Meiosis Lecture Notes

Chromosomes undergo three major activities:
- Meiotic homolog recombination
- Segregation I (based on chiasma resolution, no S-phase)
- Segregation II (based on S-phase, pairing, synapsis through loss of arm cohesion, no new cohesion, loss of centromeric cohesion)
Chromosomes spend a significant amount of time searching for each other.

Early Leptotene: Sordaria chromosomes pair at a distance of 400nm before synapsis.
Mid-Leptotene: Telomere regions begin pairing (indicated by arrows).
End Leptotene: All homologues are aligned; interlocking between two pairs of homologues is observed.
Early Bouquet: Two clusters of telomeres are present (large arrows).
Loose Bouquet: Telomeres are loosely grouped (large arrow).
Tight Bouquet: Almost all telomeres are grouped (arrow).
Zygotene Bouquet: Interlocked pairs are present (arrow).
Early Pachytene: Bouquet is very tight (arrow).
Mid-Pachytene: Bouquet is looser.

Leptotene: Homologues 1 (1m and 1p) and 2 are co-aligned over half their length.
Homologues 6, 7, and 3 are completely aligned at 400 nm.

Reversal of conjugation would only require a single division.
Meiosis involves two divisions to reduce a diploid genome (2c) to a haploid genome (1c).
- Diploid genome (2c)
- Haploid genome (1c)
- Conjugation meiosis 1c, 1c, 2c
Premeiotic S-phase results in 4c, followed by Meiosis I and II, resulting in 1c.

Rec8 is identified as meiotic cohesin, with stepwise loss in MI & MII ([Klein et al., Cell, 1999]).
Chiasmata consist of crossovers plus arm cohesion.
Removing arm cohesion in Meta/Ana 1 transition causes homologs to separate.

Stepwise removal of Rec8:
- First from chromosome arms
- Second, from centromeres
Prophase I: Cohesin along paired homologs.
Removal of arm cohesin triggers the 1st meiotic division.
Removal of Cen-cohesin triggers the 2nd meiotic division.

Prediction 1: If distal arm cohesin (Rec8) connects recombined homologs, the Metaphase-Anaphase transition should require removal (cleavage) of Rec8.
Experiment: REC8-N allele (Separase cleavage sites destroyed).
- REC8-N can't be cleaved.
Prediction II: How should cells with blocked chiasmata arrest?

Wild type:
- Metaphase: Centromeres point to poles, telomeres trail.
- Anaphase: Sister-telomeres separate and move to (mostly opposite) poles.
REC8-N:
- No Anaphase I, no separation of homologs.
- Cells arrest in Metaphases I, with centromeres at the poles and telomeres stuck at the metaphase plate ([Buonomo et al., Cell, 2000]).
Cleavability of Rec8 is required for metaphase-anaphase transition in M1

2 Homologs (mom, dad)
4 Chromatids (2mom, 2dad)
A chiasma is a physical connection between meiotic homologs, formed by a strand switch (CO) and distal sister chromatid cohesion!
Only 2 non-sister chromatids are involved per connection
Chromatids NEVER switch sides.

Show the same final phenotype as Rec8N (NO nuclear segregation but spindle assembly proceeding)
suffer from unresolved holliday junctions, ([Xaver, et al. 2015])
Yeast has no checkpoint to stop the division upon failure of JM(HJ) resolution.
DNA damage checkpoint (relies on ssDNA or DNA ends) or the SAC (lack of tension or attachment).
None of these can catch unresolved HJs.

Pachytene:Synaptonemal complex connects homologs; double Holliday junctions (dHJs) are not yet resolved.
After Pachytene: dHJs are converted to crossing over; bivalents held together by distal SCC forming chiasmata.

CHIASMATA are reversible links between homologs created by CROSSOVERs together with distal SISTER CHROMATID COHESION until Anaphase I.
Only two of the four sister chromatids of the bivalent are involved in forming a chiasma; involved chromatids are non-sisters.
Each bivalent has one chiasma at a minimum but can have many more.
All chiasmata are independent of each other in respect to which chromatids connect, as long as they are non-sisters.
CROSSOVER INTERFERENCE prevents chiasmata close to each other.

Incidence of trisomic pregnancies: 3% for women in their 20s, >30% for women in their 40s.
Mice have a similar process, but decades faster (1 vs 2.5 years).
Reduced ovulations reduces chromosome bridges

Reduced ovulations reduces CENP erosion and chromosome abnormalities.
CREST antibody recognizes key proteins at the centromere (mainly CENP-A, B, and C).
Hormonal contraception reduces the incidence of chromosomal abnormalities in MII eggs of aged females.

Requires 3 modifications compared to mitosis:
- Chiasmata: Physical connection between homologs.
- Cohesin protection at Anaphase I centromeres (for another round of segregation).
- Shugoshin recruits a phosphatase (PP2A) to the kinetochore, which antagonizes phosphorylation of Rec8 by Cdc5 (polo kinase), which prepares Rec8 for being cleaved by separase. Shugoshin and the PP2A mechanism are evolutionary conserved.
Synthelic bi-oriented attachment
Amphitelic attachment blocks division because Rec8 can’t be cleaved at that stage
Bi-orientation is required for tension to satisfy the SAC: Spindle attachment Checkpoint
Aurora B kinase universally regulates bi- orientation in meiosis and mitosis by making kinetochore attachment dependent on tension.

The SC connects axial elements by a liquid like phase compartment.
The SC works closely with a specialized DSB repair pathway called ZMM, to ensure the ‘obligatory crossover’ .
It can distinguish 0 from ≥1 via ‘interference’ .

All mutants, defective in the synapsis pathway, show a reduction (often ~50%) of CO, but normal gene conversion (NCO-non-cross-over)
The synapsis pathway antagonizes Bloom’s helicase ([Börner, Kleckner, Hunter, Cell 2004]).
Axial associations Blooms (Sgs1) helicase removes Axial associations
Sgs1 is Bloom's helicase

Involved in Homologous Recombination (HR) repair maintaining genomic stability
BLM: BLOOM‘s syndrome – elevated SCE (sister chromatid exchange)
BLM associated features:
- Broad spectrum of cancers in a early age
- Hypersensativity to genotoxic compounds