Biology · 17 Inheritance

17.3 Meiosis

Before you start

A diploid nucleus contains two sets of chromosomes; a haploid nucleus contains a single set (17.1).
A human diploid cell contains 23 pairs of chromosomes, so 46 in total.
Gametes are the sex cells — sperm and egg cells in animals, and the nuclei of pollen grains and ovules in plants.
Fertilisation is the fusion of two gamete nuclei.
Mitosis is nuclear division giving genetically identical cells, and maintains the chromosome number (17.2).

Syllabus objectives covered (2)

State that meiosis is involved in the production of gametes
Describe meiosis as a reduction division in which the chromosome number is halved from diploid to haploid resulting in genetically different cells (details of the stages of meiosis are not required)

Part 1 of 4

The arithmetic problem that meiosis solves

Start with a number you already know. A human body cell is diploid: 23 pairs, so 46 chromosomes. At fertilisation, two gamete nuclei fuse to make the first cell of a new person.
Now suppose gametes were ordinary body cells with 46 chromosomes each. Fertilisation would give a cell with 92, that person's children would have 184, and the number would double every generation. Since it does not, something must halve the number before fertilisation.
$46 + 46 = 92 (wrong — this is not what happens)$
The fix is that gametes carry half the number. Two halves fuse and the full diploid number is restored, generation after generation.
$23 + 23 = 46 (a sperm nucleus and an egg nucleus fusing)$
That halving is the job of meiosis. The syllabus point is short and worth stating exactly as it is written: meiosis is involved in the production of gametes.
So the two divisions have completely separate jobs. Mitosis makes body cells for growth, repair, replacement and asexual reproduction. Meiosis makes gametes, in the testes and ovaries of animals and in the anthers and ovules of flowering plants.

Part 2 of 4

Reduction division: diploid in, haploid out

The formal name for what meiosis does is a reduction division — a division that reduces the chromosome number. That phrase is a marking point on its own, so use it rather than "a division that halves things".
Say precisely what is reduced. The chromosome number is halved from diploid to haploid: the starting cell has two sets of chromosomes, and each cell produced has a single set.
Halving means taking one chromosome from each pair, not slicing the set in half at random. A human gamete gets one chromosome from each of the 23 pairs — a complete single set, not 23 chromosomes chosen by chance.
$diploid 46 (23 pairs) \overset{\overset{}{meiosis}}{\to} haploid 23 (one of each pair)$
The halving takes two divisions, one after the other, which is why one diploid cell yields four haploid cells. You are told explicitly that the stages of meiosis are not required, so do not spend revision time on them — but knowing there are two divisions is the cleanest way to remember why meiosis reduces and mitosis does not.
Watch the words diploid and haploid: they describe the nucleus, not the organism's size or sex. A sperm nucleus and an egg nucleus are both haploid.

Part 3 of 4

Why the cells are genetically different, and why that matters

Here is the second half of the definition, and the half people forget: meiosis results in cells that are genetically different from each other. Not identical — different.
Think about why that is possible. Each pair of chromosomes contains one chromosome that came from the mother and one from the father, carrying different alleles. When one of each pair goes into a gamete, which one goes where varies from gamete to gamete, so no two gametes carry the same combination of alleles.
You are not required to explain the mechanism in any more detail than that — the stages are outside the syllabus. What you are required to state is the outcome: the cells produced are genetically different.
This is where variation in a species comes from. Because every gamete differs and fertilisation is random, brothers and sisters are not identical. Meiosis is one of the sources of genetic variation you meet again in 18.1.
Use this as a check on your own answers. If a question is about offspring that differ, or about variation, or about gametes, it is meiosis. If a question is about cells that must match the ones already there, it is mitosis.

Part 4 of 4

Mitosis or meiosis — the comparison examiners keep asking for

This comparison is worth learning as four clean contrasts, because a Compare question wants differences stated in pairs — one side and then the other — not two separate essays.
Chromosome number. Mitosis maintains it: a diploid cell gives diploid cells. Meiosis halves it: a diploid cell gives haploid cells. This is the difference to write first, every time.
Same starting cell, different finish: two identical diploid cells, or four different haploid ones.
Number of divisions and cells produced. Mitosis is one division producing two cells. Meiosis is two divisions producing four cells.
Genetic content. Mitosis gives genetically identical cells. Meiosis gives genetically different cells — the source of variation.
Where it happens and what for. Mitosis happens throughout the body for growth, repair of damaged tissues, replacement of cells and asexual reproduction. Meiosis happens only in the reproductive organs — testes and ovaries, anthers and ovules — for the production of gametes.
When you write a comparison, pair the statements: "mitosis produces genetically identical cells whereas meiosis produces genetically different cells". A list of facts about meiosis alone often scores half of what it could, because the examiner cannot award a comparison you have not made.

Worked examples

Example 1. A body cell of a certain plant contains 20 chromosomes. Determine the number of chromosomes in a nucleus produced by meiosis in this plant, and explain your answer.

Show the working

Identify what you have been given. A body cell is diploid, so 20 chromosomes means 10 pairs.
Apply the definition. Meiosis is a reduction division: the chromosome number is halved from diploid to haploid.
$\frac{20}{2} = 10$
Say what the halving means physically, because that is where the explanation mark lives: each nucleus produced receives one chromosome from each of the 10 pairs, so it holds a single complete set.
Finish by naming what these nuclei are for. They are gamete nuclei — in a plant, the nuclei in pollen grains and ovules — and at fertilisation two of them fuse to restore the diploid number of 20.

Answer: 10 chromosomes. Meiosis is a reduction division that halves the chromosome number from diploid (20) to haploid (10), each nucleus receiving one chromosome from each pair. Fusion of two such nuclei at fertilisation restores the diploid number of 20.

Example 2. A female cat produces a litter of six kittens with one male cat. The kittens are all different from each other. Explain how meiosis leads to these differences.

Show the working

Name the cells meiosis makes: the sperm and egg cells, the gametes.
State the property of those cells that answers the question. Meiosis results in genetically different cells, so every sperm cell and every egg cell carries a different combination of alleles.
Explain where that difference comes from, in the one line the syllabus supports: each gamete receives one chromosome from each pair, and which one it receives varies.
Add the second stage, because meiosis alone is not the whole answer: which sperm fertilises which egg is random, so each kitten is formed from a different pairing of gametes.
Contrast it with the alternative to make the point unmissable: if the kittens had been produced by mitosis, as in asexual reproduction, they would be genetically identical.

Answer: The gametes are produced by meiosis, which results in genetically different cells: each sperm and each egg carries a different combination of alleles, because each gamete receives one chromosome from each pair. Fertilisation then combines a different sperm with a different egg for each kitten, so all six kittens differ.

Example 3. Compare mitosis and meiosis. Give three differences.

Show the working

Decide on your three axes before you start writing. The safest three are chromosome number, genetic content, and what the cells are for. Each of these is a genuine, statable difference.
Write each difference as one sentence containing both halves. "Mitosis maintains the chromosome number, whereas meiosis halves it from diploid to haploid."
Second: "Mitosis produces genetically identical cells, whereas meiosis produces genetically different cells."
Third: "Mitosis produces cells for growth, repair, replacement and asexual reproduction, whereas meiosis produces gametes." If you want a fourth in reserve, mitosis is one division giving two cells and meiosis is two divisions giving four.
Check your answer for one-sidedness. If a sentence mentions only meiosis, it is a statement, not a comparison, and comparison marks require both sides.

Answer: 1. Mitosis maintains the chromosome number (diploid to diploid), whereas meiosis is a reduction division that halves it from diploid to haploid. 2. Mitosis produces genetically identical cells, whereas meiosis produces genetically different cells. 3. Mitosis produces cells for growth, repair of damaged tissues, replacement of cells and asexual reproduction, whereas meiosis produces gametes.

Where people go wrong

Mistake: Writing that meiosis produces genetically identical cells, or copying the mitosis definition across with the word "meiosis" swapped in.

Instead: Meiosis results in genetically different cells. Identical cells come from mitosis.

The two definitions are similar in shape and are easy to blur under time pressure. The word identical or different is usually a whole mark on its own, and writing the wrong one contradicts the rest of your answer.

Mistake: Saying that meiosis "halves the cell" or "halves the DNA" rather than naming what is halved.

Instead: Meiosis halves the chromosome number, from diploid to haploid.

The marking point names chromosome number, diploid and haploid. Vague halving language cannot be credited because it does not show you know which quantity changes.

Mistake: Thinking a haploid gamete gets any 23 chromosomes, as though the set were cut in half at random.

Instead: A gamete receives one chromosome from each pair — a complete single set.

If chromosomes were shared out at random, some gametes would be missing whole chromosomes. One from each pair is also what makes each gamete genetically different from the next.

Mistake: Using meiosis to explain growth, healing, or how a plant grown from a cutting matches its parent.

Instead: Those are mitosis. Meiosis is used only in the production of gametes.

Growth and repair need cells that match the ones already present and keep the full chromosome number — exactly what mitosis gives. A cell in your arm never undergoes meiosis.

Check yourself

State the role of meiosis in an organism.
Answer
Meiosis is involved in the production of gametes.
Describe meiosis, using the terms reduction division, diploid and haploid.
Answer
Meiosis is a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells.
A diploid cell of a mouse contains 40 chromosomes. State the number of chromosomes in a mouse egg cell.
Answer
20 — half of 40, because meiosis halves the chromosome number from diploid to haploid.
Explain why the chromosome number must be halved during the production of gametes.
Answer
Two gamete nuclei fuse at fertilisation. If each gamete were diploid the chromosome number would double every generation, so each gamete must be haploid for fertilisation to restore the normal diploid number.
Give two differences between mitosis and meiosis.
Answer
Mitosis maintains the chromosome number whereas meiosis halves it from diploid to haploid; mitosis produces genetically identical cells whereas meiosis produces genetically different cells. (Also acceptable: mitosis is for growth, repair, replacement and asexual reproduction, whereas meiosis produces gametes.)
Identify which type of cell division causes variation between the offspring of two parents, and state why.
Answer
Meiosis, because it results in genetically different cells, so each gamete carries a different combination of alleles.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. In a species of flowering plant, a cell taken from a leaf contains 18 chromosomes. Meiosis takes place in the anthers and ovules of this plant.

(a) State the role of meiosis in this plant. 1 mark

Mark scheme

B1 it is involved in the production of gametes; accept named plant gametes, e.g. nuclei of pollen grains and ovules

(b) Describe meiosis. 3 marks

Mark scheme

B1 a reduction division
B1 the chromosome number is halved, from diploid to haploid
B1 the cells produced are genetically different (from each other and from the parent cell); ignore any description of the stages of meiosis

(c) Determine the number of chromosomes in the nucleus of a pollen grain of this plant. 1 mark

Mark scheme

B1 9

(d) Explain why the chromosome number in the gametes of this plant must be half the number found in a leaf cell. 2 marks

Mark scheme

B1 two gamete nuclei fuse at fertilisation
B1 so that the diploid number is restored / otherwise the chromosome number would double each generation

Question 2. A gardener produces new plants in two ways: by taking cuttings from one parent plant, and by collecting seeds produced after pollination between two different parent plants.

(a) Identify the type of nuclear division involved in producing the gametes that formed the seeds. 1 mark

Mark scheme

B1 meiosis

(b) Compare mitosis and meiosis. Give three differences. 3 marks

Mark scheme

B1 mitosis maintains the chromosome number whereas meiosis halves it from diploid to haploid
B1 mitosis produces genetically identical cells whereas meiosis produces genetically different cells
B1 mitosis is used for growth / repair / replacement of cells / asexual reproduction whereas meiosis produces gametes; or mitosis is one division giving two cells whereas meiosis is two divisions giving four cells. One-sided statements do not score

(c) The plants grown from the seeds are all slightly different from each other, but the plants grown from cuttings are not. Explain why. 3 marks

Mark scheme

B1 the gametes are produced by meiosis, which gives genetically different cells / each gamete has a different combination of alleles
B1 fertilisation combines a different pair of gametes each time, so each seed has a different combination of alleles
B1 the cuttings grow by mitosis, which gives genetically identical cells, so those plants are clones of the one parent

How this is examined

The full definition of meiosis, stated in syllabus language: a two- or three-mark Describe, usually the opening part of an inheritance question Three separable points: reduction division; chromosome number halved from diploid to haploid; cells produced are genetically different. Missing the third point is the most common way to lose a mark here, and "halves the DNA" does not score in place of chromosome number.
Telling mitosis and meiosis apart: a two- to four-mark Compare, or a table with one column left blank for you to fill in Paired statements, one mark per pair: chromosome number maintained whereas halved; genetically identical whereas genetically different; growth/repair/replacement/asexual reproduction whereas production of gametes; one division giving two cells whereas two divisions giving four. A one-sided statement scores nothing in a comparison.
Working out a chromosome number after meiosis from a given diploid number: a one- or two-mark Determine or State — "a body cell of this species has 24 chromosomes; give the number in a gamete" The halved number, plus the reason if asked: meiosis halves the chromosome number from diploid to haploid, so the gamete receives one chromosome from each pair. Read whether the number given is for a body cell (diploid) or a gamete (haploid) before dividing.
Why gametes have to be haploid at all: a two-mark Explain, often following a question about fertilisation Two gamete nuclei fuse at fertilisation, so if gametes were diploid the chromosome number would double each generation; halving it in meiosis means fertilisation restores the diploid number.

Words to know

Meiosis: A reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells.
Reduction division: A nuclear division that reduces the chromosome number — meiosis.
Diploid: Describes a nucleus containing two sets of chromosomes.
Haploid: Describes a nucleus containing a single set of chromosomes.
Gamete: A sex cell, with a haploid nucleus, produced by meiosis; two gamete nuclei fuse at fertilisation.
Genetically different: Carrying a different combination of alleles from another cell.

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