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MeiosisIB MYP Biology: Revision notes

Section 1

Why we need meiosis

Most human cells are diploid: they have 46 chromosomes arranged as 23 pairs, one of each pair from each parent. If an egg and a sperm each had 46, the zygote would have 92. To prevent this, the sex cells (gametes) are made by a special cell division called meiosis, which halves the chromosome number to 23. Cells with one set of chromosomes are haploid.

Meiosis happens only in the reproductive organs: the testes (making sperm) and the ovaries (making eggs).

Key termsmeiosishaploiddiploidgamete

Section 2

What happens in meiosis

  1. The cell's DNA is copied, as before any division.
  2. The chromosomes pair up and are shuffled, so each ends up with a different mixture of the chromosomes that came from the person's mother and father.
  3. The cell divides twice. The first division separates the pairs and halves the chromosome number.
  4. The second division finishes the job.

The result is four cells, each haploid and genetically different from each other.

Key termschromosome
Exam tip

Remember: meiosis = two divisions, four different haploid cells.

Section 3

Comparing mitosis and meiosis

  • Where: mitosis in body cells; meiosis in the testes and ovaries only.
  • Purpose: mitosis for growth, repair and replacing cells; meiosis for making gametes.
  • Number of divisions: mitosis one; meiosis two.
  • Cells made: mitosis two; meiosis four.
  • Chromosome number: mitosis keeps it the same (diploid to diploid); meiosis halves it (diploid to haploid).
  • Genetic make-up: mitosis cells are identical to the parent cell; meiosis cells are all different.
Key termsmitosis
Common mistake

Mitosis does not make gametes, and meiosis does not make skin cells. Check which cells you are asked about.

Section 4

Fertilisation restores the diploid number

At fertilisation a haploid sperm (23 chromosomes) fuses with a haploid egg (23 chromosomes) to form a diploid zygote with 46 chromosomes. The zygote then divides by mitosis to form an embryo.

Because the chromosome number is halved at meiosis and restored at fertilisation, it stays the same from one generation to the next.

Key termsfertilisationzygote

Section 5

Why meiosis makes variation

Meiosis shuffles chromosomes, so every gamete is different. At fertilisation any sperm can fuse with any egg at random. A single person can make over eight million different gametes, so the number of possible combinations in a child is huge. This is why brothers and sisters (except identical twins) are not identical.

Variation helps a species survive when the environment changes or a new disease appears.

Key termsvariation

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Meiosis

  1. A biology teacher in Mumbai tells her class that human body cells each contain 46 chromosomes, but the sex cells (gametes) that parents pass on are made by a special type of cell division.
    Distinguish between the terms haploid and diploid.2 marks
  2. A student in Toronto is making a revision table that compares cell division in the skin with cell division in the testes of a man. Skin cells divide to heal cuts, while cells in the testes divide to make sperm.
    Describe two differences, other than the number of cells made, between mitosis and meiosis.2 marks
  3. A cell in the testes of a man contains 46 chromosomes. This cell divides by meiosis to make sperm cells, and one of the sperm later fertilises an egg cell from his partner.
    State the number of chromosomes in each sperm cell made from this cell, and explain why gametes must have this number.3 marks
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).