Genetics

Meiosis

Meiosis is the division that makes gametes. It halves the chromosome number and shuffles the genes.

CAPS

Grade 12 · Life Sciences · Genetics

Life Sciences is two papers. Use the strand to know which one.

Where it is examined
Paper 1 is reproduction, responding to the environment, and homeostasis. Paper 2 is DNA, meiosis, genetics and evolution. Each paper is 150 marks.
How to write it
Name the structure, then say what it does. A labelled diagram only scores if the labels are the biological names. “It helps the body” is not a function.
How a marker reads it
A common mix-up is to say meiosis happens in a healing wound, or that it produces two identical cells.

A 2-mark “explain” wants two facts, or one fact and its reason. Extra sentences that contradict the first one can cancel a mark.

Diagram

Worked theory

Worked theory. A human cell with 46 chromosomes forms sperm. After meiosis each sperm has 23. An egg also has 23. The zygote has 46 again. Two brothers are not identical, even with the same parents, because crossing over and the way pairs lined up differed in each meiosis. Identical twins are the exception that proves the point: they came from one zygote that later split by mitosis, so meiosis had already finished. If gametes were made by mitosis, a child would receive 46 from each parent and the number would double every generation.

More worked examples

Chromosome 21 fails to separate
In one meiosis the pair of chromosome 21 both go into the same egg. The egg has 24 chromosomes, not 23. A normal sperm brings 23. The zygote has 47, with three copies of chromosome 21. That is Down syndrome, and it is a failed meiosis, not a failed mitosis. The other eggs from a normal meiosis still have 23. Fertilisation does not halve anything. It adds.
Four sperm, one ovum
One meiosis in a testis ends as four sperm, each with 23 chromosomes. One meiosis in an ovary ends as one ovum and small polar bodies, and the ovum also has 23. Crossing over, earlier, swapped pieces between a pair, so the chromatids are no longer identical copies. The ovum is not a sperm. Both are haploid. Fertilisation adds 23 and 23. It does not halve anything.

In ordinary words

The parent cell is diploid, two sets of chromosomes. DNA is copied once. The nucleus divides twice. Crossing over, in the first division, swaps pieces between matching chromosomes. The chromosomes also line up in different combinations. The four cells that result are haploid, one set each. Fertilisation joins two haploid gametes and restores the diploid number. Mitosis does none of this halving.

In South Africa

The reason children in one family are not copies of each other is meiosis plus which gametes met.

Part 1 / 4

What it is

Meiosis is the division that makes gametes. It halves the chromosome number and shuffles the genes.

More worked examples

Board walkthroughs on this topic. Open one, copy the working, then try the next.

Example 1 / 20

A cell with 2n = 46 undergoes meiosis. How many cells form, and how many chromosomes does each contain?

How we got there

Read this like a teacher at the board. We go slowly, name every number, and say why we take each step.

Explain in another language · 11

1. What is the question asking?

Read the question twice. Circle the unknown. Underline every number and unit. The paper is asking: A cell with 2n = 46 undergoes meiosis. How many cells form, and how many chromosomes does each contain?

In other words — as if you have never seen this before

If you have never seen this before: Meiosis halves the chromosome number. We will name a method in words, write the formula with no numbers, then substitute one line at a time.

2. What do we already know?

2n=46.

3. Which method — and why this one?

Meiosis halves the chromosome number.

4. Step-by-step working

01 / 05
Identify the given
Meiosis I and II give 4 cells.
Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
02 / 05
Divide
462=23 chromosomes (n).
Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
03 / 05
Step G — Sense-check the size
Ask: is this the kind of number a Grade 10–12 paper would want? A speed of 3 000 ms for a learner walking, or a probability bigger than 1, means a unit or algebra slip. Estimate first: round the numbers and do the sum in your head.
Why this step: Markers write ‘unrealistic’ in the margin when the method is right but the size is wild. A 10-second head check often finds a missing 10² or a unit that never changed.
04 / 05
Step F — Box the answer with its unit
Four haploid cells with 23 chromosomes each.
Why this step: A number without a unit is half a mark on a Physics paper, and a marker can miss a Maths answer that is buried in working.
05 / 05
Step H — What the marker is looking for
On an NSC memo you usually earn: (1) a method mark for naming the right formula or idea in words, (2) an accuracy mark for substituting the given numbers with units, (3) an accuracy mark for the boxed answer with its unit. Write those three lines even if you are unsure of the arithmetic — blank pages score zero.
Why this step: First-timers chase the final number and leave the method blank. The memo still pays out for the idea and the substitution even when the last line is wrong.

5. The answer

Four haploid cells with 23 chromosomes each.

Copy onto your answer sheet — in this order

Answer: Four haploid cells with 23 chromosomes each.

6. Check it like a marker

Mitosis gives 2 diploid cells.

7. A trap first-timers fall into

First-timers often grab the first formula they remember, mix units (cm with m, minutes with seconds), or drop a minus sign. Write the formula in words first: “this equals that, because…”

Share on WhatsApp

Next lesson →

Reproduction in vertebrates

Life Sciences

Take topic quiz