17.3 Meiosis

2026 Syllabus Objectives

  1. State that meiosis is involved in the production of gametes
  2. Describe meiosis as a reduction division in which the chromosome number is halved from diploid to haploid resulting in genetically different cells (details of the stages of meiosis are not required)

What is Meiosis? 🔬

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Meiosis is a special type of cell division that occurs in the reproductive organs (ovaries and testes) to produce gametes (sex cells). Unlike mitosis, which produces genetically identical cells, meiosis produces cells that are genetically different from the parent cell and from each other.

Key Purpose of Meiosis

Meiosis is essential for sexual reproduction because:

  • It produces haploid gametes (cells with half the number of chromosomes)
  • It ensures that when two gametes fuse during fertilization, the resulting zygote has the correct diploid number of chromosomes
  • It generates genetic variation in offspring

Meiosis as a Reduction Division ⚡

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What Does "Reduction Division" Mean?

Meiosis is called a reduction division because:

The number of chromosomes is halved (or reduced) during the process.

Before meiosis: The parent cell is diploid – it contains two complete sets of chromosomes (one set from each parent).

After meiosis: The daughter cells are haploid – they contain only one set of chromosomes.

The Chromosome Number Changes

In humans, for example:

  • A diploid cell contains 46 chromosomes (23 pairs)
  • After meiosis, each haploid gamete contains 23 chromosomes (one from each pair)

This can be represented as:

Diploid parent cell (2n)→meiosisHaploid gametes (n)\text{Diploid parent cell (2n)} \xrightarrow{\text{meiosis}} \text{Haploid gametes (n)}

For humans: 2n=46→n=232n = 46 \rightarrow n = 23

How Meiosis Works 📌

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Although details of the stages are not required, understanding the basic process helps explain why meiosis is a reduction division.

The Two-Stage Division Process

Meiosis involves two consecutive divisions:

First Division (Meiosis I):

  • Homologous chromosomes (chromosome pairs – one from mother, one from father) pair up together
  • These paired chromosomes then separate from each other
  • Each intermediate cell receives one chromosome from each homologous pair
  • This is the reduction division – the chromosome number is halved

Second Division (Meiosis II):

  • The chromatids of each chromosome separate
  • Each chromatid becomes an individual chromosome in a daughter cell
  • This division is similar to mitosis

The Overall Result

One diploid parent cell→meiosisFour haploid daughter cells\text{One diploid parent cell} \xrightarrow{\text{meiosis}} \text{Four haploid daughter cells}

Each of the four daughter cells:

  • Contains half the number of chromosomes of the parent cell
  • Is genetically different from the parent cell and from each other

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