Cell division: meiosis

What you'll learn

This revision guide covers meiosis, the type of cell division that produces gametes (sex cells) in sexually reproducing organisms. You'll understand how meiosis halves chromosome number, creates genetic variation, and differs fundamentally from mitosis. This topic is essential for understanding inheritance and variation in AQA GCSE Biology.

Key terms and definitions

Meiosis — a type of cell division that produces four non-identical daughter cells, each with half the number of chromosomes of the parent cell

Gamete — a sex cell (sperm or egg) that contains half the normal number of chromosomes (haploid)

Haploid — a cell containing one set of chromosomes (n), such as gametes in humans containing 23 chromosomes

Diploid — a cell containing two sets of chromosomes (2n), such as normal body cells in humans containing 46 chromosomes (23 pairs)

Fertilisation — the fusion of male and female gametes to form a zygote, restoring the diploid chromosome number

Chromosome — a structure made of DNA that carries genetic information in the form of genes

Genetic variation — differences in characteristics between individuals of the same species caused by differences in genes

Zygote — a fertilised egg cell formed when two gametes fuse, containing the full diploid number of chromosomes

Core concepts

Why meiosis is necessary

Sexual reproduction involves the fusion of two gametes during fertilisation. If gametes contained the full number of chromosomes (diploid), the resulting zygote would have double the normal number. This would double again in each generation, which is clearly impossible.

Meiosis solves this problem by:

• Halving the chromosome number in gametes (producing haploid cells)
• Ensuring that when gametes fuse, the normal diploid number is restored
• Maintaining a constant chromosome number across generations

In humans:

• Normal body cells contain 46 chromosomes (23 pairs) — diploid (2n)
• Meiosis produces gametes with 23 chromosomes — haploid (n)
• At fertilisation: 23 (egg) + 23 (sperm) = 46 (zygote) — diploid restored

The process of meiosis

Meiosis occurs only in the reproductive organs (testes and ovaries in animals). The process involves two consecutive divisions producing four cells from one parent cell.

Before meiosis begins:

• DNA replication occurs, copying all chromosomes
• Each chromosome now consists of two identical chromatids joined at the centromere
• The cell still appears diploid but with duplicated chromosomes

First division (Meiosis I):

1. Chromosomes condense and become visible
2. Homologous pairs (matching chromosomes, one from each parent) line up in the centre of the cell
3. The pairs are separated and pulled to opposite poles of the cell
4. The cell divides, producing two cells
5. Each cell contains one chromosome from each homologous pair (but each is still made of two chromatids)

Second division (Meiosis II):

1. The chromosomes line up in the centre of each cell
2. The chromatids are separated and pulled to opposite poles
3. Each cell divides again
4. Four haploid cells are produced, each genetically different

Key outcome: One diploid parent cell produces four haploid daughter cells, each containing different combinations of genetic information.

How meiosis creates genetic variation

Meiosis produces gametes that are genetically different from each other and from the parent cell. This genetic variation is crucial for evolution and survival of species.

Two main mechanisms create variation:

1. Independent assortment:

• During the first division, homologous pairs line up randomly at the cell centre
• Which chromosome from each pair goes to which daughter cell is random
• In humans, with 23 pairs of chromosomes, this creates 2²³ (over 8 million) possible combinations
• This means each gamete receives a unique mix of maternal and paternal chromosomes

2. Crossing over (also called recombination):

• During the first division, homologous chromosomes pair up very closely
• Sections of DNA are exchanged between the paired chromosomes
• This creates new combinations of alleles on each chromosome
• The chromosomes now contain a mixture of genetic material from both parents
• Note: AQA GCSE requires awareness of crossing over but not detailed mechanisms

Why variation matters:

• Increases genetic diversity in populations
• Provides raw material for natural selection
• Allows populations to adapt to changing environments
• Reduces the likelihood of genetic diseases affecting entire populations

Meiosis vs mitosis — key differences

Understanding the differences between these two types of cell division is essential for exam success.

Feature	Mitosis	Meiosis
Purpose	Growth, repair, asexual reproduction	Production of gametes for sexual reproduction
Number of divisions	One	Two
Number of daughter cells	Two	Four
Chromosome number in daughter cells	Diploid (same as parent)	Haploid (half the parent)
Genetic identity	Genetically identical to parent and each other	Genetically different from parent and each other
Where it occurs	Throughout the body	Only in reproductive organs
Variation produced	No variation (except mutations)	Variation through independent assortment and crossing over

Common exam focus:

• Mitosis maintains chromosome number; meiosis halves it
• Mitosis produces clones; meiosis produces variation
• Both involve DNA replication before division begins

The role of meiosis in sexual reproduction

Meiosis is essential for the sexual reproduction cycle:

1. Gamete production: Meiosis in reproductive organs produces haploid gametes
2. Fertilisation: Two gametes (one from each parent) fuse
3. Zygote formation: The diploid number is restored in the zygote
4. Development: The zygote divides by mitosis to form an embryo
5. Growth: Continued mitosis produces a fully developed organism
6. Maturity: The organism eventually produces its own gametes through meiosis

Advantages of sexual reproduction involving meiosis:

• Creates genetic variation in offspring
• Variation allows adaptation to changing environments
• Increases chances of survival if environment changes
• Reduces impact of harmful genetic mutations

The importance of maintaining chromosome number:

• Errors in meiosis can lead to gametes with wrong chromosome numbers
• This can cause genetic conditions (e.g., Down syndrome from an extra chromosome 21)
• Normal meiosis ensures each species maintains its characteristic chromosome number

Gametes in different organisms

While meiosis follows the same basic pattern, gametes differ between organisms.

In animals:

• Males produce sperm cells (small, mobile, produced in large numbers)
• Females produce egg cells (large, non-mobile, contain food reserves)
• Both are specialised for their roles in fertilisation

In flowering plants:

• Pollen grains contain the male gametes
• Ovules contain the female gametes (egg cells)
• Both are produced by meiosis in the reproductive organs of flowers

Human gamete production:

• Sperm: produced continuously in testes from puberty onwards
• Eggs: all eggs are present at birth but complete meiosis only after ovulation
• Both processes involve meiosis but with different timings and cell numbers

Worked examples

Example 1: Chromosome numbers

Question: A cat body cell contains 38 chromosomes.

a) How many chromosomes would be in a cat's gamete? (1 mark)

b) Explain why gametes must have half the chromosome number of body cells. (2 marks)

c) How many chromosomes would be present in a fertilised cat egg cell? (1 mark)

Mark scheme answers:

a) 19 chromosomes (1 mark)

• Gametes are haploid, containing half the diploid number

b) So that when two gametes fuse during fertilisation (1 mark), the normal diploid number is restored / maintained across generations (1 mark)

• Alternative: to prevent chromosome number doubling each generation

c) 38 chromosomes (1 mark)

• Fertilisation restores the diploid number (19 + 19 = 38)

Example 2: Comparing mitosis and meiosis

Question: A student is comparing mitosis and meiosis.

a) State two ways in which meiosis differs from mitosis. (2 marks)

b) Explain why meiosis produces genetic variation but mitosis does not. (3 marks)

Mark scheme answers:

a) Any two from:

• Meiosis involves two divisions, mitosis involves one (1 mark)
• Meiosis produces four daughter cells, mitosis produces two (1 mark)
• Meiosis produces haploid cells, mitosis produces diploid cells (1 mark)
• Meiosis produces genetically different cells, mitosis produces identical cells (1 mark)
• Meiosis only occurs in reproductive organs, mitosis occurs throughout the body (1 mark)

(Maximum 2 marks)

b) In meiosis, chromosomes are randomly assorted / distributed into daughter cells (1 mark), so each gamete receives a different combination of chromosomes / alleles (1 mark). Crossing over / recombination occurs, mixing genetic material from homologous chromosomes (1 mark).

In mitosis, chromosomes are copied exactly and distributed equally, so daughter cells are genetically identical (accept for 1 mark if not already awarded).

(Maximum 3 marks)

Example 3: Application to inheritance

Question: In humans, the diploid chromosome number is 46.

a) How many chromosomes are present in a human sperm cell? (1 mark)

b) Name the type of cell division that produces sperm cells. (1 mark)

c) Explain why this type of cell division is important for sexual reproduction. (3 marks)

Mark scheme answers:

a) 23 (1 mark)

b) Meiosis (1 mark)

c) Meiosis halves the chromosome number / produces haploid gametes (1 mark), so that when gametes fuse at fertilisation (1 mark), the normal diploid number is restored / the zygote has 46 chromosomes (1 mark).

Alternative points:

• Meiosis produces genetic variation (1 mark), which is important for adaptation / evolution / survival (1 mark)

(Maximum 3 marks — must include reference to chromosome number for full marks)

Common mistakes and how to avoid them

• Confusing the number of divisions: Meiosis involves TWO divisions, not one. Remember: "MEIosis = TWO divisions" as a memory aid. Don't state that meiosis is just one division that halves chromosome number.

• Getting chromosome numbers wrong: The parent cell is diploid (2n), after the first division cells are still diploid but with single chromatids, and only after the second division are cells haploid (n). Many students incorrectly state cells are haploid after the first division.

• Mixing up purposes: Mitosis is for growth and repair (maintaining chromosome number); meiosis is for gamete production (halving chromosome number). Don't state that mitosis produces gametes or that meiosis is used for growth.

• Forgetting about variation: Meiosis produces genetically DIFFERENT cells through independent assortment and crossing over. Don't state that meiosis produces identical cells — that's mitosis.

• Incomplete explanations of why meiosis is needed: Always link halving chromosome number to fertilisation restoring the diploid number. Simply stating "to produce gametes" without explaining the chromosome number issue loses marks.

• Confusing fertilisation with meiosis: Fertilisation is when gametes FUSE (joining together). Meiosis is the division that PRODUCES gametes. These are opposite processes happening at different times.

Exam technique for "Cell division: meiosis"

• Command word focus: "Explain" questions require you to give reasons WHY something happens, not just describe WHAT happens. For example, explain WHY meiosis is needed (to halve chromosome number so fertilisation can restore it), not just that it produces gametes.

• Use correct terminology: Always use "haploid" and "diploid" rather than "half the chromosomes" or "normal number." Use "gametes" not "sex cells" where appropriate. Precision in terminology gains marks and demonstrates understanding.

• Compare and contrast structure: When asked to compare mitosis and meiosis, use comparative language ("whereas," "however," "in contrast"). Make sure differences are clearly paired — state what happens in BOTH processes for each point.

• Mark allocation guides detail: A 3-mark question requires three distinct points. Don't repeat the same idea in different words. For meiosis questions, common mark points include: halving chromosome number, producing variation, two divisions, four daughter cells, and linking to fertilisation.

Quick revision summary

Meiosis is cell division producing four genetically different haploid gametes from one diploid parent cell through two consecutive divisions. It halves chromosome number, ensuring fertilisation restores the diploid number and maintains constant chromosomes across generations. Genetic variation arises through independent assortment of chromosomes and crossing over. Unlike mitosis (which produces identical diploid cells for growth and repair), meiosis only occurs in reproductive organs and is essential for sexual reproduction, providing variation crucial for adaptation and evolution.