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Human body

Genes and DNA

The DNA moleculeThe DNA molecule Every cell in your body carries your own, unique set of coded instructions—unless you are an identical twin, in which case you share this code with your twin. The instructions, called genes, are found in two sets of 23 chromosomes located inside the nucleus of body cells. Chromosomes are made primarily of a substance called DNA. The genes are "written" in the chemical structure of the long DNA molecule. They contain all the information a living thing needs to develop, grow and maintain itself through life. Your body has about 23,000 genes: the human genome. They determine your hair colour, your height, any tendency to develop certain diseases, and so on.


Chromosomes inside a cell nucleusChromosomes inside a cell nucleusYou inherited one set of 23 chromosomes from your mother and one set from your father. Together they make up the 46 paired chromosomes inside your cells. Each set contains the full range of around 23,000 genes. You might inherit an eye colour gene for blue eyes from your mother, and a matching gene, but for brown eyes, from your father. One pair of chromosomes, the sex chromosomes, determine whether you are female or male. Females have the same kind of sex chromosome, called XX, while males have two distinct sex chromosomes, called XY.


Protein-coding genes make up just 2% of all your DNA, so what is the rest of it for? It used to be called "junk DNA" but scientists now call it non-coding DNA. It includes “switches" that turn some genes on and others off.

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