Deoxyribonucleic acid (DNA) is a nucleic acid that encodes genetic instruction essential to the development and function of living organisms.
Structure and replication
The structure of DNA is an antiparallel double helix
The linear backbone of a DNA molecule consists of alternating sugar residues and phosphate groups. The bond linking an individual sugar residue to the neighboring sugar residues is a 3', 5'-phophodiester bond, a phosphate group links a carbon atom 3' of a sugar to carbon atom 5' of the neighboring sugar.
The structure of DNA is a double helix in which two DNA molecules, DNA strands, are held together by weak hydrogen bonds to form a DNA duplex. Hydrogen bonding occurs between laterally opposed bases, base pairs (bp), of the two strands of the DNA duplex according to Watson-Crick rules: Adenine specifically binds to Thymine and Cytosine specifically binds to Guanine. As a result, the base composition of DNA from different cellular sources is not random: the amount of adenine equals that of thymine, and the amount of cytosine equals that of guanine. The base composition of DNA can therefore be specified unambiguously by quoting its %GC (= %G + %C) composition. For examples, if a source of cellular DNA is quoted as being 42% GC, the base composition can be inferred to be : G, 21%; C, 21%; A, 29%; T, 29%.
DNA can adopt different types of helical structure. A-DNA and B-DNA are both right-handed helices (ones in which the helix spirals in a clockwise direction as it moves away from the observer). They have respectively 11 and 10 bp per turn. Z-DNA is a left-handed helix which has 12 bp per turn. Under physiological conditions, most of the DNA in a bacterial or eukaryotic genome is of the B-DNA form. Here each helical strand has a pitch (the distance occupied by a single turn of the helix) of 3.4 nm. As the phosphodiester bonds link carbon atoms number 3' and number 5' of the successive sugar residues, one end of each DNA strand, the so called 5' end, will have a terminal sugar residue in which carbon atom number 5' is not linked to a neighboring sugar residue. The other end is defined as the 3' end because of a similar absence of phosphodiester bonding at carbon atom number 3' of the terminal sugar residue. The two strands of a DNA duplex are said to be antiparallel because they always associate (anneal) in such a way that the 5'→3' direction of one DNA strand is the opposite to that of its partner.