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D2.1.9—Meiosis as reduction division

Meiosis is a reduction division in which homologous chromosomes separate first, producing haploid nuclei after two divisions in gamete formation.

Syllabus
First assessment 2025
Objective
D2.1.9
Level
HL

Exam analysis

Chance of appearing12%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper1
Typical marks1

Common command terms

  • State
  • Identify
  • Outline

Scoring notes

Common mistake
Saying sister chromatids separate in meiosis I instead of homologous chromosomes.

Recent exam appearances

November 2025Paper1A ["HL"] · TZ319[ 1 ]D2.1.9—Meiosis as reduction division
November 2024Paper1 ["HL"] · TZ033[ 1 ]D2.1.9—Meiosis as reduction division
May 2024Paper2 ["HL"] · TZ13(a)(i)[ 1 ]D2.1.9—Meiosis as reduction division
November 2023Paper1 ["HL"] · TZ232[ 1 ]D2.1.9—Meiosis as reduction division
May 2022Paper1 ["HL"] · TZ112[ 1 ]D2.1.9—Meiosis as reduction division
Practice this objective

Coverage 2012–2025 · Updated 16 Jul 2026

Meiosis Reduces Chromosome Number

Meiosis is a reduction division: one diploid nucleus undergoes two nuclear divisions after one DNA replication to produce four haploid nuclei.

Diploid nuclei contain two homologous chromosome sets; haploid nuclei contain one. Homologous chromosomes separate in meiosis I, halving the number of sets, and sister chromatids separate in meiosis II without another round of replication.

Diploid nucleus → DNA replication → homologous pairs segregate in meiosis I → two haploid nuclei with duplicated chromosomes → chromatids segregate in meiosis II → four haploid nuclei.

In a sexual life cycle, haploid gametes produced by meiosis fuse at fertilization, restoring the diploid chromosome number instead of doubling it in every generation.

Chromosome number is reduced in meiosis I, even though each chromosome still has two chromatids until meiosis II.

Meiosis as reduction division

Assessment in practice

1 marks
How it is assessed

This objective is assessed through structured response, commonly using State / Identify / Outline.

Command terms

State / Identify / Outline

What earns marks

Build the answer around this relationship: Meiosis halves chromosome number to produce haploid nuclei.

Watch for

Saying sister chromatids separate in meiosis I instead of homologous chromosomes.

Representative question

Question 1

[Maximum number: 5]

Outline what occurs in cells in the first division of meiosis.

Core Cell Division

  • Cell division produces daughter cells for growth, repair or reproduction; cytokinesis divides cytoplasm by a contractile ring in animals or a cell plate in plants.
  • DNA replication creates sister chromatids joined at centromeres before nuclear division.
  • Mitosis preserves chromosome number: chromosomes condense, align, sister chromatids separate and nuclei reform, producing genetically identical nuclei.
  • Meiosis follows one replication with two divisions: homologous chromosomes separate in meiosis I and sister chromatids in meiosis II, producing haploid cells.
  • Crossing over, independent orientation and random fertilization generate allele combinations.
  • Non-disjunction is failed chromosome separation and can produce aneuploid cells, including trisomy 21.
  • Identify stages in micrographs from chromosome condensation, equatorial alignment, separation and nuclear-envelope cues.

Concept essentials

  • Meiosis halves chromosome number to produce haploid nuclei.
  • Homologous chromosomes separate during meiosis I.
  • Sister chromatids separate during meiosis II.
  • One DNA replication is followed by two meiotic divisions.
  • Meiosis can produce four haploid cells from one diploid cell.