Mitosis

Your body is made of trillions of cells, and new cells are being made all the time. The way most of your body cells make copies of themselves is by a type of cell division called mitosis.

In mitosis, one cell divides to make two new cells. The two new cells are called daughter cells, and the cell they came from is called the parent cell.

The key feature of mitosis is this:

The two daughter cells contain identical sets of chromosomes — exactly the same as each other and exactly the same as the parent cell.

Chromosomes are the long, thread-like structures inside the nucleus of a cell. They are made of DNA and carry the genes (the instructions that control the cell). Because both daughter cells get a complete, identical set of chromosomes, they end up carrying exactly the same genes as the parent cell.

Exam tip. The word the mark scheme looks for is identical. Mitosis produces two genetically identical daughter cells — not "similar", not "almost the same", but identical.

The cell that divides in mitosis is a diploid cell. "Diploid" means the cell has two complete sets of chromosomes — one set inherited from each parent. In humans, a diploid body cell has 46 chromosomes (23 pairs).

Here is what happens, step by step:

1. Before the cell divides, every chromosome is copied exactly. This makes an identical copy of all the DNA.
2. The copied chromosomes are then shared out equally between the two new cells.
3. Each daughter cell ends up with a full, identical set of chromosomes — the same number and the same DNA as the parent cell.

So a human cell that starts with 46 chromosomes divides to make two cells, each with 46 chromosomes. The chromosome number does not change, and the genetic information stays exactly the same.

Here is a simple diagram showing one cell dividing by mitosis into two identical cells:

Exam tip. If a question gives a chromosome number (e.g. "a cell with 12 chromosomes divides by mitosis"), each daughter cell has the same number — 12. The number is kept the same, not halved.

Because the two daughter cells contain identical sets of chromosomes, they carry exactly the same genes as each other and as the parent cell. Cells (or organisms) that are genetically identical are called clones.

So we can say:

The daughter cells produced by mitosis are genetically identical to the parent cell — they are clones.

This is very different from sexual reproduction, where offspring inherit a mixture of genes from two parents and are not identical. In mitosis there is only one cell making copies of itself, so there is no mixing of genetic information and no variation between the cells.

This "no variation" feature is why mitosis is used whenever an organism needs exact copies — for example, when growing, repairing tissue, or reproducing asexually.

Exam tip. Learn the link as a chain: identical chromosomes → identical genes → genetically identical → clones. Being able to use the word clones and explain why (identical chromosomes) is worth easy marks.

The specification names four situations where mitosis is used. You should be able to list all four and explain why each one needs identical cells.

Use of mitosis	What happens	Why identical cells matter
Growth	More cells are made so the organism gets bigger	New cells must be identical so the body works correctly
Repair	Damaged or worn-out cells are replaced (e.g. healing a cut)	Replacement cells must be the same type as the ones lost
Cloning	Identical copies of a cell or organism are produced	The whole point is to make exact genetic copies
Asexual reproduction	One parent produces offspring on its own (e.g. bacteria, some plants)	Offspring are genetically identical to the single parent

In all four cases the same rule applies: mitosis makes genetically identical cells, so the new cells (or whole new organisms) are clones of what they came from.

A handy way to remember the four uses is "G-R-A-C": Growth, Repair, Asexual reproduction and Cloning.

Exam tip. A common 4-mark question is "state four processes in which mitosis is involved" — the answer is simply growth, repair, cloning and asexual reproduction. Memorise all four so you never lose a mark by leaving one out.

Let us pull the whole topic together into one clear picture.

1. Mitosis starts with one diploid parent cell (two sets of chromosomes).
2. The chromosomes are copied so there is an identical set for each new cell.
3. The cell divides into two daughter cells.
4. Each daughter cell has an identical set of chromosomes — the same number and the same genes as the parent.
5. The daughter cells are therefore genetically identical — they are clones.
6. This division is used for growth, repair, cloning and asexual reproduction.

A short summary line you can use in the exam:

Mitosis divides a diploid cell into two genetically identical (clone) daughter cells, and is used for growth, repair, cloning and asexual reproduction.

Exam tip. If you only remember two things from this topic, make them: (1) the daughter cells are identical / clones, and (2) the four uses are growth, repair, cloning, asexual reproduction. Those two ideas answer the large majority of mitosis questions.