Mitosis makes two genetically identical cells with the same chromosome number as the parent. Meiosis makes four genetically different cells with half the chromosome number. This contrast appears in short recall questions and in longer questions about fertilisation and variation.
It is the first lesson in cell division and inheritance, and the later lessons on alleles and crosses depend on it.
Why do cells divide in two different ways?
Body cells divide to grow, repair tissue and, in some organisms, reproduce asexually. These jobs need new cells that are exact copies, so mitosis is used.
Sex cells (gametes) have a different job. Two gametes will fuse at fertilisation, so each must carry half the usual set. Meiosis does this, and it also shuffles the genetic material, which is why offspring differ from their parents.
How to compare them, step by step
Use the same four checks every time.
- Where does it happen? Mitosis happens in body cells. Meiosis happens only in the organs that make gametes.
- How many cells result? Mitosis gives 2. Meiosis gives 4.
- What is the chromosome number? Mitosis keeps it the same (diploid to diploid). Meiosis halves it (diploid to haploid).
- Are the cells identical? Mitosis cells are identical. Meiosis cells are genetically different from each other.
Worked example
A fictional insect has a body cell with 12 chromosomes (6 pairs). Describe the products of mitosis and of meiosis, then state the chromosome number of the zygote.
Mitosis: the cell divides once to give 2 cells. Each keeps 12 chromosomes and is genetically identical to the parent.
Meiosis: the cell gives 4 cells. The number is halved, 12 ÷ 2 = 6, so each gamete has 6 chromosomes. The four gametes are genetically different from each other.
Fertilisation: a sperm with 6 chromosomes fuses with an egg with 6 chromosomes. The zygote has 6 + 6 = 12 chromosomes, which restores the normal number.
Check: 12 ÷ 2 = 6 and 6 + 6 = 12, so the numbers are consistent. The zygote then grows by mitosis, so every body cell of the new insect also carries 12.
The mistake to watch for
A common slip is to say that meiosis “divides the cell in half”, so there are 2 cells with 6 chromosomes each.
Mistaken answer: “Meiosis makes two cells with 6 chromosomes.”
The chromosome number is right but the cell count is wrong, and the cells are not identical.
The correction is to remember that the cell divides twice in meiosis, which gives four cells. A quick way to check is to ask: “how many cells, how many chromosomes, identical or different?” and answer all three every time.
Check yourself
1. A human skin cell has 46 chromosomes. How many chromosomes are in each cell after it divides by mitosis?
Show answer
Mitosis keeps the number the same. Each of the two new cells has 46 chromosomes.
2. A fictional fish has 20 chromosomes in a body cell. How many gametes are made from one cell by meiosis, and how many chromosomes does each have?
Show answer
Meiosis gives 4 gametes. The number is halved: 20 ÷ 2 = 10 chromosomes in each.
3. Explain why meiosis must halve the chromosome number.
Show answer
Fertilisation joins two gametes. If each gamete had the full number, the zygote would have double, and the number would keep doubling each generation. Halving in meiosis means the zygote receives the normal number again.
Where this leads next
The next step is to name what sits on the chromosomes, in distinguishing gene, allele and chromosome. After the five lessons, test yourself with the cell division and inheritance practice set, and use the inheritance model board to see gametes combine in a fictional model.
Some students remember the facts but mix up the counts when the numbers change. That is the kind of pattern our teachers look for in online one-to-one Biology tuition.