The double-helix structure of DNA, enabling it to act as a stable information-carrying molecule, in terms of • the components of DNA nucleotides: deoxyribose, phosphate and the bases adenine, cytosine, guanine and thymine • two sugar-phosphate back bones held together by hydrogen bonds between base pairs • specific base pairing

DNA is often compared to a ladder that has been twisted, this is because it consists of two back bones bonded together and then twisted into a double helix.

The back bone is made up of a sugar, deoxyribose, and a phosphate group.

These two molecules are bonded together along with an organic base; together they are called a nucleotide.

Organic bases are the molecules that make up the code of DNA.

There are four different bases: cytosine (C); thymine (T); adenine (A); guanine (G).

C and T are single ring-bases. A and G are double-ring bases (so they are twice as long.)

Single rings only ever join with double rings so that the 'rungs of the ladder' are always three rings long.

The pairing goes: C with G; A with T.

The bases are joined together by hydrogen bonds. Two for A to T; three for C to G.

• Bases code for genes
• Hydrogen bonds can be easily broken for when DNA needs to replicate
• The back bone protects the bases
• The twisting makes it smaller so more information can fit
• Covalent bonds between the phosphate and deoxyribose make the back bone strong

NB: often deoxyribose is drawn as a pentagon.