The salient features of the Double-helix structure of DNA are as follows:
It is made of two polynucleotide chains , where the backbone is constituted by sugar-phosphate, and the bases project inside .
The two chains have antiparallel polarity.
It means, if one chain has the polarity 5'Ã 3', the other has 3'Ã 5'. Hence statement 1 is correct.
The bases in two strands are paired through hydrogen bonds (H-bonds ) forming base pairs (bp).
Hence statement 2 is correct.
Adenine forms two hydrogen bonds with Thymine from the opposite strand and vice-versa.
Similarly, Guanine is bonded with Cytosine with three H-bonds .
As a result, always a purine comes opposite to a pyrimidine.
This generates an approximately uniform distance between the two strands of the helix.
The two chains are coiled in a right-handed fashion.
Hence statement 3 is correct.
The pitch of the helix is 3.4 nm (a nanometre is one-billionth of a metre, that is
10−9 m ) and there is roughly 10 bp in each turn. Consequently, the distance between a bp in a helix is approximately 0.34 nm.
The plane of one base pair stacks over the other in a double helix . This, in addition to H-bonds, confers stability of the helical structure.
The proposition of a double helix structure for DNA and its simplicity in explaining the genetic implication became revolutionary.
Francis Crick proposed the Central dogma in molecular biology , which states that the genetic information flows from DNA to RNA to Protein.
In some viruses the flow of information is in the reverse direction , that is, from RNA to DNA.