What you'll learn
Meiosis is a specialized form of cell division that produces gametes (sex cells) with half the chromosome number of the parent cell. This topic appears frequently on CXC CSEC Biology papers, particularly in questions about reproduction, inheritance, and variation. You'll master the stages of meiosis, understand how it creates genetic variation, and explain its biological significance for sexual reproduction.
Key terms and definitions
Meiosis — a type of nuclear division that reduces the chromosome number by half, producing four non-identical haploid daughter cells from one diploid parent cell.
Diploid (2n) — a cell containing two complete sets of chromosomes, one from each parent (e.g., 46 chromosomes in humans, 20 in maize).
Haploid (n) — a cell containing only one complete set of chromosomes (e.g., 23 in human gametes, 10 in maize pollen).
Homologous chromosomes — pairs of chromosomes that carry genes for the same characteristics in the same positions, one inherited from each parent.
Crossing over (recombination) — the exchange of genetic material between non-sister chromatids of homologous chromosomes during meiosis I, creating new gene combinations.
Gamete — a sex cell (sperm or egg/ovum) produced by meiosis, containing half the chromosome number of body cells.
Genetic variation — differences in genetic makeup between individuals of the same species, essential for natural selection and evolution.
Independent assortment — the random distribution of maternal and paternal chromosomes into daughter cells during meiosis, producing different chromosome combinations.
Core concepts
The need for meiosis in sexual reproduction
Body cells (somatic cells) in organisms are diploid — they contain two sets of chromosomes. In humans, this means 46 chromosomes (23 pairs). If gametes were also diploid, fertilization would double the chromosome number each generation: 46 + 46 = 92 in offspring, then 184 in the next generation, and so on. This would be lethal.
Meiosis solves this problem by producing haploid gametes. When two haploid gametes fuse during fertilization, the diploid number is restored:
- Human sperm (n = 23) + human egg (n = 23) = zygote (2n = 46)
- Maize pollen (n = 10) + maize egg (n = 10) = maize embryo (2n = 20)
This maintains a constant chromosome number across generations, which is essential for species survival.
Where meiosis occurs
In animals (including humans):
- Males: Testes produce sperm through meiosis (spermatogenesis)
- Females: Ovaries produce eggs through meiosis (oogenesis)
In flowering plants:
- Anthers produce pollen grains containing male gametes
- Ovules (inside ovaries) produce egg cells
Examples from the Caribbean: Meiosis occurs in the anthers of hibiscus flowers (Hibiscus rosa-sinensis, Jamaica's national flower) and in the reproductive organs of breadfruit trees (Artocarpus altilis), though cultivated breadfruit in the region rarely reproduce sexually.
The two stages of meiosis: Meiosis I and Meiosis II
Meiosis consists of two consecutive divisions with one DNA replication, producing four haploid cells from one diploid cell.
Meiosis I — the reduction division
This division separates homologous pairs and reduces chromosome number from diploid to haploid.
Prophase I (longest phase):
- Chromosomes condense and become visible
- Homologous chromosomes pair up (synapsis) to form bivalents
- Crossing over occurs — non-sister chromatids exchange segments of DNA
- Centrioles move to opposite poles
- Nuclear membrane breaks down
- Spindle fibers form
Metaphase I:
- Bivalents line up along the cell equator
- Spindle fibers attach to centromeres
- Orientation of each pair is random (independent assortment)
Anaphase I:
- Homologous chromosomes separate and move to opposite poles
- Sister chromatids remain attached at centromeres
- Each pole receives a haploid set of chromosomes
Telophase I and Cytokinesis:
- Chromosomes may decondense slightly
- Nuclear membranes may reform briefly
- Cytoplasm divides, producing two haploid cells
- Each cell contains chromosomes still consisting of two sister chromatids
Meiosis II — the equational division
This division resembles mitosis, separating sister chromatids.
Prophase II:
- Chromosomes condense (if they had decondensed)
- New spindle forms at right angles to the first
- Nuclear membrane breaks down (if reformed)
Metaphase II:
- Chromosomes line up individually along the equator
- Spindle fibers attach to centromeres
Anaphase II:
- Centromeres divide
- Sister chromatids separate and move to opposite poles
- Each chromatid is now an independent chromosome
Telophase II and Cytokinesis:
- Chromosomes decondense
- Nuclear membranes reform
- Cytoplasm divides
- Four haploid daughter cells produced, each genetically unique
How meiosis creates genetic variation
Meiosis produces genetically different cells through three mechanisms:
1. Crossing over (Prophase I)
During synapsis, non-sister chromatids of homologous chromosomes exchange genetic material. This creates new combinations of alleles on the same chromosome.
Example: Consider genes for fruit color and texture on the same chromosome in tomato plants (commonly grown in Trinidad and Tobago):
- Original chromosome 1: Red color (R), Smooth texture (S)
- Original chromosome 2: Yellow color (r), Rough texture (s)
- After crossing over: New combinations like Red-Rough (R-s) or Yellow-Smooth (r-S)
Multiple crossover points can occur on each chromosome pair, creating billions of possible combinations.
2. Independent assortment (Metaphase I)
Each bivalent lines up randomly at the equator. For a human cell with 23 pairs, there are 2²³ = 8,388,608 possible combinations of maternal and paternal chromosomes in each gamete, even without crossing over.
In organisms with fewer chromosome pairs, the number is smaller but still significant:
- Maize (10 pairs): 2¹⁰ = 1,024 combinations
- Pea plants (7 pairs): 2⁷ = 128 combinations
3. Random fertilization
Any sperm can fertilize any egg. Combined with independent assortment:
- Humans: 8,388,608 × 8,388,608 = 70+ trillion possible offspring combinations
- This explains why siblings (except identical twins) are genetically unique
Differences between mitosis and meiosis
Understanding these differences is crucial for CXC CSEC Biology exams:
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One | Two (Meiosis I and II) |
| Number of daughter cells | 2 | 4 |
| Chromosome number in daughter cells | Diploid (2n) — same as parent | Haploid (n) — half the parent |
| Genetic identity of daughter cells | Genetically identical to parent | Genetically different from parent and each other |
| Pairing of homologous chromosomes | No pairing | Pairing occurs (synapsis) |
| Crossing over | Does not occur | Occurs in Prophase I |
| Function | Growth, repair, asexual reproduction | Produces gametes for sexual reproduction |
| Where it occurs | Somatic (body) cells | Gonads (testes, ovaries) or reproductive structures |
Significance of meiosis
Maintains chromosome number across generations
By halving the chromosome number in gametes, meiosis ensures that fertilization restores the diploid number rather than doubling it. This is essential for species survival.
Generates genetic variation
Through crossing over, independent assortment, and random fertilization, meiosis creates unique offspring. This variation is vital for:
- Adaptation: Populations can evolve in response to environmental changes
- Disease resistance: Some individuals may possess alleles conferring resistance
- Agricultural improvement: Plant and animal breeders select for desirable traits
Caribbean example: Variation in cocoa trees (Theobroma cacao) produced through sexual reproduction in Trinidad and Tobago has allowed farmers to select disease-resistant varieties, supporting the region's chocolate industry.
Enables natural selection and evolution
Variation provides the raw material for natural selection. Individuals with advantageous trait combinations survive and reproduce, passing these traits to offspring. Without meiosis, populations would lack the variation necessary for evolutionary change.
Example: The diverse varieties of Scotch Bonnet peppers (Capsicum chinense) cultivated across Jamaica arose partly through genetic variation generated by meiosis, followed by selection by farmers.
Allows sexual reproduction
Sexual reproduction combines genetic material from two parents, producing offspring that are neither clones nor completely unrelated to their parents. This balance maintains species identity while promoting diversity.
Worked examples
Question 1: A cell from a maize plant undergoes meiosis. The diploid number for maize is 20.
(a) State the number of chromosomes in: (i) the cell at the start of meiosis [1 mark] (ii) each cell after meiosis I [1 mark] (iii) each cell after meiosis II [1 mark]
(b) Explain how meiosis produces genetic variation in maize pollen grains. [4 marks]
Solution:
(a)(i) 20 chromosomes (diploid/2n = 20) [1]
(a)(ii) 10 chromosomes (haploid/n = 10) [1]
(a)(iii) 10 chromosomes (haploid/n = 10) [1]
(b) Award marks for any four points:
- Crossing over/recombination occurs [1]
- During Prophase I, homologous chromosomes pair up [1]
- Non-sister chromatids exchange genetic material/segments of DNA [1]
- This creates new combinations of alleles on the same chromosome [1]
- Independent assortment occurs [1]
- Random orientation of bivalents at the equator during Metaphase I [1]
- Maternal and paternal chromosomes distributed randomly to daughter cells [1]
- This produces different chromosome combinations in each pollen grain [1]
(Maximum 4 marks)
Question 2: The diagram shows a pair of homologous chromosomes during meiosis. Gene A codes for seed color, with alleles A (yellow) and a (green). Gene B codes for seed shape, with alleles B (round) and b (wrinkled).
[Assume diagram shows crossing over between genes A and B]
(a) Name the stage of meiosis when this event occurs. [1 mark]
(b) State the name of this process. [1 mark]
(c) Before the event shown, one chromosome carried alleles A and B, while its homologue carried alleles a and b. Identify two new allele combinations that could result from this event. [2 marks]
(d) Explain the significance of this process for pea plant populations in Jamaica. [3 marks]
Solution:
(a) Prophase I [1]
(b) Crossing over/recombination [1]
(c) A with b [1] a with B [1] (Accept Ab and aB)
(d) Award marks for any three points:
- Produces new combinations of alleles [1]
- Increases genetic variation in offspring/population [1]
- Provides raw material for natural selection/evolution [1]
- Allows plants to adapt to changing environmental conditions [1]
- Some combinations may be more advantageous for survival [1]
- Helps maintain healthy, diverse populations [1]
(Maximum 3 marks)
Question 3: Explain why meiosis is necessary for sexual reproduction in cocoa plants grown in Trinidad. [4 marks]
Solution:
Award marks for any four points:
- Sexual reproduction involves fusion of gametes/fertilization [1]
- Meiosis produces gametes with half the chromosome number/haploid gametes [1]
- Without meiosis, gametes would be diploid [1]
- Fertilization would double the chromosome number each generation [1]
- This would be lethal/chromosomes wouldn't function properly [1]
- Meiosis maintains constant chromosome number across generations [1]
- When haploid gametes fuse, diploid number is restored [1]
(Maximum 4 marks)
Common mistakes and how to avoid them
• Mistake: Stating that meiosis produces two daughter cells instead of four. Correction: Meiosis involves two consecutive divisions (Meiosis I and II), producing four haploid cells from one diploid parent cell. Mitosis produces two cells.
• Mistake: Confusing the products of meiosis I and II. Students often say both divisions produce haploid cells, but describe them identically. Correction: After Meiosis I, two haploid cells form, but each chromosome still consists of two sister chromatids joined at the centromere. After Meiosis II, four haploid cells form, with chromosomes consisting of single chromatids.
• Mistake: Stating that crossing over occurs between sister chromatids. Correction: Crossing over occurs between non-sister chromatids of homologous chromosomes. Sister chromatids are identical copies, so exchanging material between them wouldn't create variation.
• Mistake: Writing that meiosis produces genetically identical cells. Correction: Meiosis produces genetically different cells due to crossing over, independent assortment, and the random distribution of genetic material. Only mitosis produces genetically identical cells.
• Mistake: Forgetting that chromosome number is halved in Meiosis I, not Meiosis II. Correction: Meiosis I is the reduction division where homologous pairs separate, reducing chromosome number from diploid to haploid. Meiosis II separates sister chromatids, like mitosis, but doesn't change the chromosome number.
• Mistake: Stating vague benefits of variation like "variation is good" without explaining why. Correction: Always link variation to specific biological advantages: adaptation to environmental changes, disease resistance, natural selection, evolution, or species survival. Use examples where possible.
Exam technique for "Cell Division: Meiosis and Its Significance"
• "State" questions (1 mark each) require brief, precise answers. For "State where meiosis occurs," write "testes and ovaries" or "anthers and ovules" — no explanation needed. For "State the chromosome number," give only the number, though adding "diploid" or "haploid" shows understanding.
• "Explain" questions (3-6 marks) require cause-effect reasoning. Use connective words like "because," "therefore," "this results in," and "consequently." When explaining how meiosis creates variation, describe the mechanism (crossing over/independent assortment) AND state the outcome (new allele combinations/different chromosome distributions).
• Comparison questions often ask you to distinguish mitosis from meiosis. Present answers clearly using a table or parallel statements. Always include: number of divisions, number and type of daughter cells, genetic identity, and function. Don't just describe one process.
• Diagram questions may show chromosomes at different stages. Identify the stage by key features: pairing and crossing over (Prophase I), bivalents at equator (Metaphase I), homologous chromosomes separating (Anaphase I), individual chromosomes at equator (Metaphase II), chromatids separating (Anaphase II). Mark examiners look for stage-specific terminology.
Quick revision summary
Meiosis produces four genetically different haploid gametes from one diploid cell through two divisions. Meiosis I separates homologous pairs (reduction division); Meiosis II separates sister chromatids (equational division). Crossing over in Prophase I and independent assortment in Metaphase I create genetic variation. This variation enables adaptation, natural selection, and evolution. Meiosis maintains constant chromosome numbers across generations by halving chromosome number in gametes; fertilization restores the diploid number. Occurs in testes, ovaries (animals) and anthers, ovules (plants).