Meiosis and Gamete Formation
Meiosis is a specialized type of cell division that reduces the chromosome number by half, creating four genetically unique haploid cells (gametes). This process is fundamental to sexual reproduction, ensuring that the offspring receives a complete, diploid set of chromosomes upon fertilisation.
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- The meiotic cell cycle consists of an initial interphase (for cell growth and DNA replication) followed by two successive divisions: Meiosis I and Meiosis II.
- One diploid (2n) parent cell produces four haploid (n) daughter cells.
- The daughter cells are genetically different from both the parent cell and each other, which introduces genetic variation into a population.
- In humans, diploid body cells have 46 chromosomes, and the haploid gametes (sperm and egg) produced by meiosis have 23 chromosomes.
- Fertilisation is the fusion of two haploid gametes to form a diploid zygote, restoring the full chromosome count for the new organism.
Formulae
Gamete chromosome number (n) = Parent cell chromosome number (2n) / 2 Use this simple relationship to find the chromosome count in a haploid cell when you know the count in the diploid parent cell.
Definitions
- Diploid (2n)
- A cell containing two complete sets of chromosomes, one inherited from each parent. Most somatic (body) cells are diploid.
- Haploid (n)
- A cell containing a single set of unpaired chromosomes. Gametes are haploid.
- Gamete
- A mature haploid reproductive cell (e.g., sperm or egg) that can fuse with another gamete during fertilisation.
- Zygote
- The initial diploid cell formed when two gametes are joined by means of sexual reproduction.
Worked example
The diploid number (2n) of chromosomes in a fruit fly is 8. A single diploid cell from this fly undergoes meiosis. How many daughter cells are produced, and what is the chromosome number in each daughter cell?
- 1
Identify the starting information:
The parent cell is diploid (2n) and has 8 chromosomes.
- 2
Recall the outcome of meiosis:
it produces four daughter cells and halves the chromosome number.
- 3
Calculate the chromosome number in the daughter cells.
These cells are haploid (n).
- 4 n = 2n / 2 = 8 / 2 = 4 chromosomes
- 5
Combine the results:
Meiosis results in four daughter cells, each containing 4 chromosomes.
Answer: 4 daughter cells, each with 4 chromosomes.
Common mistakes
- ×Off-by-a-factor error: Confusing the outcomes of mitosis and meiosis. Meiosis involves two divisions and produces four cells, not one division producing two cells. It also halves the chromosome number (2n → n), it does not keep it the same.
- ×Forgetting the genetic outcome: A critical feature of meiosis is that it produces genetically different cells. Stating they are identical is a common mistake and describes mitosis instead.
- ×Incorrect chromosome counting: Remembering that meiosis starts with a diploid cell (2n) and ends with haploid cells (n). A common error is to divide the chromosome number by four because there are four cells, which is incorrect.
No-calculator tips
- ✓Chromosome math in meiosis is almost always just halving or doubling. If the question gives the diploid number, divide by 2 to get the haploid number. If it gives the haploid number, multiply by 2 to get the diploid number.
- ✓Track the cell count mentally: Start with 1 cell → (Meiosis I) → 2 cells → (Meiosis II) → 4 cells. This simple flow prevents confusion with mitosis.