Lesson 7 of 9 / Mutation, cell division and cancer
Meiosis II and random fertilisation
Why are gametes genetically varied rather than smaller clones?
I can describe meiosis II and link meiotic processes to variation and fertilisation.
I can describe the meiotic cycle and main stages.Syllabus
Syllabus 9477, Core 2(s). Describe the meiotic cycle and main stages.
I can explain meiosis and random fertilisation as sources of variation.Syllabus
Syllabus 9477, Core 2(t). Explain meiosis and random fertilisation as sources of variation.
About 7 min
Make a guess. You are not marked.
Is DNA replicated again between meiosis I and meiosis II?
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There is one S phase.
There is only one S phase. Meiosis II separates the sister chromatids already present.
The key idea
Meiosis 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.
Watch out for this
A student says: "DNA must replicate again before meiosis II." What is wrong with this?
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A second S phase would disrupt the normal reduction process. Meiosis II uses the chromatids made before meiosis I.
Check your understanding
At the end of normal meiosis from a diploid cell, the products are...
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haploid with genetic combinations that can differ.
Homologue separation reduces ploidy and meiotic processes generate variation.
Quick recall
Why are gametes genetically varied rather than smaller clones?