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What are we made of?

In this section you can learn about what we are made of - cells, proteins, genes, DNA, chromosomes - and how these are involved in cancer.


All complex life, from plants to pandas, is made up of clusters of cells - tiny bags of life containing proteins, fats, sugars and DNA, all working together.

Each one of us is made up of roughly a hundred million million (100,000,000,000,000) cells, and there are more than 200 different types of cells in the body.

Cells group together to form tissue that makes up organs like the brain, liver and muscles.

Cells within different tissues are highly specialised to do a particular job. The pictures below show different types of cells. The cells on the left are red blood cells, the ones in the middle are hair cells from inside the ear, while the cells on the right are fat cells.

Different types of cells

New cells don't just appear from nowhere. They are produced by existing cells dividing in two. This process is called the cell cycle.


A computer-generated image of a protein called p97. This protein is thought to be involved in the breakdown of other proteins.Proteins are the bits of the cell that actually do things. There are many thousands of different proteins with different jobs. They can:

  • act as scaffolding to maintain the cell's shape;
  • transport other proteins around the cell;
  • control chemical reactions;
  • turn genes on and off;
  • or even transmit messages from outside the cell to its control centre, the nucleus.

Proteins are made from long folded chains of smaller molecules called amino acids. The instructions for building each type from amino acids are all contained in the cell's DNA, which is found in the nucleus.


The lengths of DNA found in the nucleus of human cells, and are known as chromosomes. If all of the DNA in a single human cell was stretched out end to end, it would be about two metres long.

To pack its DNA into a manageable size, the cell winds its chromosomes around special proteins to form 'beads' called nucleosomes. These pack the chromosomes into a dense structure called chromatin. Human cells have 46 chromosomes in their nucleus, 23 that come from the person's mother, and 23 from their father.


A 3D model of the structure of a DNA double helix.A cell's DNA (which stands for Deoxyribo Nucleic Acid) is the 'manual' that tells a cell how to build new proteins. Like proteins, DNA is made of long chains of smaller molecules, known as nucleotides or bases. There are four different nucleotides: adenine (A), guanine (G), cytosine (C) and thymine (T).

These nucleotides have a special ability to stick together in pairs. T will bind tightly to A, but not C or G, Likewise, C will stick to G but not A or T. This property allows DNA molecules to exist as two long strands of 'complementary' molecules, which wrap around each other to form the famous double helix.

Find out more about the structure of DNA.

Because DNA contains the instructions that our cells needs to grow and function correctly, any damage to DNA can be extremely harmful to a cell. So there are a variety of safety measures in place. Cells contain special proteins that 'proofread' the genetic code and repair any small errors. And if the damage is severe, the cell commits suicide, a process called apoptosis.

Sometimes, the damage to DNA is severe enough to make a faulty protein, but not severe enough to trigger apoptosis. The resulting faulty cell can divide, passing on its damaged DNA to its daughter cells.

Cancer Research UK is leading the world in understanding how DNA damage occurs, what causes it, and how it can be prevented.


The region of a chromosome that tells a cell how to make an individual protein is called a gene; the collection of all the 30,000 genes on our 46 chromosomes is called the human genome. The sequence of bases in each gene forms a code. This is called the genetic code, common to almost all life on the planet.

The genetic code tells the cell in which order to join the amino acids to build a particular protein chain. Special molecules 'read' the gene sequence and 'decode' it to read off the correct sequence of amino acids to build the protein, a bit like an old-fashioned punch-card computer.


Occasionally, DNA can become damaged, or mutated, and the sequence of nucleotides in a gene is altered. This damage can cause the gene to make a protein that may not work properly or is overactive, or to make no protein at all.

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Updated: 25 September 2009