🎓 Prepared by students from Boğaziçi University

What is Meiosis and Sexual Reproduction?

Meiosis is the process of cell division that produces gametes (sperm and eggs) for sexual reproduction. Unlike mitosis, which creates identical copies, meiosis shuffles genetic material through two divisions and crossing over, ensuring every offspring is genetically unique.

Short answer

Meiosis is reduction division: a diploid cell (2n) divides twice to form four haploid gametes (n). Homologous chromosomes pair and exchange genetic material (crossing over), then segregate, producing genetic variation essential for evolution and species survival.

Meiosis: Two Divisions

1
Interphase
DNA replicates; cell is diploid (2n).
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2
Meiosis I
Homologous chromosomes pair and exchange segments (crossing over). Homologs separate → 2 haploid cells.
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3
Meiosis II
Sister chromatids separate → 4 genetically unique haploid gametes (n).
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4
Gametes
Sperm or egg cells ready for fertilisation.

Step-by-step worked examples

A human cell with 46 chromosomes undergoes meiosis. How many chromosomes in each gamete?

Diploid human cell = 2n = 46
After Meiosis I: n = 23 (one set)
After Meiosis II: gamete = 23 chromosomes

Crossing over occurs during which phase of meiosis?

Prophase I of Meiosis I
Homologous chromosomes pair (synapsis)
Non-sister chromatids exchange alleles
This increases genetic variation

From one diploid cell, how many genetically distinct haploid gametes are produced?

Meiosis produces 4 haploid cells
Crossing over + independent assortment
= each is genetically unique

Flashcards

Quick quiz

Q1.In humans, meiosis reduces chromosome number from 46 to:

Correct answer: A. From diploid (2n=46) to haploid (n=23). Each gamete has one set.

Q2.Crossing over takes place during:

Correct answer: B. Prophase I: homologs synapse and non-sister chromatids exchange segments.

Q3.Sister chromatids separate during:

Correct answer: C. Meiosis II, specifically Anaphase II, when sister chromatids move to opposite poles.

Q4.How many gametes result from one meiotic event?

Correct answer: D. Two consecutive divisions produce four haploid cells.

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Common mistakes

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Meiosis and mitosis produce the same result. — Correct: Mitosis = identical diploid copies; meiosis = unique haploid gametes.

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Crossing over makes sister chromatids identical. — Correct: Crossing over exchanges alleles between homologous chromosomes, increasing variation.

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Meiosis occurs in all cells. — Correct: Meiosis occurs only in germ cells (gonads) to produce gametes.

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Gametes are diploid. — Correct: Gametes are haploid (n)—fertilisation restores the diploid state.

FAQ

What is the main purpose of meiosis?

To produce haploid gametes (sex cells) for sexual reproduction and create genetic variation.

How is meiosis different from mitosis?

Mitosis produces two identical diploid cells; meiosis produces four unique haploid gametes.

What is the significance of crossing over in meiosis?

It exchanges alleles between homologous chromosomes, increasing genetic diversity in offspring.

When do homologous chromosomes separate?

During Anaphase I of Meiosis I.