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Genes and DNA

This page is about what genes are and how they work. There is information about

 

Genes, chromosomes and DNA

We all hear about genes, chromosomes and DNA, but many people don’t really know how these fit together.

All body cells contain chromosomes. They are contained in the control centre of the cell – the nucleus. There are 23 pairs of chromosomes in human cells, making 46 in all.

Diagram of a chromosome in a cell

We have two copies of each chromosome - one which we inherit from our mother, and one which we inherit from our father.

The chromosomes are made of DNA. This is the genetic code – the blueprint for a human being. DNA is a bit like an instruction manual for building the body and keeping it healthy.

Each strand of DNA is really a long string of genes.

Diagram of a gene on a chromosome

As we inherit a copy of each chromosome from each parent, we also inherit a copy of each gene from each parent. Scientists now think there are about 25,000 different human genes. Over 99% of the genetic code is identical in all of us. There are small variations in DNA between people. It is these small variations – having different genes - that make us all different from each other.

Cells in the body are constantly growing and dividing to replace old or damaged cells. Proteins in the cell control this process. They are the substances that do all the work in the cell. Each gene has the code to make one particular protein.

 

How genes work

Each string of DNA has pairs of chemicals joined together all the way along it. There are 4 of these chemicals, known as bases. They are called

  • Adenine (A)
  • Thymine (T)
  • Guanine (G)
  • Cytosine (C)

Diagram showing a double helix of a chromosome

The bases in the DNA strings always link up in the same way.  C always joins up with G, and A with T. A particular series of these pairs in the DNA makes up one gene. They vary in length. There may be thousands of these pairs in a gene.

The order of the bases is the code for a protein. Each gene contains the code to make one protein. Rather amazingly, the pairs of bases can be arranged in enough ways to make all the different proteins we need. These proteins

  • Control what type of cell it is and what it does (its function)
  • Give us particular characteristics – for example, eye colour
  • Control the way the body functions
  • Turn other proteins on and off

Some genes work on their own to produce a protein while some genes need to work with other genes to make a protein.

 

How genetic mistakes come about

The body often needs new cells, to repair damage from injury or replace ageing cells. To make them, existing cells in the affected area divide into two.

First, the cell copies all of its DNA to make 2 complete sets of the genetic code – 1 set for each new cell. This means copying the sequence of thousands of A, T, C and G bases in exactly the right order. But during the copying process, mistakes can happen – in a similar way to someone introducing spelling mistakes when copying out a long manuscript.

Some of these mistakes have no effect. And some are repaired by the cell. Other mistakes can change the way a gene behaves. For example, it may mean that the gene is permanently switched on – or switched off. So the protein it makes may be made in far too large quantities, or not at all. As each protein has a job to do, these gene changes can directly affect how the cell behaves. 

Cells need multiple mistakes to change from a normal cell into a cancer cell. Some of these mistakes can mean that the cell is one step further along the road to becoming cancerous. If you look at the difference between cancer cells and normal cells, you’ll see that there are several specific differences that could come about because of changes to specific genes.

These genetic mistakes can happen to any body cells during the course of our lives. They are called acquired gene faults. They may happen by chance as new cells replace old, or because we have been exposed to things that damage cells and DNA. They do not affect all body cells and cannot be passed on in the family.

Genetic mistakes can also happen when eggs and sperm are being made. In this case, the mistake may be passed on to children in the genetic blueprint contained in either the sperm or egg. Once they’ve been passed on, they will be present in all the body cells of the children that inherit the gene fault. So the mistake can then be passed on to their children and down through the generations. This is how inherited gene faults originally happen.

 

How genetic mistakes can lead to cancer

Cells divide and grow throughout our lives. So as we get older our chance of getting cancer is higher. Our cells have divided more often, so there have been more chances for mistakes in the genetic code to be introduced.

Some behaviour also increases the number of chances for genetic mistakes to be introduced. If you smoke, you will be constantly damaging the cells lining your airways. So the body will need to make more new cells to repair the damage. This increased turnover of cells increases your risk of cancer. And some chemicals in the smoke are directly harmful to DNA, which further increases the chance of genetic mistakes.

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Updated: 24 July 2013