TL;DR:
Mitosis makes 2 genetically identical cells, each with 46 chromosomes in a human. Meiosis makes 4 different cells with 23 each.
Meiosis halves the chromosome number so fertilisation restores 46 (
23 + 23 = 46).Independent assortment alone allows
2²³combinations of chromosomes in one person's sex cells, and crossing over adds more.
Your body started as one cell and now has trillions. Each new cell needs a full copy of your DNA. Yet to make a baby, a sperm and an egg must each carry only half of it. These two jobs need two different kinds of cell division: mitosis for growth and repair, and meiosis for making sex cells.
Chromosome counts: diploid and haploid
A normal human body cell has 46 chromosomes, arranged as 23 homologous pairs. One chromosome of each pair came from your mother and one from your father. A cell with two sets is diploid (2n). A cell with a single set, 23 chromosomes, is haploid (n).
Before any division, the cell copies its DNA. Each chromosome then consists of two identical sister chromatids joined together. A cell with 46 chromosomes still has 46 chromosomes after copying, but each one is doubled, so there are 92 chromatids.
Mitosis: one cell becomes two copies
Mitosis has four main phases, followed by cytokinesis, where the cell pinches in two.
Prophase: chromosomes condense and become visible, and the nuclear envelope breaks down.
Metaphase: chromosomes line up along the middle of the cell.
Anaphase: the sister chromatids are pulled apart to opposite ends.
Telophase: new nuclei form around the two sets, and the chromosomes relax.
Result for a human cell: the 92 chromatids separate into two groups of 46. Each new cell has 46 chromosomes, diploid, and is genetically identical to the parent. Check: 46 → 2 cells × 46.
Meiosis: one cell becomes four different cells
Meiosis has two rounds of division after a single copying of DNA.
Meiosis I: homologous chromosomes pair up, and the pairs are separated. Each daughter cell gets one chromosome from each pair, so the count drops from 46 to 23, though each chromosome is still doubled.
Meiosis II: sister chromatids separate, much like in mitosis. The result is four cells with 23 single chromosomes each.
Track a human cell: 46 chromosomes → copy → meiosis I → 2 cells × 23 (doubled) → meiosis II → 4 cells × 23. These are haploid sex cells. At fertilisation, 23 + 23 = 46, restoring the diploid number.
Worked example: why meiosis creates variety
There are two sources of variety. The first is crossing over: during meiosis I, homologous chromosomes swap pieces, so each chromosome is a mix of both parents' DNA.
The second is independent assortment. When the 23 pairs line up in meiosis I, each pair faces either way, independently of the others. For each pair there are 2 choices, so the number of different combinations is 2 × 2 × ... × 2 with 23 factors, which is 2²³ = 8,388,608.
That is the count from this one source in one person, before crossing over is added. Two parents each contribute a sex cell, so the combinations from both sides multiply: 2²³ × 2²³ = 2⁴⁶, which is about 7.0 × 10¹³. This is why siblings, other than identical twins, differ from each other.
Feature | Mitosis | Meiosis |
|---|---|---|
Purpose | Growth and repair | Making sex cells |
DNA copies | 1 | 1 |
Divisions | 1 | 2 |
Cells produced | 2 | 4 |
Chromosomes per cell (human) | 46 | 23 |
Genetically identical? | Yes | No |
Errors in meiosis can change chromosome numbers. If a pair fails to separate, a sex cell can end up with one chromosome too many or too few, and a fertilised egg then has a wrong total. Some of these conditions are well known in genetics, and they show how carefully the process normally runs. Your course may cover specific examples.
One more habit helps: when you read a question, first decide whether it is about a body cell or a sex cell, since that tells you whether to expect 46 or 23 chromosomes in the products. Cells also control when to divide. A cell cycle has a growth stage, a DNA copying stage and checkpoints that stop the process if something is wrong. When those controls fail and cells divide without limit, the result can be a tumour, which is why mitosis matters for understanding cancer.
How to check your answer
Count cells. Mitosis gives 2, meiosis gives 4.
Count chromosomes. Mitosis keeps 46, meiosis halves it to 23.
Fertilisation. Two haploid cells must add up to the diploid number.
Chromosomes versus chromatids. After copying, 46 chromosomes have 92 chromatids.
Mistakes that cost marks
Saying DNA is copied twice in meiosis. It is copied once and the cell divides twice.
Saying the chromosome number doubles after copying. The chromatid number doubles, not the chromosome count.
Mixing up what separates when: homologous pairs in meiosis I, sister chromatids in anaphase of mitosis and in meiosis II.
Thinking mitosis makes variety. Apart from rare mutations, its products are identical.
Skipping cytokinesis as part of the process.
Where you will use it next
Cell division connects to genetics, inheritance and cancer, which involves division that has lost its controls. Division uses energy made by cellular respiration and ATP, and the sugar behind that comes from photosynthesis.
Frequently asked questions
Do all human cells have 46 chromosomes?
Most body cells do. Sperm and egg cells have 23, and a few specialised cells, such as mature red blood cells, have no nucleus at all.
Why are identical twins identical?
They come from one fertilised egg that split in two early on, and mitosis copied the same DNA into both.
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