Lesson 6 of 9 / Mutation, cell division and cancer
Meiosis I separates homologues
How does a diploid cell halve its chromosome-set number?
In this lesson: Describe meiosis I, crossing-over and independent orientation.
About 8 min
The key ideaHomologues 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.
Is this true? "Crossing-over exchanges segments between identical sister chromatids only."
The variation-producing exchange described here is between non-sister chromatids of homologous chromosomes.