What is a genome?

Your genome is the instructions for making and maintaining you. It is written in a chemical code called DNA. All living things have a genome; plants, bacteria, viruses and animals.

Your genome is all 3.2 billion letters of your DNA. It contains around 20,000 genes. Genes are the instructions for making the proteins our bodies are built of – from the keratin in hair and fingernails to the antibody proteins that fight infection.

Genes make up about 1-5% of your genome. The rest of the DNA, between the genes, used to be called ‘junk’ DNA as it wasn’t thought to be important. But we now know that DNA between genes is important for regulating the genes and the genome, switching them on and off at the right time. There is still much more to learn about what it all does.

What is DNA?

DNA (deoxyribose nucleic acid) is a long molecule with a twisted, double helix shape. DNA is made up of four different chemicals, or bases. These are represented by the letters A, T, C and G. The bases are attached to two phosphate backbones. The bases are paired together; A with T, G with C. The two backbones twist around each other to give the characteristic double helix.

As well as being helix shaped, DNA is tightly packed so it takes up less space. If you stretched the DNA in one cell all the way out, it would be about 2m long.

What is a genome?

What is genome sequencing?

Sequencing is a technique that is used to ‘read’ DNA. It finds the order of the letters of DNA (A, T, C and G), one by one.

Sequencing a human genome means finding the sequence of someone’s unique 3 billion letters of DNA. Find out more about sequencing.

Why sequence a genome?

Learning more about genomes can help us to identify the cause of genetic diseases.

Some rare diseases are caused by as little as a single change (variant), like a spelling mistake, in someone’s DNA. Looking at the genome of a person affected by a rare disease can help find which DNA changes might be causing the problem.

For people with cancer, the cancer cells have developed a different genome to the healthy cells. Comparing the normal and cancer genomes may give clues about possible ways to treat the cancer. Find out more about genomics and cancer.

For some patients, knowing more about their genome may mean that a particular treatment can be recommended.

When the genome sequences of patients with the same condition are compared, it is possible to see patterns. These patterns can be put together with health information. Once this is done we may be able to link particular patterns with whether people are likely to become ill and, if so, how severe their illness is likely to be.

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