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#meiosis

135 public questions tagged with this topic.

Cell Division

What happens during anaphase II of meiosis?

Meiosis consists of two divisions following single S phase, with meiosis II equationally separating sisters after reductional meiosis I segregated homologs. Cells exit meiosis I entering interkinesis without replication, centrosomes duplicate, chromosomes recondense, and spindle reforms often orthogonal to first division axis. At metaphase II plate, sister chromatids align with kinetochores bioriented to opposite poles similar to mitosis, achieved despite haploid chromosome number. Protection of centromeric cohesion established in meiosis I by Shugoshin SGO2 bound to PP2A phosphatase that dephosphorylates Rec8 cohesin preventing separase cleavage is now removed as SGO2 degrades from centromeres. Upon APC/C-Cdc20 activation at anaphase II, securin degraded, separase active cleaves remaining centromeric Rec8, dissolving cohesion. Sister chromatids, now genetically non-identical due to crossing over in prophase I exchanging non-sister segments, separate toward opposite poles driven by kinetochore microtubule depolymerization and spindle elongation, generating four haploid daughter cells essential for sexual reproduction and complementation after fertilization. This circuitry is highly conserved across eukaryotes, integrating growth factor signals, DNA damage surveillance, and developmental cues, and its disruption frequently underlies oncogenesis, providing targets for checkpoint inhibitors and cancer therapeutics.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 21: Meiosis II Sister Chromatids.

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Cell Division

Which of the following is NOT a characteristic of meiosis?

Meiosis encompasses specialized sequence absent from mitosis distinguishing germline division that produces haploid gametes maintaining constant chromosome number across sexual generations. Hallmark events include synapsis where homologous maternal and paternal chromosomes align along entire length via synaptonemal complex comprised of SYCP2, SYCP3 lateral elements and SYCP1 transverse filaments, providing intimate pairing necessary for homology search. Parallel programmed double-strand breaks catalyzed by Spo11 recruit DMC1 recombinase promoting crossing over between non-sister chromatids, generating recombinant chromosomes with novel allele combinations contributing to evolution and adaptation. Two successive divisions without intervening S phase reduce chromosome complement from diploid 2n to haploid n: meiosis I segregates homologs reductionally, meiosis II separates sisters equationally. Independent assortment adds further shuffling. Producing diploid daughters would duplicate ploidy each generation leading to polyploid lethality, contradicting reduction purpose. Therefore generation of haploid diversity and halving underpin meiosis definition. This circuitry is highly conserved across eukaryotes, integrating growth factor signals, DNA damage surveillance, and developmental cues, and its disruption frequently underlies oncogenesis, providing targets for checkpoint inhibitors and cancer therapeutics.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 21: Overview of Meiosis Characteristics.

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Cell Division

What is the function of the chiasmata in meiosis?

Chiasmata become cytologically visible during diplotene and diakinesis after synaptonemal complex disassembly, appearing as cross-shaped connections between homologs under microscope. They represent late consequence of crossover recombination established chemically in pachytene between non-sister chromatids, where exchange of DNA segments creates physical interlock. Cohesion between sister chromatids distal to crossover site mediated by cohesin Rec8 maintains association, converting molecular exchange into durable tether linking homologs. This tether provides resistance to spindle pulling forces exerted on bioriented bivalents at metaphase I, generating tension that satisfies spindle assembly checkpoint and stabilizes correct attachment via Aurora B mediated error correction. As separase cleaves arm cohesin during anaphase I, chiasmata dissolve, permitting homolog separation toward opposite poles. Thus functions are twofold: genetic consequence facilitating allele shuffling underlying diversity, and mechanical function ensuring accurate segregation. Absence leads to achiasmate chromosomes that segregate randomly, increasing aneuploidy risk. This circuitry is highly conserved across eukaryotes, integrating growth factor signals, DNA damage surveillance, and developmental cues, and its disruption frequently underlies oncogenesis, providing targets for checkpoint inhibitors and cancer therapeutics.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 21: Chiasmata and Homolog Cohesion.

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Cell Division

The separation of homologous chromosomes occurs in:

Segregation patterns distinguish meiosis from mitosis through reductional division where homologous chromosomes rather than sister chromatids separate. After S phase, each chromosome consists of two sisters held by cohesin containing meiosis-specific kleisin Rec8. During prophase I homologs pair and cross over forming bivalents. At metaphase I, homologous kinetochores achieve bipolar attachment but sister kinetochores exhibit monopolar orientation mediated by monopolin complex in yeast and fused geometry plus MEIKIN-MoA1 in mammals, causing both sisters of each homolog to attach to same pole. Shugoshin SGO2 paired with PP2A protects centromeric Rec8 from separase cleavage by dephosphorylation. Upon APC/C-Cdc20 activation, securin degrades, separase cleaves arm Rec8 distal to chiasmata, dissolving link between homologs while centromeric cohesion persists. Consequently, maternal and paternal homologs composed of two chromatids segregate to opposite poles in anaphase I, halving chromosome number. Nondisjunction at meiosis I produces disomic gametes causing trisomies such as Down syndrome. This circuitry is highly conserved across eukaryotes, integrating growth factor signals, DNA damage surveillance, and developmental cues, and its disruption frequently underlies oncogenesis, providing targets for checkpoint inhibitors and cancer therapeutics.

Ref: Petronczki et al., Cell 2003, Meiotic Homolog Separation. Alberts 7th ed., Chapter 21, Meiosis I.

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Cell Division

Which substage of prophase I is characterized by crossing over?

Pachytene represents longest stable meiotic prophase stage where fully synapsed bivalents thicken and crossing over implemented. Molecular hallmark is homology-directed repair of programmed double-strand breaks introduced by Spo11 topoisomerase-like transesterase in leptotene that generates 3 prime single-strand overhangs via MRE11-mediated resection. Overhangs invade homologous duplex mediated by meiosis-specific recombinases DMC1 and RAD51, forming D-loop intermediates. Decision between non-crossover and crossover fate controlled by ZMM proteins MSH4-MSH5 heterodimer stabilizing joint molecules and MLH1-MLH3 heterodimer functioning as resolvase cleaving Holliday junctions as crossover. At least one obligate crossover per homolog pair ensures physical connection via chiasma required for correct biorientation at metaphase I. Pachytene therefore concentrates recombination machinery within synaptonemal complex central region. Defects preventing crossover formation cause achiasmate chromosomes, trigger aneuploidy, and underlie human trisomy risk increasing with maternal age. This circuitry is highly conserved across eukaryotes, integrating growth factor signals, DNA damage surveillance, and developmental cues, and its disruption frequently underlies oncogenesis, providing targets for checkpoint inhibitors and cancer therapeutics.

Ref: Hunter, Cold Spring Harb Perspect Biol 2015, Meiotic Recombination in Pachytene. Alberts 7th ed., Chapter 21.

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Cell Division

Which phase of meiosis involves the formation of the synaptonemal complex?

Prophase of meiosis I is extended into five substages to accomplish pairing, synapsis, and recombination unique to germ cells. Leptotene initiates with chromosome condensation into thin threads and formation of axial elements along each chromatid containing cohesin Rec8 and HORMA proteins. Zygotene marks onset of homologous pairing facilitated by telomere bouquet clustering at nuclear envelope driven by cytoskeletal forces and homology search mediated by DMC1 recombinase. Synaptonemal complex assembly begins in zygotene: lateral elements SYCP2 and SYCP3 polymerize along axes, transverse filament protein SYCP1 dimerizes bridging homologs, central element SYCE1-3 completes tripartite zipper extending along entire chromosome length. This structure stabilizes homolog junction and provides platform for recombination nodules containing SPO11-induced double-strand breaks processed into crossovers. By end of zygotene, most homologs fully synapsed, failure causing pachytene checkpoint arrest and apoptosis of defective spermatocytes or oocytes leading to infertility phenotypes in humans. This circuitry is highly conserved across eukaryotes, integrating growth factor signals, DNA damage surveillance, and developmental cues, and its disruption frequently underlies oncogenesis, providing targets for checkpoint inhibitors and cancer therapeutics.

Ref: Page & Hawley, Annu Rev Cell Dev Biol 2004, Meiotic Synapsis. Alberts 7th ed., Chapter 21, Meiosis I Prophase.

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