Which of the following are the salient features of the Double-helix structure of DNA? 1. The two chains have antiparallel polarity. 2. The bases in two strands are paired through hydrogen bond (H-bonds) forming base pairs (bp). 3. The two chains are coiled in a right-handed fashion.
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'à 3', the other has 3'à 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.