNA gap repair
What does Branch Migration of the Holliday Model cause?
- it causes a heteroduplex to form
- heteroduplexes contain mismatches and will be repaired by the cell
