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, or strand, 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 DNA molecule is directional, due to the
assymetrical structure of the sugars which constitute the
skeleton of the molecule. Each sugar is connected to the
strand upstream (i.e. before it in the chain) in its fifth carbon
and to the strand downstream (i.e. after it) in its
third carbon. Therefore we say that the DNA strand goes from
5' (read five prime) to 3'
(read three prime). The directions of the two
complementary DNA strands are reversed to one another.
The total length of the human DNA is estimated to be
base pairs
(abbreviated bp). The amount of DNA varies between different
organisms. The organism Amoeba dubia (a single celled
organism), for example, has more than 200 times the DNA than human.