Lesson 7 of 9 / Mutation, cell division and cancer
Meiosis II and random fertilisation
Why are gametes genetically varied rather than smaller clones?
In this lesson: Describe meiosis II and link meiotic processes to variation and fertilisation.
About 7 min
The key ideaMeiosis II separates sisters without another S phase; random gamete fusion adds another layer of genetic variation.
Track the mechanism
What changes without a second replication?
In each haploid product, a spindle forms and any re-formed nuclear envelope breaks down. Chromosomes still have two chromatids; DNA has not replicated again.
Original animal-cell schematic, not the human chromosome complement. Teal and purple distinguish homologues; crossed rods are replicated chromosomes, not two homologues. Brown marks spindle poles. Meiosis II omits exchanged tip colours to focus on segregation; it does not reverse earlier recombination.
Explanation
There is no second DNA replication between meiosis I and meiosis II. In prophase II, chromosomes condense, spindle apparatus forms and any re-formed nuclear envelopes break down. Chromosomes line up individually at metaphase II, with sister chromatids attached towards opposite spindle poles. At anaphase II, sister chromatids separate and move to opposite poles. Nuclear envelopes re-form during telophase II and cytokinesis produces haploid products.
In an animal-cell model, centrioles/centrosomes organise spindle poles and membrane constriction partitions cytoplasm; plant meiosis uses different spindle organisation and cell plates. The central shared logic is chromosome segregation, not the presence of centrioles in every species.
Crossing-over can make sister chromatids genetically different after meiosis I, and independent orientation of homologues produces different combinations. With n independently assorting pairs, 2^n chromosome combinations are possible in gametes before including crossing-over, under the simple model.
Random fertilisation combines one gamete from each parent, restoring diploidy and multiplying possible combinations. Variation supplies material for selection; meiosis does not make every combination beneficial. Nondisjunction is an error, not the normal source of the required chromosome-number reduction.
Step by step
- 1
Check DNA replication history
One S phase supports two divisions.
- 2
Track each chromatid
Recombined chromatids can carry different allele combinations.
- 3
Combine independent events
Multiply gamete possibilities for random fertilisation.
Worked example
Work through the evidence
Each parent has n = 3 independently assorting chromosome pairs. Ignoring crossing-over, how many chromosome combinations can fertilisation produce?
One way to explain it
Each parent can make 2^3 = 8 combinations; random fusion gives 8 x 8 = 64 combinations.
Why this answer works
- The 2^n calculation assumes independent assortment.
- Crossing-over would increase possible allele combinations further.
Is this true? "DNA must replicate again before meiosis II."
A second S phase would disrupt the normal reduction process. Meiosis II uses the chromatids made before meiosis I.