Lesson 6 of 9 / Mutation, cell division and cancer
Meiosis I separates homologues
How does a diploid cell halve its chromosome-set number?
I can describe meiosis I, crossing-over and independent orientation.
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 8 min
Make a guess. You are not marked.
In which division of meiosis does the chromosome number halve?
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Meiosis I, when homologous chromosomes separate.
Homologues go to opposite poles in anaphase I. Each nucleus then holds one of each pair.
The key idea
Homologues pair and can exchange segments, then separate in meiosis I while sister chromatids remain together.
Track the mechanism
Which structures separate in meiosis I?
Homologues pair as a bivalent. Non-sister chromatids may exchange corresponding segments; chiasmata maintain connections. The nuclear envelope breaks down and a spindle forms.
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
DNA replicates once before meiosis begins. In prophase I, homologous chromosomes pair as bivalents. Non-sister chromatids can exchange corresponding DNA segments through crossing-over, producing new allele combinations. Visible chiasmata mark crossover-associated connections as homologues begin to separate.
The nuclear envelope breaks down and the spindle forms. At metaphase I, bivalents align at the equator. Each pair can orient independently relative to the poles, so maternal and paternal homologues can be combined differently in daughter cells.
In anaphase I, homologous chromosomes move to opposite poles while sister chromatids remain joined. This is the reduction division: each resulting nucleus has one member of each homologous pair. Telophase I and cytokinesis may occur before the second division, with details varying among organisms.
Crossing-over exchanges equivalent regions rather than simply joining random chromosomes. Independent assortment reshuffles whole homologues; crossing-over reshuffles alleles within a chromosome. Both generate combinations of existing alleles rather than necessarily creating new alleles.
Step by step
- 1
Identify homologues
They carry the same gene loci, potentially different alleles.
- 2
Track non-sister exchange
Only the chromatids participating become recombinant.
- 3
Reduce the sets
One homologue of each pair reaches each pole.
Worked example
Work through the evidence
A cell with 2n = 8 completes meiosis I normally. What is the chromosome state in each product?
One way to explain it
Each product has n = 4 chromosomes, each still consisting of two sister chromatids.
Why this answer works
- Homologues have separated.
- Sister chromatids will separate in meiosis II.
Watch out for this
A student says: "Crossing-over exchanges segments between identical sister chromatids only." What is wrong with this?
Show the answer
The variation-producing exchange described here is between non-sister chromatids of homologous chromosomes.
Check your understanding
Which event directly halves the chromosome-set number?
Show the answer
Separation of homologues in meiosis I
Each product receives one chromosome from each homologous pair.
Exam-style questions
Check your understanding
Which statements are true of meiosis I but NOT of mitosis? 1: Homologous chromosomes pair. 2: Sister chromatids separate. 3: Crossing over occurs. 4: The chromosome number is halved.
Show the answer
1, 3 and 4 only
Pairing, crossing over and halving happen in meiosis I. Sister chromatids separate in mitosis and meiosis II, not meiosis I.
Quick recall
How does a diploid cell halve its chromosome-set number?