Chromosomes, mitosis and the cell cycle – GCSE Combined Science Biology

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Your body is made of billions of cells, and new ones are needed all the time. This topic is about how one cell makes two new cells that are exact copies of itself. This is called mitosis.

It matters because it is how you grow, how a cut heals and how some living things reproduce. It also explains what goes wrong in cancer.

By the end you will be able to describe the steps of the cell cycle and mitosis. You will say why mitosis is useful. You will also work out how many cells there are after a number of divisions.

Genes, DNA and chromosomes

Inside the nucleus of a cell is DNA. DNA is a long molecule that stores the instructions for building and running the body. Think of it as a huge recipe book.

A gene is one short section of DNA. It is like a single recipe, and it holds the code for one protein. The DNA is packed into long threads called chromosomes.

Human body cells have 46 chromosomes, arranged in 23 pairs. One chromosome in each pair comes from each parent. The two in a pair carry the same genes in the same places.

The cell cycle

The cell cycle is the series of stages a cell goes through before it divides. A cell cannot split in half straight away. It has to get ready first.

First the cell grows. It makes more organelles, such as mitochondria and ribosomes. Organelles are the small parts inside a cell. It also makes the proteins and the energy it needs for dividing.

Next the DNA is copied, so there are two full sets of chromosomes. Last comes mitosis, and then the cell splits in two.

Mitosis

Mitosis is the stage where the nucleus divides. The copied chromosomes line up in the middle of the cell. The two copies of each chromosome are pulled to opposite ends. Two new nuclei form.

After that the cytoplasm and cell membrane divide, which gives two new cells. These are called daughter cells. Each has the full 46 chromosomes in humans.

Think of photocopying a worksheet and giving one copy to each of two friends. Both friends end up with the whole worksheet. The daughter cells are genetically identical to each other and to the parent cell.

The stages have names. Interphase is when the cell grows and copies its DNA. In prophase the chromosomes become visible and the nuclear membrane breaks down. In metaphase the chromosomes line up along the middle of the cell. In anaphase the two copies of each chromosome are pulled to opposite ends. In telophase a new nuclear membrane forms around each set. Cytokinesis is when the cytoplasm and cell membrane divide.

The two daughter cells are diploid. They have two sets of chromosomes, 46 in humans, and are identical to the parent cell. Mitosis is used for growth, repair and, in some organisms, asexual reproduction.

Why mitosis matters

Mitosis is how a many-celled organism grows. It is also how damaged cells are replaced, such as skin cells after a graze.

Some living things use mitosis to reproduce. This is asexual reproduction, where there is only one parent. The offspring are clones, which means they have exactly the same genes as the parent.

Because mitosis makes identical cells, it cannot produce variation. Variation means differences between individuals. That comes from sexual reproduction instead.

Counting cells

Each division doubles the number of cells. One cell becomes 2, then 4, then 8. After n divisions there are 2ⁿ cells, if you started with one cell.

Think of folding a sheet of paper in half again and again. One fold gives 2 layers, two folds give 4 and three folds give 8. It doubles each time. It does not go up by 2 each time.

Try it. A cell divides once every 20 minutes. In one hour that is 3 divisions, so 2³ = 8 cells.

Cancer

Genes control cell division. If these genes change, a cell can divide without control. Think of a car with a stuck accelerator. The cell cannot slow down.

The cells build up into a lump called a tumour. A benign tumour stays in one place. A malignant tumour invades other tissues. Its cells can travel in the blood and form secondary tumours. This is cancer.

Some risk factors are smoking, ultraviolet radiation and certain genes that are passed on in families.

Recap

DNA is packed into chromosomes in the nucleus. Human body cells have 46.

The cell grows and copies its DNA first. Then mitosis makes two identical cells.

Mitosis is used for growth, repair and asexual reproduction. After n divisions there are 2ⁿ cells.

Uncontrolled mitosis forms a tumour. A malignant tumour spreads and causes cancer.