A model that shows molecular structure of DNA was proposed by Watson and Crick in 1953. Two polynucleotide chains operated in antiparallel direnctions in a DNA molecule. They form a spiral helical structure by twisting around each other. A phosphor-diaster bond links a nucleotide to another. Two adjacent nucleotides are separated in a distance of 3.4 A°, meanwhile the two opposite chains are separated in a distance of 20 A°.
Phosphate and sugar are linked to nitrogenous bases in the chain, in which weak hydrogen bonds link nitrogenious base of purine type nitrogenous base of pyrimidine type. The ration is 1:1, meaning to say that the number of pyrimidine and purine in a DNA molecule is always the same.
Complimentary pairing is a pairing consisting Adenine(A) which is linked to Thymine(T) by two weak hydrogen bonds and Guanine(G) which is linked to Cytosine(C) by three weak hydrogen bonds. A helix has 10 nitrogenous base pairs. Its length is 34 A°.
