Comprehensive reference on gamete formation and clinical significance
| Stage | Phase | Key Events & Mechanisms | Chromosome Status | Embryology Relevance & Clinical Significance |
|---|---|---|---|---|
| PRE-MEIOSIS | Interphase (Pre-Meiotic) |
Germ cell growth
DNA replication (S phase)
Centrosome duplication
|
Diploid (2n)
46 chromosomes
92 chromatids (4C DNA)
|
Foundation for embryonic development.
Errors cause catastrophic DNA damage.
Sets stage for proper chromosome segregation.
|
| MEIOSIS I | Prophase I |
Chromosome condensation
Synapsis
CROSSING OVER (chiasmata)
Chiasmata visible
Max condensation
|
Tetrad formation (bivalents)
23 tetrads (46 chromosomes)
92 chromatids
|
MOST CRITICAL STAGE: Primary source of genetic diversity.
Diplotene arrest in females → maternal age effect.
Chiasma failure → nondisjunction.
|
| MEIOSIS I | Metaphase I |
Tetrads align at equator
Independent assortment
Spindle attachment
|
23 tetrads aligned
Maternal/paternal orientation random
|
Second major diversity source: 8.4M+ chromosome combinations.
Random alignment errors → aneuploid gametes.
|
| MEIOSIS I | Anaphase I |
Homologous chromosomes separate
Sister chromatids remain attached
Nondisjunction possible
|
Separating to haploid sets
Each pole: 23 chromosomes (46 chromatids)
|
PRIMARY SOURCE OF EMBRYONIC ANEUPLOIDY: ~90% of Trisomy 21.
Maternal nondisjunction risk ↑ with age.
|
| MEIOSIS I | Telophase I & Cytokinesis |
Unequal division
Polar body formation
|
Two haploid (n) cells:
Secondary oocyte (23 chromosomes)
First polar body
|
Female-specific: Unequal cytokinesis conserves cytoplasm.
Secondary oocyte arrests at Metaphase II until fertilization.
|
| MEIOSIS II | Interkinesis |
Brief interphase
NO DNA replication
|
Haploid (n)
23 chromosomes (46 chromatids)
|
Critical 'no replication' prevents polyploidy.
Short duration reduces error risk.
|
| MEIOSIS II | Metaphase II |
Chromosomes align singly
Spindle attachment
|
23 chromosomes aligned
Each = 2 chromatids
|
Female clinical checkpoint: Oocytes arrested here until fertilization triggers completion.
|
| MEIOSIS II | Anaphase II |
Sister chromatids separate
Nondisjunction possible
|
Separating to single chromatids
Each pole: 23 chromosomes (23 chromatids)
|
Causes ~10% of aneuploidies.
Paternal errors more common here than Meiosis I.
|
| MEIOSIS II | Telophase II & Cytokinesis |
Nuclear envelopes reform
Cytoplasm divides
|
Four haploid (n) cells:
23 chromosomes each
23 chromatids each (1C DNA)
|
Sex-specific outcomes: Males - 4 functional sperm.
Females - 1 functional ovum + 2-3 polar bodies.
|
| SUMMARY | Core Functions |
Chromosome number control (23+23=46)
Genetic diversity engine
Gamete quality control
|
Ensures proper diploid number in zygote
|
#1 cause of miscarriage: Aneuploidy (>50% first-trimester losses).
Leading congenital disorders: Down, Edwards, Patau syndromes.
|