Cell Cycle and Cell Division · Lesson 6 of 9
Meiosis I: Homologue Separation and Interkinesis
“Trace bivalent alignment, reductional segregation, telophase I and the brief DNA-free interval before meiosis II.”
• Explain how bivalents orient at metaphase I. • Distinguish homologous-chromosome separation in anaphase I from chromatid separation. • Describe telophase I and interkinesis accurately. • Calculate chromosomes, chromatids and DNA content after meiosis I. • Relate independent orientation of homologues to genetic variation.
A bivalent must connect to opposite spindle poles
After the five substages of prophase I, homologous chromosomes remain associated in bivalents. By metaphase I each bivalent moves towards the equatorial region of a spindle. The crucial difference from mitotic metaphase is the unit being oriented: here the maternal and paternal homologues of each pair are attached in a way that directs them towards opposite poles. Sister chromatids of each homologue remain joined. This arrangement prepares for a reduction in chromosome-set number rather than an immediate separation of sisters.
Metaphase I: pairs, not independent X-like chromosomes
At metaphase I, the bivalents line up along the spindle's equatorial plane. Fibres from opposite poles connect to chromosome-associated kinetochore structures. In a simplified drawing, each paired unit can look like two X-shaped replicated homologues side by side. Their orientations can vary independently of other homologous pairs: the maternal member of one pair may face the left pole while the maternal member of another faces the right. There is no necessary rule that every maternal chromosome travels together in one direction.
The stage of meiosis I when bivalents align around the spindle equator so homologous chromosomes of each pair face opposite poles.
When there are two homologous pairs, the relative orientations of the two bivalents can yield multiple combinations of maternal and paternal chromosomes in different daughter nuclei. This independent assortment, together with crossing over, contributes to genetic variation. The orientations are not controlled by which parent a chromosome came from; each pair attaches and segregates as a functional unit.
Anaphase I: homologues separate while sisters remain paired
At anaphase I, each bivalent is resolved as its two homologues move toward opposite poles. Sister chromatids remain associated at their centromeric regions. Because one complete homologue from each pair travels to each pole, each future daughter nucleus receives one chromosome from each homologous pair. This halves the number of chromosome sets. It does not mean the DNA amount instantly becomes one-fourth of its original G1 content: the chromosomes are still replicated and contain two DNA molecules each.
A division in which homologous chromosome sets are separated so that the number of chromosome sets per resulting nucleus is halved.
Consider a diploid cell with 2n = 6 chromosomes. Before meiosis I, there are six replicated chromosomes and twelve chromatids in total. At anaphase I the homologous chromosomes move apart, but their sister pairs remain intact. At the end of the first division each daughter receives three chromosomes, each still duplicated and therefore containing six chromatids. A correctly labelled meiotic anaphase I diagram should show whole X-like replicated chromosomes moving to opposite poles, not isolated sisters.
Telophase I: two haploid nuclei or cells
At the poles, chromosome sets can become enclosed by re-forming nuclear membranes, and a nucleolus may reappear. Cytokinesis may follow and produce a dyad of two cells. Some organisms do not re-form completely dispersed interphase-like nuclei before the next division, so the microscopic appearance varies. The key consequence is ploidy reduction: each daughter nucleus is haploid (n), even though every chromosome still consists of sister chromatids. By the end of cytokinesis, each product ordinarily has n chromosomes and 2C DNA relative to an unreplicated haploid C.
Interkinesis does not include a second S phase
A usually short interval between meiosis I and meiosis II, not accompanied by another round of DNA replication.
After telophase I there may be a brief interval called interkinesis. It can involve cellular reorganisation, but chromosomal DNA is not normally replicated again. This distinction is essential: if DNA were copied again before meiosis II, the second division would no longer perform the expected sister-chromatid separation from the products of meiosis I. The cell therefore enters meiosis II with n chromosomes, each still replicated and containing two sister chromatids.
Tracking n and C through the first division
| Stage | Chromosome complement per cell or nucleus | DNA amount per cell or nucleus | Chromatid relationship |
|---|---|---|---|
| Before S, diploid mother cell | 2n | 2C | One chromatid per chromosome |
| After S, before meiosis I | 2n | 4C | Two sister chromatids per chromosome |
| Metaphase I | 2n | 4C | Bivalents, homologues paired |
| Anaphase I before cytokinesis | Total cell still 2n; n replicated chromosomes move to each pole | 4C for undivided cell; 2C allocated to each future nucleus | Sisters remain together |
| After telophase I and cytokinesis | n per daughter cell | 2C per daughter cell | Two sister chromatids per chromosome |
| Interkinesis | n | 2C | No new replication |
The table separates counts for the whole, undivided cell from counts per daughter nucleus. At anaphase I, saying 'the cell has only n chromosomes' is premature unless specifying one pole or future nucleus. Before cytokinesis, the parent cytoplasm still contains both haploid groups. When cells are partitioned, each group becomes one haploid daughter. The DNA quantity assigned to each product is 2C because each haploid chromosome remains duplicated.
Mitosis and meiosis I differ in the identity of what separates
In mitotic anaphase, the sister chromatids of each duplicated chromosome part, with each now counted as a daughter chromosome. In anaphase I, corresponding homologues part while the sisters of each homologue remain together. This difference alone accounts for the reduction in ploidy. The order of the chromosome mechanics is not arbitrary: crossing over and chiasmata in prophase I help set up homologous pairing, while sister-chromatid separation is reserved for meiosis II.
Problem
A cell begins as 2n = 8 in G1. What are chromosome number, chromatid number and DNA quantity in each daughter after the first meiotic division?
- 1.G1 has eight chromosomes with 2C DNA; S makes eight replicated chromosomes and 4C DNA.
- 2.Meiosis I separates four homologous pairs, giving four chromosomes to each daughter.
- 3.Each daughter has n = 4 chromosomes, eight chromatids and 2C DNA; meiosis II has not happened yet.
Do not draw sister chromatids separating in anaphase I. Do not insert a second S phase between meiosis I and II. The first division halves ploidy while leaving chromosomes replicated.
Quick Quiz
Quiz
What aligns at the equator in metaphase I?
Which structures separate in anaphase I?
A 2n = 8 cell has completed meiosis I. Each product has:
What happens to DNA replication during interkinesis?
What is the DNA amount per daughter after meiosis I when G1 was 2C?
Why does orientation of bivalents matter?
How does anaphase I differ from mitotic anaphase?
A 2n = 6 cell at the end of meiosis I has how many chromatids per daughter?
Practice Problems
- Draw metaphase I and anaphase I for a diploid cell with 2n = 4 and label homologues.
- Explain why telophase I daughter cells are haploid despite possessing duplicated chromosomes.
- Why is interkinesis not equivalent to a conventional G1–S–G2 interphase?
- For a 2n = 10 cell, calculate bivalents at metaphase I, chromosomes at each pole in anaphase I, and chromatids in each post-meiosis-I daughter.
- Compare independent assortment and crossing over as sources of variation.
Draw two bivalents at the equator in metaphase I, each containing two replicated homologues. During anaphase I show one complete duplicated homologue from each pair travelling to each pole. The resulting two poles have n = 2 replicated chromosomes each.
Key Takeaways
• Bivalents, not individual sister chromatids, align during metaphase I. • Homologues separate in anaphase I while sisters stay joined. • Telophase I yields haploid products whose chromosomes are still duplicated. • Interkinesis is not a second S phase. • Independent assortment combines parental homologues in varied arrangements.