BiologyCBSE Class 11 · Cell Cycle and Cell Division

Meiosis: Prophase I Sub-stages, Meiosis I and II and Chromosome Counts

Updated 17 October 2026 · Fact-checked

Meiosis is a reductional division in which a diploid cell gives four haploid cells through two divisions, meiosis I and II, with a single round of DNA replication. Prophase I has five sub-stages (leptotene, zygotene, pachytene, diplotene, diakinesis) with synapsis and crossing over. Meiosis gives variation and keeps the species chromosome number constant.

Understand Meiosis: Phases and Chromosome Numbers

Meiosis. Occurs in meiocytes (germ cells) to produce haploid gametes or spores. Two successive divisions, meiosis I (reductional) and meiosis II (equational), after one S phase. Result: four haploid cells.

Meiosis I.

Prophase I is long; five sub-stages:
Leptotene: chromosomes condense and become visible as long thin threads.
Zygotene: homologous chromosomes pair up in synapsis, aided by the synaptonemal complex; the paired homologues are bivalents (tetrads).
Pachytene: bivalents are clearly visible as tetrads; recombination nodules appear and crossing over takes place between non-sister chromatids of homologous chromosomes, catalysed by the enzyme recombinase; it gives genetic recombination.
Diplotene: the synaptonemal complex dissolves; homologues separate except at the chiasmata, the sites of crossing over. Can last months or years in some vertebrate oocytes.
Diakinesis: terminalisation of chiasmata is complete; chromosomes are fully condensed; spindle assembles; nucleolus disappears and the nuclear envelope breaks down.
Metaphase I: bivalents align on the equatorial plate; spindle fibres from opposite poles attach to the kinetochores of homologous chromosomes.
Anaphase I: homologous chromosomes separate; sister chromatids remain together at their centromeres.
Telophase I: nuclear membrane and nucleolus reappear; cytokinesis follows to give two haploid cells (dyad of cells).
Interkinesis: short gap between the two divisions; no DNA replication.

Meiosis II resembles mitosis: prophase II, metaphase II (chromosomes at the equator), anaphase II (centromeres split and sister chromatids separate), telophase II (nuclear envelope reforms, cytokinesis), giving four haploid cells (tetrad of cells).

Significance. Keeps the chromosome number constant over generations (gametes are haploid, fertilisation restores diploid); variation through crossing over and independent assortment of homologues; raw material for evolution.

Counting in a human meiocyte (2n = 46). G₁: 46 chromosomes, 2C. After S: 46 chromosomes (92 chromatids), 4C. End of meiosis I: each of the two cells has 23 chromosomes (each of two chromatids), 2C. End of meiosis II: four cells with 23 chromosomes each, 1C. So 1 meiocyte gives 4 gametes; 100 meiocytes give 400 haploid cells.

Draw and label: a bivalent at pachytene showing chiasmata and two homologues with sister chromatids.

Meiosis: Phases and Chromosome Numbers: key facts

Prophase I: $L e pt o t e n e ∣ z y g o t e n e ∣ p a c h y t e n e ∣ d i pl o t e n e ∣ d iak in es i s$; Five sub-stages
Counting: Meiocyte 2n -> 2 cells n (meiosis I) -> 4 cells n (meiosis II); Chromosome number halves in meiosis I
DNA (2n = 46): G1 2C
after S 4C
after meiosis I 2C
after meiosis II 1C; Per cell
Output: 1 meiocyte -> 4 haploid cells; N meiocytes -> 4N; Counting

How to answer Meiosis: Phases and Chromosome Numbers questions

Questions give a stage description, a diploid number or ask for outputs.

1Name the sub-stage from the key event: thin threads, pairing, crossing over, chiasmata, terminalisation.
2For counts, halve the chromosome number at meiosis I and keep it at meiosis II.
3For DNA, track 2C, 4C, 2C, 1C.
4Remember sister chromatids separate only in meiosis II.
5Note that no S phase occurs between the divisions.

Meiosis: Phases and Chromosome Numbers in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Siblings differ

Suppose two brothers from the same parents look different despite having the same parents.

Explain using meiosis.

Try it first, then show the solutionSolution

Crossing over exchanges segments between homologous chromosomes.
Homologues assort independently into gametes.
So each gamete carries a different combination of genes.

Answer: Meiotic recombination and assortment give variation.

Memory hook: Shuffle then deal.

Tricks to remember Meiosis: Phases and Chromosome Numbers

Short memory aids. Say each one aloud once, then use it on a question the same day.

Lazy Zebras Play Dull Dances
Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis.
Use it when: Prophase I
Pachy = crossing
Pachytene: crossing over.
Use it when: Recombination
I homologues, II chromatids
Anaphase I separates homologues; Anaphase II chromatids.
Use it when: Anaphase

Meiosis: Phases and Chromosome Numbers: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Zygotene and pachytene · 2 marks · Very short answerBoard level

Distinguish zygotene from pachytene in prophase I.

Try it first, then show the solutionSolution

Zygotene: homologues pair (synapsis) with the synaptonemal complex, forming bivalents (1 mark).
Pachytene: crossing over occurs at recombination nodules, catalysed by recombinase (1 mark).

Answer: Pairing versus crossing over.

Example 2: Anaphase I vs II · 3 marks · Short answerBoard level

State what separates in anaphase I and anaphase II and the chromosome number at each pole in a cell with 2n = 8.

Try it first, then show the solutionSolution

Anaphase I: homologous chromosomes separate (1 mark).
Anaphase II: sister chromatids separate (1 mark).
Each pole at the end of anaphase I has 4 chromosomes (1 mark).

Answer: Homologues; chromatids; 4.

Example 3: Human numbers · 4 marks · Short answerHarder board level

A human primary spermatocyte (2n = 46) undergoes meiosis. Give chromosome number and DNA (C) at G1, after S, at the end of meiosis I (per cell) and at the end of meiosis II (per cell).

Try it first, then show the solutionSolution

G1: 46, 2C (1 mark).
After S: 46 (92 chromatids), 4C (1 mark).
End of meiosis I: 23 chromosomes, 2C (1 mark).
End of meiosis II: 23 chromosomes, 1C (1 mark).

Answer: 46,2C; 46,4C; 23,2C; 23,1C.

Example 4: Output · 3 marks · Short answerHarder board level

How many haploid cells are produced from 100 meiocytes? How many meiotic divisions occur in total?

Try it first, then show the solutionSolution

Each meiocyte gives 4 cells, so 100 x 4 = 400 (1 mark).
Each meiocyte divides twice (meiosis I and II) (1 mark).
First divisions: 100; second divisions: 200; total 300 divisions (1 mark).

Answer: 400 cells; 300 divisions in all.

Example 5: Variation · 5 marks · Long answerHOTS / competency

Explain how crossing over and independent assortment each contribute to variation, and why interkinesis lacks DNA replication.

Try it first, then show the solutionSolution

Crossing over exchanges segments between non-sister chromatids, forming new gene combinations (1 mark).
At metaphase I homologous pairs align randomly, so maternal and paternal chromosomes assort independently (1 mark).
Both occur in meiosis I and increase the variety of gametes (1 mark).
Interkinesis lacks S phase because DNA was already replicated and the second division only separates chromatids (1 mark).
If DNA replicated again, the chromosome number would not be halved and gametes would be diploid or polyploid (1 mark).

Answer: Recombination plus assortment; no second S phase.

Where students lose marks in Meiosis: Phases and Chromosome Numbers

The slips examiners see most often, and what to write instead.

Mistake: Chromosome number halves in meiosis II
Divisions mixed.
Wrong working
Number halves in anaphase II.
Right working
It halves in meiosis I (homologues separate).
Do this: I reductional.
Usually costs: 1 mark.
Mistake: S phase in interkinesis
Replication assumed.
Wrong working
DNA replicates in interkinesis.
Right working
No DNA replication.
Do this: No S.
Usually costs: 1 mark.
Mistake: Crossing over between sisters
Wrong chromatids.
Wrong working
Occurs between sister chromatids.
Right working
Between non-sister chromatids of homologues.
Do this: Non-sister.
Usually costs: 1 mark.

Exam tips

Use C values and chromosome numbers separately.
Write the five sub-stages with one event each.
Mention synaptonemal complex and recombinase.
Compare with mitosis in a table.

Practice questions on Meiosis: Phases and Chromosome Numbers

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Chiasmata are visible at
(a) leptotene (b) zygotene (c) diplotene (d) telophase I
Hint
Homologues separate except at chiasmata.
Answer
(c).
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Meiosis I is called reductional division.
Reason (R): The chromosome number is halved in meiosis I.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Homologues separate.
Answer
(a).
Q3 · 2 marks · Very short answerHarder board level
A cell has 2n = 20. How many bivalents form at metaphase I, and how many chromatids does each bivalent have?
Hint
Pairs.
Answer
10 bivalents; each bivalent has 4 chromatids.
Q4 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a cell has 2n = 12. After meiosis I, two cells form.
(a) Chromosomes per cell after meiosis I. (1)
(b) Chromatids per chromosome then. (1)
(c) Chromosomes per cell after meiosis II. (1)
(d) Why is DNA content per cell 1C at the end? (1)
Hint
Count carefully.
Answer
(a) 6. (b) 2. (c) 6. (d) Chromatids separated in meiosis II, and no replication occurred, so each cell has half the 2C of meiosis I.
Q5 · 3 marks · Short answerHOTS / competency
Suppose a species has 2n = 6. Find the number of different combinations of maternal and paternal chromosomes possible in gametes by independent assortment (ignore crossing over), and give the chromosome number per gamete.
Hint
2 to the power n.
Answer
Each of the 3 homologous pairs can orient in 2 ways, so 2 x 2 x 2 = 8 combinations. Each gamete has 3 chromosomes.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Where does crossing over occur?Show answer
Pachytene.
2.What are chiasmata?Show answer
Sites of crossing over seen in diplotene.
3.Does DNA replicate in interkinesis?Show answer
No.
4.What separates in anaphase I?Show answer
Homologous chromosomes.
5.Result of meiosis?Show answer
Four haploid cells.
6.What is synapsis?Show answer
Pairing of homologous chromosomes.

Meiosis: Phases and Chromosome Numbers: frequently asked questions

What is meiosis?

Reductional division giving haploid cells.

Why is meiosis important?

Keeps chromosome number constant and gives variation.

What is a bivalent?

A pair of synapsed homologues.

What is terminalisation?

Movement of chiasmata toward the ends.

How is meiosis II like mitosis?

Sister chromatids separate.