Lecture 10 : HR in Meiosis

What initiates programmed DSBs in meiosis?

SPO11 nuclease creates covalent protein-linked DSBs by cutting both DNA strands; DSB formation is programmed and genome-wide to initiate recombination.

Why are meiotic DSBs important?

They initiate homologous recombination that produces crossovers (COs), which promote accurate homolog pairing and segregation and increase genetic diversity.

How does SPO11 create a DSB biochemically?

SPO11 forms a transient covalent phosphotyrosyl linkage to the 5' DNA ends after cleavage, leaving protein-linked DSB ends.

How is SPO11 released from DSB ends?

Endonucleolytic cleavage (by MRN/CtIP-like activities) removes SPO11 bound to short oligonucleotides, producing short 3' overhangs that are substrates for resection.

How does meiotic resection proceed after SPO11 removal?

Resection extends 5'→3' to generate long 3' ssDNA tails; Exo1 and helicase–nuclease pairs perform long-range resection, similar to mitosis but often more extensive.

How are RAD51 and DMC1 involved in meiosis?

Both recombinases form filaments on resected ssDNA; DMC1 is meiosis-specific and promotes inter-homolog strand invasion bias, while RAD51 supports filament formation and recombination machinery.

What enforces the inter-homolog bias in meiosis?

Meiosis-specific factors (DMC1, HOP2–MND1, Mek1 kinase signaling, chromosome axis proteins) favor using the homologous chromosome rather than sister chromatid as the repair template.

How does strand invasion differ in meiosis vs mitosis?

In meiosis, strand invasion is biased toward homologs and often stabilized to form double Holliday junctions (dHJs) that are later resolved into COs or NCOs; mitosis favors sister chromatids and non-crossovers.

What are the major outcomes of meiotic HR?

Crossovers (reciprocal exchange between homologs) and gene conversion tracts (nonreciprocal sequence transfer); COs are required for proper homolog segregation.

How do dHJs form during meiosis?

After strand invasion and DNA synthesis, second-end capture and ligation can produce paired HJs (dHJs) that are stabilized and processed into CO or NCO products.

Which enzymes/processes direct CO formation in meiosis?

ZMM (Zip/Mer/Mei) proteins (e.g., Zip1–4, Msh4–Msh5) promote stabilization and maturation of meiosis-specific dHJs into Class I COs through interference-regulated pathways.

What are Class I and Class II crossovers?

• Class I: ZMM-dependent, interference-sensitive COs that are evenly spaced.

• Class II: MUS81-dependent, interference-insensitive COs that act as backup events.

How is crossover number and distribution regulated?

CO interference (spacing mechanism), homeostasis (maintains CO number despite DSB variability), and axis/chromatin structure influence where and how many COs form.

What is the role of MSH4–MSH5 in meiosis?

These MutSγ complex proteins stabilize recombination intermediates (single-end invasions/dHJs) promoting maturation into Class I COs.

How are meiotic COs resolved into chiasmata?

Resolution of dHJs by resolvases (e.g., Yen1/GEN1, MUS81 complexes) and/or regulated dissolution/decatenation yields physical COs manifested as chiasmata between homologs.

Why is inter-homolog CO formation favored despite risk of LOH?

LOH is largely irrelevant in gametogenesis (meiosis produces haploid gametes); inter-homolog COs are essential for correct homolog disjunction and genetic diversity.

How is Spo11 activity regulated temporally and spatially?

Controlled by meiosis-specific protein complexes, chromosome axis organization, and checkpoint kinases to ensure DSBs form at appropriate times/locations and number.

How is DSB repair timing coordinated with meiotic progression?

DSB formation occurs in early prophase I; resection, strand invasion, and crossover designation proceed during prophase, with resolution timed before metaphase I to permit segregation.

How do meiotic pathways contribute specifically to crossover formation?

ZMM pathway channels some DSBs toward stabilized dHJs that become interference-sensitive COs; alternative pathways (MUS81) provide additional COs when needed.

Compare HR in meiosis vs mitosis (concise):

• Template choice: meiosis uses homologs (inter-homolog bias); mitosis uses sister chromatids.

• Outcome bias: meiosis promotes crossovers for segregation and diversity; mitosis favors non-crossovers to preserve genome sequence.

• Proteins: meiosis has DMC1, ZMM, and meiosis-specific regulators; mitosis relies more on RAD51, Srs2/BLM, and dissolution mechanisms to avoid COs.

• Regulation: meiosis tightly programs DSB formation, CO number/distribution; mitosis suppresses COs and prioritizes genome stability.