The double helical structure of DNA was proposed by James Watson and Francis Crick in 1953.
Each DNA molecule is made of 2 polynucleotide chains joined by hydrogen bonds between the bases.
The mononucleotides are linked together by a phosphodiester linkage. Sugar and phosphate are seen around the helix, (outside surface). They are exposed to the aqueous environment. The hydrophobic bases are located in the interior of the helix.
The position on the purine and pyrimidine rings are numbered and the position on the sugar carry a prime symbol to differentiate them from the ring position of the base.
The purine or pyrimidine bases are attached to 1’ carbon of the sugar. The phosphate is attached to 5’ carbon. The 3’ carbon is used for making bond with the phosphate of another nucleotide forming 3’ → 5’ phosphodiester bond.
The 2 chains of DNA are antiparallel. One chain runs in 3’ → 5’ direction and the other runs in 5’ → 3’ direction.
The base sequences of 2 strands are complementary.
Adenine of one chain pairs with the thymine on the other chain. There are 2 hydrogen bonds between adenine and thymine.
Guanine of one chain pairs with the cytosine of the other. There are 3 hydrogen bonds between guanine and cytosine.
The double stranded molecule is twisted to form a helix with major and minor grooves.
The diameter of the double helix is 20 Ao (angstrom) units. Each turn of the helix is 34 Ao units with 10 pairs of nucleotides.
The purine or pyrimidine bases are bonded to carbon atom 1 of the sugar whereas the individual nucleotides are united by means of a phosphatediester linkage between carbon atom 3 of one sugar molecule and a terminal carbon 5 of the next.
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