DNADeoxyribonucleic acid (DNA) is a large molecule which stores the genetic information in organisms. It is composed of two strands, arranged in a double helix form. Each strand is composed of a chain of molecules called nucleotides, composed of a phosphate group, a five carbon sugar (pentose) called deoxyribose and one of four different nitrogen containing bases.

Each nucleotide is connected to the next by way of covalent bonding between the phosphate group of one nucleotide and the third carbon in the deoxyribose ring. This gives the DNA strand a “direction” – from the 5’ (“five prime”) end to the 3’ (“three prime”) end. By convention, a DNA sequence is always read from 5’ -> 3’ ends.

DNA nucleotides contain one of four different nitrogenous bases:

Chemical structure of adenine


Chemical Structure of guanine


Chemical structure of cytosine


Chemical structure of thymine


Each of these bases jut off the sugar-phosphate “backbone”. If the double helix of the DNA molecule can be thought of as a “twisted ladder”, the sugar-phosphate backbones form the “rails”, while the nitrogenous bases form the “rungs”.

The two strands of DNA are bound together by hydrogen bonding between the nucleotides. Adenine always binds to thymine and guanine always binds to cytosine. This means that the two strands of DNA are complementary. The complementary nature of DNA is allows it to be copied and for genetic information to be passed on - each strand can act as a template for the construction of its complementary strand.

The order of bases along a DNA strand is called the DNA sequence. It is the DNA sequence which contains the information needed to create proteins through the processes of transcription and translation.

Each strand of DNA is anti-parallel. This means that each strand runs in a different direction to the other – as one travels down the DNA duplex, one strand runs from 5’ -> 3’, while the other runs 3’ -> 5’.

John Kyrk has an excellent animation of the structure of DNA here.