chapter 13 exam 2 : recombination Flashcards
What is homologous recombination?
Exchange of DNA through crossing over of homologous chromosomes
What is the function of homologous recombination?
- Also used for repair, especially double-stranded breaks (DSBs)
- Maintaining diversity in sexually reproducing organisms (meiosis)
- Mate-type switching
- Diversity in avoidance and detection mechanisms
- Adaptation of genes to avoid host immune system
* •Novel invasion proteins - Adaptation of genes to detect pathogens
* •Novel recognition proteins
* Antibodies detect antigens. If bacteria change antigens, host needs new antibodies.
Lesions in the template strand lead to 4 outcomes.
- Continue through the lesions –> perpetuate mutation
- Repair is initiated (partial repair) –> DSB
- Fork stalling –> wait for repair
- Lesion bypass –> single stranded gap
Homologous repair mechanism:
1.Create 3’ overhangs
2.3’ end strand invasion via recombinase
•Formation of D loop
3.3’ end strand invasion of other strand invasion
•Formation of D loop
4.Strand extension via polymerase
What are the 2 repair pathways of homologous repairs?
1.Synthesis-dependent strand annealing (SDSA)
•Homologs separate
•Finish polymerase and ligase
2. “Classic” DSBR pathway
•Homologs stay together and resolve holiday junctions
1.No crossover
2.Crossover
Holiday junctions are broken via ______
resolvase
How is recombination used in meiosis?
Recombination is also used to maintain diversity in sexually reproducing organisms
What is crossing over and when does it happen?
- Crossing over happens in prophase I of meiosis I
- Between homologous chromosomes or homologs
- 2 homologs have the same genes, but potentially different alleles
2 homologs may have the same _____ but potentially different ______
genes ; alleles
When does crossing over occur?
Prophase I of meiosis I
What does crossing over occur between?
Homologous chromosomes or homologs
Regions of cross over are spaced apart to avoid ________
interference
Does crossing over occur between homologs or sister chromatids?
homologs, (not sister chromatids)
On average, how many crossovers occur per arm?
1
Does cross over have to occur at least once per arm?
yes
In meiosis I ______ ______ separate
homologous chromosomes
What separates in meiosis II?
sister chromatids
What is the result of meiosis?
4 unidentical, haploid cells
What does haploid mean?
having only one copy of genetic material
How is haploid represented?
n
How is diploid represented
2n
what does diploid mean?
having 2 copies of genetic material
Meiotic recombination occurs during what pathway?
classic DSBR pathway
Does meiotic recombination occur during the SDSA pathway?
No, it occurs during the classic DSBR pathway
What are the steps of meiotic recombination?
1.In early prophase I, double stranded breaks are introduced
•Certain “hotspots” are more commonly broken
•Thus, non-random, but not always predictable
2.3’ overhangs created
3.Strand invasion
4.Extension via polymerase
5.Double cross over intermediate (2 holiday junctions)
6.Resolution
1.Non-crossover (same as SDSA)
2.Crossover
What is gene conversion?
Nonreciprocal transfer of genetic information
What does crossing over maintain?
Genetic diversity!
What is mitotic crossover?
- Damage during mitosis can lead to crossover events—for the sake of repair
- SDSA is most common
Is SDSA common in meiosis or mitosis?
mitotic division
Describe DSBR and mating type switching
•Yeast (S. cervisiae )exists as 2 possible mate types:
1.Matα
2.Mata
•Yeast can convert between either type through DSBR
•Each mate type is stored as a pseudogene
•HMLα
•HMRa
What is used when untimely breaks occur?
NHEJ (non homologous end joining)
When is NHEJ used?
When DSBs occur when homologous chromosomes aren’t available for recombination
Describe NHEJ:
- Common in eukaryotes
- Few prokaryotes
- Common in G0 and G1
- Times when homologous chromosomes aren’t paired
- Mutagenic process = DNA is not the same as it was
- Loss of function of NHEJ genes promotes cancer
Loss of function of NHEJ genes ______ cancer
promotes
What does a mutagenic process mean?
The DNA is not the same as it was
When is NHEJ common?
eukaryotes, and G0 and G1 (when homologous chromosomes aren’t paired)
- not as common in prokaryotes
What are the steps of NHEJ?
- Recognition of damage via Ku proteins
- Cleaning of ends via Artemis •Creation of blunt ends
- Creation of 3’ over hang
- Polymerase
- End joining via ligase
What joins the ends in NHEJ?
ligase
What cleans the ends in NHEJ?
Artemis
What recognizes damage in NHEJ?
Ku proteins
What does artemis cleaning the ends in NHEJ lead to?
Blunt ends
B cells and T cells are immune cells that must recognize an infinite diversity of ______ ______
invading proteins
Intentional NHEJ called V(D)J recombination creates random novel versions of ______ _____
Recognition proteins
- body randomly mixes recognition genes so they are not surprising by novel invading proteins
What is another term for recognition protein?
antibody
How is genome editing used in targeting DSBs?
- used to target DSBs to activate or inactivate or modify genes.
- used in CRSPR, zinc fingers, and TALENS
