What you'll learn
This topic covers meiosis, the specialised type of cell division that produces gametes (sex cells) for sexual reproduction. You must understand how meiosis differs from mitosis, the stages involved, and crucially how meiosis generates genetic variation through independent assortment and crossing over. Exam questions regularly test your ability to explain these mechanisms and calculate genetic outcomes.
Key terms and definitions
Meiosis — a type of cell division that produces four genetically different haploid cells (gametes) from one diploid cell through two successive divisions.
Gamete — a sex cell (sperm or egg/ovum) containing half the normal chromosome number (haploid).
Haploid — a cell containing one complete set of chromosomes (n), half the diploid number.
Diploid — a cell containing two complete sets of chromosomes (2n), one set from each parent.
Homologous chromosomes — a pair of chromosomes that carry the same genes in the same positions, one inherited from each parent.
Crossing over (recombination) — the exchange of genetic material between homologous chromosomes during meiosis, creating new combinations of alleles.
Independent assortment — the random distribution of maternal and paternal chromosomes into gametes during meiosis.
Genetic variation — differences in DNA sequences between individuals of the same species.
Core concepts
Why meiosis is essential for sexual reproduction
In sexual reproduction, two gametes fuse during fertilisation to form a zygote. If gametes contained the full diploid number of chromosomes, the chromosome number would double each generation. Meiosis solves this problem by halving the chromosome number.
Humans have 46 chromosomes (23 pairs) in body cells. Through meiosis:
- Gametes receive 23 chromosomes (haploid number)
- At fertilisation: 23 (egg) + 23 (sperm) = 46 (zygote)
- The diploid number is restored in the offspring
This maintains constant chromosome numbers across generations while allowing genetic variation through combining genetic material from two parents.
The key differences between mitosis and meiosis
Understanding these differences is fundamental for CIE IGCSE Biology exam questions:
Mitosis:
- One division
- Produces two daughter cells
- Daughter cells genetically identical to parent cell
- Daughter cells diploid (2n)
- Occurs in body cells for growth and repair
- No pairing of homologous chromosomes
Meiosis:
- Two successive divisions (meiosis I and meiosis II)
- Produces four daughter cells (gametes)
- Daughter cells genetically different from parent cell and each other
- Daughter cells haploid (n)
- Occurs in reproductive organs (testes and ovaries)
- Homologous chromosomes pair up during meiosis I
The stages of meiosis
Meiosis consists of two divisions: meiosis I (reduction division) and meiosis II (similar to mitosis).
Before meiosis begins:
- DNA replication occurs during interphase
- Each chromosome consists of two identical sister chromatids joined at the centromere
- The cell is diploid (2n) with replicated chromosomes
Meiosis I (the reduction division):
- Prophase I — chromosomes condense and become visible; homologous chromosomes pair up forming bivalents; crossing over occurs between non-sister chromatids; nuclear membrane breaks down
- Metaphase I — bivalents line up at the cell equator; spindle fibres attach to centromeres
- Anaphase I — homologous chromosomes (not sister chromatids) are pulled to opposite poles; each chromosome still consists of two chromatids
- Telophase I — two haploid cells form, each containing one chromosome from each homologous pair (but each chromosome consists of two chromatids)
Meiosis II (similar to mitosis):
- Prophase II — chromosomes condense; nuclear membrane breaks down (if reformed)
- Metaphase II — chromosomes line up at the equator of each cell
- Anaphase II — sister chromatids separate and move to opposite poles
- Telophase II — four haploid cells form, each genetically different
Result: four haploid gametes, each with half the chromosome number of the parent cell.
How crossing over creates genetic variation
Crossing over occurs during prophase I of meiosis and is a major source of genetic variation.
The process:
- Homologous chromosomes pair up to form bivalents
- Non-sister chromatids wrap around each other at points called chiasmata (singular: chiasma)
- Sections of DNA are exchanged between the chromatids
- This creates new combinations of alleles on each chromosome
Example: Consider two genes on a chromosome pair — one for eye colour, one for hair colour. Initially, one chromosome carries alleles for brown eyes and black hair, while its homologous partner carries alleles for blue eyes and blonde hair. Through crossing over, a chromosome might end up with alleles for brown eyes and blonde hair — a new combination not present in either parent.
The significance:
- Each crossing over event creates unique combinations of alleles
- Multiple crossing over events can occur on each bivalent
- This means no two gametes produced by an individual are genetically identical
- Offspring receive novel combinations of characteristics
Independent assortment and genetic variation
Independent assortment provides another major source of genetic variation during meiosis I.
The mechanism:
- During metaphase I, bivalents line up randomly at the cell equator
- For each pair, either the maternal or paternal chromosome can face either pole
- This orientation is completely random for each homologous pair
- The arrangement of one pair does not influence others
Mathematical impact: For an organism with n pairs of chromosomes, independent assortment can produce 2^n different combinations of chromosomes in gametes.
- Humans have 23 pairs of chromosomes
- Number of possible combinations = 2^23 = 8,388,608
- Each person can therefore produce over 8 million genetically different gametes through independent assortment alone
- When combined with crossing over, the variation is essentially limitless
Example for exam purposes: An organism with just 3 pairs of chromosomes (n=3) can produce 2^3 = 8 different chromosome combinations in its gametes through independent assortment. If we label the maternal chromosomes A, B, C and paternal chromosomes A', B', C', possible gamete combinations include: ABC, ABC', AB'C, AB'C', A'BC, A'BC', A'B'C, A'B'C'.
How genetic variation benefits populations
The genetic variation produced by meiosis has evolutionary significance:
- Adaptation — variation provides raw material for natural selection; some combinations may be advantageous in changing environments
- Survival — genetically diverse populations are more resilient to diseases and environmental changes
- Evolution — populations with greater variation can evolve faster when selection pressures change
Exam questions often ask you to explain why sexual reproduction produces variation while asexual reproduction does not. The answer centres on meiosis and the random fusion of genetically different gametes.
Worked examples
Example 1: Calculating genetic combinations
Question: A species has a diploid chromosome number of 8.
(a) State how many chromosomes will be present in a gamete. [1]
(b) Calculate the number of different chromosome combinations possible in the gametes through independent assortment alone. Show your working. [2]
(c) Explain how crossing over would affect your answer to part (b). [2]
Mark scheme answers:
(a) 4 chromosomes [1 mark]
(b) Number of pairs = 8 ÷ 2 = 4 [1 mark] Number of combinations = 2^4 = 16 [1 mark]
(c) Crossing over would increase the number of possible combinations [1 mark]; because it creates new combinations of alleles on individual chromosomes / exchanges genetic material between homologous chromosomes [1 mark]
Example 2: Comparing mitosis and meiosis
Question: Complete the table to show three differences between mitosis and meiosis. [3]
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | ||
| Genetically identical | ||
| Chromosome number in daughter cells |
Mark scheme answers:
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One [1 mark] | Two [1 mark] |
| Genetic variation in daughter cells | Genetically identical | Genetically different [1 mark] |
| Chromosome number in daughter cells | Diploid / 2n / same as parent [1 mark] | Haploid / n / half the parent [1 mark] |
[Award any 3 marks for correct completion]
Example 3: Explaining variation
Question: Meiosis produces genetic variation in gametes.
(a) Name two processes during meiosis that cause genetic variation. [2]
(b) Explain how one of these processes causes variation. [3]
Mark scheme answers:
(a) • Independent assortment / random assortment (of chromosomes) [1 mark] • Crossing over / recombination [1 mark]
(b) For independent assortment: • Homologous chromosomes / bivalents line up randomly at the equator [1 mark] • During metaphase I / first division [1 mark] • Either the maternal or paternal chromosome from each pair can end up in each gamete / gametes receive different combinations of maternal and paternal chromosomes [1 mark]
OR for crossing over: • Homologous chromosomes pair up [1 mark] • (Non-sister) chromatids exchange sections / genetic material [1 mark] • Creating new combinations of alleles (on chromosomes) [1 mark]
Common mistakes and how to avoid them
• Mistake: Stating that meiosis produces two cells instead of four. Correction: Meiosis involves two successive divisions (meiosis I and meiosis II), producing four haploid gametes from one diploid parent cell.
• Mistake: Confusing when chromosome number halves — saying it happens in meiosis II. Correction: Chromosome number is halved during meiosis I when homologous chromosomes separate. Meiosis II separates sister chromatids (similar to mitosis) but doesn't change chromosome number further.
• Mistake: Describing crossing over as "chromosomes swapping" without precision. Correction: Be specific: crossing over involves non-sister chromatids of homologous chromosomes exchanging sections of DNA / genetic material at chiasmata during prophase I.
• Mistake: Forgetting that DNA replication occurs before meiosis begins. Correction: Like mitosis, DNA replication happens during interphase before meiosis starts. This means chromosomes consist of two sister chromatids at the beginning of meiosis I.
• Mistake: Stating that independent assortment creates "new alleles" or "new genes." Correction: Independent assortment creates new combinations of existing alleles/chromosomes, but doesn't create new alleles. Only mutation creates genuinely new alleles.
• Mistake: Using vague phrases like "meiosis creates variation" without explaining the mechanism. Correction: Always specify the mechanism — either independent assortment of chromosomes during metaphase I, or crossing over between non-sister chromatids during prophase I. Exam questions demand precision.
Exam technique for "Meiosis and genetic variation"
• "Compare" questions: When asked to compare mitosis and meiosis, make direct comparisons using the same feature. Don't just describe each separately. Use comparative language: "whereas," "while," "but." For example: "Mitosis produces two cells whereas meiosis produces four cells."
• "Explain" questions about variation: Structure answers with two parts — first describe the mechanism (what happens), then state the outcome (how this causes variation). For crossing over: describe the exchange of DNA between chromatids, then explain this creates new allele combinations. Expect 2-3 marks for these questions.
• Calculation questions: Show all working when calculating possible gamete combinations (2^n formula). Even if your final answer is wrong, you can gain method marks. Always define n as the number of chromosome pairs, not the total number of chromosomes.
• Command word "State": Requires brief answers without explanation. For example, "State the number of cells produced by meiosis" needs just "four" or "4" — no elaboration needed or rewarded.
Quick revision summary
Meiosis is cell division producing four genetically different haploid gametes from one diploid cell through two divisions. It maintains constant chromosome numbers across generations in sexual reproduction. Genetic variation arises through two mechanisms: crossing over (exchange of DNA between non-sister chromatids during prophase I creating new allele combinations) and independent assortment (random distribution of maternal/paternal chromosomes during metaphase I — producing 2^n possible combinations where n = number of chromosome pairs). Unlike mitosis, which produces identical diploid cells for growth, meiosis ensures each gamete is genetically unique, providing variation essential for adaptation and evolution.