DNA: the double helix and its four bases

0:00DNA holds the instructions for every living thing. But what does it look like?

0:06DNA has two strands. Each has a backbone, a chain of sugar and phosphate.

0:13Each sugar carries one of four bases: adenine, thymine, cytosine and guanine.

0:21Across the middle, A always pairs with T, and C always pairs with G.

0:27Weak hydrogen bonds hold each pair together.

0:31Together, one sugar, one phosphate and one base make a unit called a nucleotide.

0:38The two strands twist around each other into a spiral called a double helix.

0:45In 1953, Watson and Crick described the double helix, using Rosalind Franklin's X-ray image.

0:52The order of the bases along a strand is a code for building proteins.

0:59Each group of three bases stands for one amino acid. A chain of amino acids makes a protein.

1:06To copy itself, DNA unzips. The hydrogen bonds break and the two strands come apart.

1:14Each strand is a template, a pattern for a new strand. New bases pair up, A with T and C with G.

1:23This makes two identical copies of DNA, each with one old strand and one new strand.

1:31When a cell divides, each new cell gets a copy, so nearly every cell in your body has the same DNA.

1:39So why are skin and muscle cells different? They use different parts of the same DNA.

1:45DNA is a double helix of base pairs, A with T and C with G, and the order of its bases is a code.