Cell Cycle and Cell Division · Lesson 7 of 9
Meiosis II: Sister-Chromatid Separation
“Explain how two haploid products complete a second equational division and become four haploid nuclear products.”
• Describe prophase II, metaphase II, anaphase II and telophase II. • Explain why meiosis II is equational despite occurring in haploid cells. • Track chromosome and DNA-content changes through the second division. • Relate kinetochore attachment and chromatid separation to chromosome inheritance. • Distinguish four haploid nuclear products from the final number of functional gametes.
Why a second division is necessary
By the end of meiosis I the homologous chromosome pairs have been separated. Each daughter nucleus has only one member of each original pair, but those chromosomes are still duplicated. Meiosis II completes the distribution task by separating the two sister chromatids of each retained chromosome. The second division is therefore similar in mechanism to mitosis, although the starting cells are already haploid. The result is ordinarily four haploid nuclear products from the original diploid cell, provided the divisions and cytokinesis are completed normally.
There is no additional DNA replication between meiosis I and meiosis II. The starting DNA content of each post-meiosis-I daughter is 2C, arranged as n chromosomes with two sisters each. At the end of meiosis II, each of the resulting nuclei has n unreplicated chromosomes with DNA content C. Because sister chromatids separate without further changing the chromosome-set number, meiosis II is called an equational division.
Prophase II: condense and prepare separate spindles
After the short interval of interkinesis, chromosomes in the two haploid cells compact again if they have partly dispersed. New spindle arrangements develop for each nucleus or cell, and any nuclear envelope present breaks down by the end of the stage. Unlike the long prophase I, prophase II does not include synapsis of homologues, tetrad formation or another episode of the characteristic prophase-I crossing over. The cell's task is no longer to identify and separate homologous pairs; those pairs were already divided between the daughter nuclei.
Metaphase II: sister chromatids attach to opposite poles
The replicated chromosomes align individually near each spindle equator. Each has two sister kinetochores associated with opposite spindle poles. The essential alignment resembles that of mitosis. At this stage each haploid cell still has n chromosomes, although each is duplicated. For example, if n = 3 then each metaphase-II cell has three duplicated chromosomes and six chromatids; calling these six separate chromosomes before sister separation would overcount.
Anaphase II: sister centromeres separate
At the onset of anaphase II, the association between sister chromatids is released. The chromatids are then regarded as daughter chromosomes and move towards opposite spindle poles along kinetochore-connected microtubules. The chromosome count in the undivided haploid cell temporarily doubles from n to 2n daughter chromosomes, with n moving to each pole. However, this temporary numerical description does not mean the cell has regained two homologous parental sets: it now contains separated sisters destined for different daughters.
A useful comparison is that meiosis I takes a bivalent and distributes its two duplicated homologues, whereas meiosis II takes one duplicated homologue and distributes the two sister-derived chromosomes. Both use spindles, but the relationships they resolve are different. In anaphase II, the centromeric region leads movement, with chromosome arms following. The same conceptual explanation used for mitotic anaphase can therefore be applied, provided we do not accidentally add new homologous pairing.
Telophase II and the final cytoplasmic divisions
Chromosome groups reach the poles, nuclear envelopes re-form and chromosomes decondense. Cytokinesis generally completes the separation, resulting in four haploid cells or nuclear products from one diploid starting cell. The textbook refers to a tetrad of cells. 'Tetrad' here means four meiotic products, whereas 'tetrad' in prophase I describes four chromatids in one paired homologous unit. The context determines which meaning is intended.
Counts through the second meiotic division
| Moment | Chromosomes | DNA amount | Status of sisters |
|---|---|---|---|
| Each cell after meiosis I | n | 2C | Two sisters per chromosome |
| Metaphase II, per cell | n | 2C | Joined sisters attached to opposite poles |
| Anaphase II, in one undivided cell | 2n transiently; n towards each pole | 2C total | Sisters now independent daughter chromosomes |
| Telophase II, per future daughter nucleus | n | C | One chromatid per chromosome |
| After cytokinesis, each final product | n | C | Unreplicated chromosomes |
Imagine a diploid mother cell with 2n = 8. After meiosis I, two cells each contain four duplicated chromosomes and 2C DNA. At metaphase II each still has four chromosomes, now with eight chromatids. After anaphase II, each of these two cells contains eight daughter chromosomes temporarily, four moving to each pole. Once cytokinesis is complete, the four resulting cells each have four unreplicated chromosomes and C DNA. Writing 'four cells of 2n' would contradict the purpose of meiosis.
Do the four nuclear products always become four equally sized gametes?
No. The typical textbook diagram shows four similar-sized haploid cells, which is common in forms of sporogenesis and spermatogenesis. However, the cellular outcome is different during many kinds of animal oogenesis: unequal cytokinesis directs most cytoplasm into one large egg and produces much smaller polar bodies, so four haploid nuclear products do not imply four functional eggs. In many flowering plants, meiosis produces haploid spores rather than directly producing mature gametes. The important invariant is reduction of chromosome sets, not the number of fully functional gametes made by every organism.
| Developmental setting | Typical division products | Important qualification |
|---|---|---|
| Male animal spermatogenesis | Four small haploid spermatids from one primary spermatocyte | Further differentiation is needed to form sperm |
| Animal oogenesis in many vertebrates | One large ovum lineage and small polar bodies | Unequal cytokinesis; not four equally sized eggs |
| Flowering plant microsporogenesis | Four haploid microspores | They develop into pollen grains; meiosis does not directly produce mature sperm cells |
Problem
A diploid cell has 2n = 12. How many chromosomes and DNA copies does each final normal meiotic product contain?
- 1.Six homologous pairs were present in the diploid cell, so a haploid final product contains n = 6 chromosomes.
- 2.After meiosis II each chromosome is unreplicated and contains one DNA double-helix molecule.
- 3.Each resulting cell therefore contains six chromosomes and C DNA, half the 2C G1 amount of the diploid original.
A normal meiotic sequence creates four haploid nuclear products, but their sizes and developmental fates can differ. In oogenesis, only one typically becomes the large functional egg.
Quick Quiz
Quiz
What separates in anaphase II?
What is the DNA amount in a haploid cell just before metaphase II if G1 of the original diploid mother was 2C?
What is the DNA amount in each final product after normal meiosis II?
Which event does not normally occur in prophase II?
A haploid metaphase-II cell with n = 4 has:
How many haploid nuclear products ordinarily result from one complete meiosis?
Which is accurate about animal oogenesis?
Why is meiosis II termed equational?
Practice Problems
- Explain every stage of meiosis II, highlighting the event distinguishing it from meiosis I.
- A cell is 2n = 16 at G1. Calculate counts in each cell at metaphase II and in each final product.
- Why is the stage between meiotic divisions not another S phase?
- Compare four equal-sized haploid products with asymmetric meiotic products using biological examples.
- What is the difference between 'tetrad of chromatids' and 'tetrad of meiotic products'?
Prophase II: condensed chromosomes and spindles, without new synapsis. Metaphase II: individual replicated chromosomes line up with sister kinetochores facing opposite poles. Anaphase II: sisters separate. Telophase II: daughter nuclei and cells form. Homologues were separated in meiosis I.
Key Takeaways
• Meiosis II follows meiosis I without additional DNA replication. • Sister chromatids separate in anaphase II, unlike homologues in anaphase I. • Metaphase-II cells have n replicated chromosomes and 2C DNA. • Final products normally have n unreplicated chromosomes and C DNA. • Four haploid products do not necessarily become four functional gametes.