DNA

The determination of the structure of DNA by Watson and Crick in 1953 is often said to mark the birth of modern molecular biology. The DNA is a large molecule composed of four basic units called nucleotides. Each nucleotide contains phosphate, sugar and one of the four bases: Adenine, Guanine, Cytosine and Thymine (usually denoted as A,G,C and T). The structure of DNA is described as a double helix. Each helix is a polymer of nucleotides chained together by phosphodiester bonds. The two helices are held together by hydrogen bonds. These bonds are formed by pairs of bases, with each base pair consisting of one purine base (A or G) and one pyrimidine base (C or T), paired according to the following rule: G pairs with C, and A pairs with T. The total length of the human DNA is about $3\times 10^9$ base pairs (abbreviated bp).

  

Figure: DNA