Chromosomes, genes and proteins : Cancer Research UK
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Chromosomes, genes and proteins


Human chromosomes stained with a dye and viewed with a microscope. Courtesy of Denise Sheer at the Cancer Research UK Human Cytogenetics Laboratory. The DNA inside our cells is very long - nearly 2 metres - so it is tightly coiled up and packaged into structures called chromosomes. Human cells have 23 pairs of chromosomes. One set of chromosomes are passed on from our mother and the other set is passed on from our father. Our chromosomes are arranged into 22 different pairs plus the sex chromosomes X and Y, that determine if you are male or female. Males have an X and a Y chromosome and females have two X chromosomes.

All living cells have DNA but not all cells have the same number of chromosomes. For example, dogs have 39 pairs of chromosomes in each cell and tomatoes have 12 pairs of chromosomes. Bacteria have less DNA than humans and it isn't packaged into chromosomes.


Your DNA carries all the instructions needed to build your body and maintain its functions. The information stored in our DNA would fill 200 telephone directories! Each instruction is carried on a unique piece of DNA called a gene.

Genes can be turned on or off depending on the job a cell needs to get done. This process is a bit like opening or closing an instruction manual, so that the instructions can be read or not. If a gene is damaged or faulty the cell may not be able to read the instructions, or it might get them wrong.

Each cell contains a full set of all our genes - there are around 30,000 of them!


A gene carries the instructions for making a particular protein. Proteins are the molecules in cells that actually do most of the work. Proteins are built from building blocks called amino acids.

There are only 20 different amino acids but they can be arranged in many different ways to make up the hundreds of thousands of proteins that exist in our bodies.

Examples of proteins are enzymes, such as pepsin, and hormones, such as insuln, that carry out specific jobs in the cell. Proteins also make up the structure of our tissues such as collagen in skin and bones and keratin in hair and nails.

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Updated: 25 September 2009