Meiosis and homologous recombination Flashcards
Is meiosis or mitosis more accurate?
Mitosis
What two events occur in meiosis but not mitosis?
- synapsis and crossing over
- separation of homologs
Describe the important steps of meiosis I prophase.
- leptonema: condensation of DNA and contact with kinetochores
- zygonema: homolog interactions become visible
- pachynema: homologs are held together by transverse filaments
*during these steps, homolgous recombination is taking place. During the last step, synaptonemal complex forms and crossing over takes place*
Describe the synaptonemal complex (SC)
- sister chromatids of each homolog are held together by cohesin
- cohesin also holds the homologs to the axial core proteins, which themselves are held together via transverse filaments.
Describe the cohesin complex.
- Consists of two smc proteins, each with a hinge and N and C termini at the same end of the tertiary structure. Hinges are important for dimer formation of the smc’s.
- Scc proteins help to open and close the ring structure. Scc1 is the target of separase, which opens cohesin to release sister chromatids.
When does crossing over occur, and when is it visible?
It occurs during pachynema, but becomes visible during the diffuse stage.
What is the resulting structure of crossing over?
The chiasmata is a result of crossing over, leaving a physical link between homologs.
What is the timeline of meiosis in female development?
- prophase I begins at the embryonic stage, and is arrested until ovulation.
- during ovulation, the cell reaches prophase II, at which point it is arested until fertilization by a sperm cell.
What is homologous recombination and what is its purpose?
- Homologous recombination is the process by which homologous DNA molecules interact and exchange genetic information.
- it is important for the repair of double stranded breaks (in somatic cells)
- it promotes the accurate segregation of homologs during anaphase I via the establishment of chiasma (in germ cells) which pairs homologs
- it promotes genetic diversity (in germ cells)
What is the effect of HR mutant organisms on meiosis?
They fail to form the synaptonemal complex, which is important for crossing over.
How is the segregation of homologous chromosomes directed?
Segregation of homologs is directed by homologous recombination which generates cross overs (chiasma). These physical connections are where force is applied by the mitotic spindle upon separation.
What would happen in an organism which cannot generate meiotic crossovers?
Their germ cells will undergo meiotic nondisjunction and sterility will result.
What are the two steps of cohesion release in meiosis?
There is coheison between the sister chromatids of each homolog at the centromere (protected by shugoshins) and along the arms.
- Meiosis I: the bivalent is held together by the chiasma resulting from crossing over. Cleavage of the cohesin between arms by separase releases bivalents from one another.
- Meiosis II: centromere cohesin is released by separase to release sister chromatids from one another.
Why does aneuploidy increase exponentially with the age of a woman?
Crossover events take place very early in development (prophase I), before birth. With time, the cohesins between chromosomes become defective.
Which recombination events can lead to meiotic nondisjunction?
- no crossover: bivalent is unstable, so segregation occurs randomly. This would take place during meiosis I.
- distal crossover: crossover takes place at the very ends of the homologs, resulting in unstable bivalent because there is not enough cohesion. Results in random segregation, and takes place during meiosis I
- proximal crossover: crossover occurs very close to centromeres of homologs. the bivalent becomes entangled, so there is a reduction in division at meiosis II.
*to circumvent these problems, crossovers are distributed non-randomly*
How are double stranded breaks achieved in meiosis I? How does it work?
Double stranded breaks are catalyzed by Spo11 (very highly conserved!) at its tyrosine residue:
- Spo11 acts by forming a complex with other proteins, including Mer2 (important to regulation!)
- Mer2 is regulated by phosphorylation, activating it an thereby promoting its interaction with Spo11, Xrs2, Mei4, and Rec114
- the complex activates exonuclease to cause DSB
What proteins are responsible for resecting DSB in HR?
Mrx, ExoI, Sae2, Sgs1 (helicase)
Which proteins are responsible for strand invasion during DSB HR? How is this catalyzed?
Rad51 (large helical protein) and Dmc1 catalyze strand invasion by each binding to one of the sister chromatids of the broken chromosome, allowing a sister to search for a homologous sister.
What occurs after strand invasion during DSB HR?
Strand synthesis and extension by DNA polymerase.
What step occurs after strand extension in DSB HR? What protein is responsible?
capture of the second end of the broken chromosome occurs after extension of the first. This is catalyzed by Rad52.
After DSB HR, how are the two DNA homologs separated from one another?
The cutting of the two holliday junctions.
What are the two ways in which DHJs can be resolved?
- crossover formation: Holliday junctions are not cut in the same direction
- non-crossover formation: both holliday junctions are cut in the same direction
Why might non-crossover resulting homologs not be a result of the DSBR pathway?
NCO homologs appear before CO homologs. So, perhaps NCO homologs come from a different pathway, such as one which does not have second end capture after elongation of the first strand. Instead, the first syntesized strand would dissociate from its annealing homolog and act as the template for elongation of the second strand. This proposed pathway is called synthesis-dependent strand annealing (SDSA) model.
