Key points

• DNA is the genetic code which makes up genes, which are responsible for giving an organism a specific characteristic.

• Watson and Crick, with help from Franklin and Wilkins, discovered the double helix structure of DNA in 1953.

Video - The importance of DNA

The structure of DNA

stands for deoxyribonucleic acid. It is a chemical made up of two long strands, arranged in a spiral. This is the double-helix structure. DNA carries genetic information - the genetic code. It has all the instructions that a living organism needs to grow, reproduce and function. DNA is passed on from parents to their offspring during fertilisation.

DNA is made from four . These are made from the : A and T, G and C. These can be either way around, but A always pairs with T and G with C. The base pairs are held together by weak chemical hydrogen bonds.

DNA is present in the of almost every cell in your body. In fact, there are around two metres of DNA packed into each nucleus of your body cells. Your sex cells or gametes, sperm or ova, only have half of this, because they join the other cell during .

All life on Earth has the same genetic code. The same four letters make up bacteria, fungi, animals - including humans -, plants and single-celled .

From DNA to the genome

are small sections of DNA that are the genetic code for an inherited characteristic such ear shape or eye colour. You inherit pairs of genes for most characteristics from each of your parents. So you have two genes for ear shape and two for eye colour. are pairs of genes that are inherited – one from each parent.

There are around 23 000 genes that make up a human.

Genes are arranged into larger coiled structures called . Humans have 46 chromosomes in total. We inherit 23 from each of our parents. Other organisms have different amounts. The fruit fly has only 8 chromosomes and is often used to study patterns of inheritance, while some ferns have over one hundred.

All of the chromosomes of an organism add up to make its genome. This is one copy of all its DNA. Every human on Earth has a unique genome unless they are identical twins who were made from the same fertilised ovum (egg cell). Non-identical twins were made from two separate sperm fertilising two different ova (egg cells) and so have different genomes.

DNA makes up genes which make up chromosomes which make up the genome. A good model for this is thinking about money. Pennies make up £1 coins which make up £5 notes which make up £20 notes.

The discovery of DNA

Scientists worked out the structure of DNA in the 1950s. Rosalind Franklin made ‘X-ray diffraction’ images of DNA. James Watson and Francis Crick used information from one of her images to work out a model for the structure of DNA. Work by Maurice Wilkins, a colleague of Franklin, supported their model.

Video - Brian Cox explains DNA

Can you answer these questions based on the video?

1. Which two scientists discovered the structure of DNA?

2. Who helped them by taking Photograph 51?

Test your knowledge

Quiz - What is DNA?

Test questions

1. Describe how genetic information is stored in living cells.

2. Describe how the structure of DNA was discovered.