Double stranded DNA forms a helix structure and is found in this form in the nucleus of a eukaryotic cell. It is comprised of a sense and antisense strand that are complementary to each other and these bind to each other according to Watson-Crick base pairing rules. The nucleotide bases that make up DNA are guanine, cytosine, adenine and thymine. These bases are abbreviated to G, C, A and T respectively. Guanine always forms three hydrogen bonds with cytosine and adenine always forms two hydrogen bonds with thymine. Therefore the [G] = [C] and [A] = [T]. Double stranded DNA is split into single stranded DNA in the cell during processes such as DNA replication and transcription of DNA into mRNA. For a cell to fit all of its DNA into the nucleus, it must compress it down into chromosomes.