Deoxyribonucleic acid (DNA) is the information-carrying molecule found in all living organisms. In most animal, plant and fungal cells DNA is stored in the nucleus coiled up in thread-like structures called chromosomes.
The nucleus of one of your skin cells contains about two metres of DNA, so a chromosome is a very large molecule compacted into a very small space.
The information DNA contains is the instructions that the cell uses to make proteins. Proteins play a big part in determining the characteristics of specialised cells and whole organisms.
Your eye colour, muscle mass, height and even your ability to learn new skills all result from the activity of specific proteins. The building blocks of DNA are called nucleotides.
There are four different types of nucleotide. The part of a nucleotide that can make it different from others is called the base.
The four DNA bases are adenine, thymine, cytosine and guanine. These base names are abbreviated to A, T, C and G.
Nucleotides are linked together to form strands. DNA consists of two strands of nucleotides twisted around each other to form a shape called a double helix.
The two strands are held together by weak bonds between pairs of bases. Only certain pairs of bases have complementary shapes that let them form bonds with each other to make the double helix.
Base A bonds with base T and base G bonds with base C. These are complementary.