Question

How many strands make up a DNA double helix?

Answer 1

Watson and Crick first described the structure of the DNA double helix in 1953 using X-ray diffraction data of DNA fibers obtained by R.Franklin and M.Wilkins. Watson, Crick, and Wilkins were awarded the 1962 Nobel prize for Medicine for discovering the molecular structure of DNA. R.Franklin died of cancer before the Nobel Prize was awarded.

Complete answer:
The Watson-Crick double helix model describes the features of the B form of DNA. However, there are many other forms of DNA which are more distinct from the B form and are biologically important too. The conformation that DNA would adopt primarily depends on hydration level, DNA sequence, chemical modification of the bases, the type and concentration of metal ions in solution.

In human beings B form of DNA is present.
B-form of DNA has following major features:

1)Two polynucleotide strands make up a DNA double helix.
2)Two long polynucleotide strands are coiled around a central axis.
3)Strands are wrapped plectonemically in a right handed helix.
4)Strands are antiparallel i.e., one strand is oriented in the 5’to 3’ direction and the other in the 3’to 5’ direction.

Strands interact by hydrogen bonds between complementary base pairs.
G forms three hydrogen bonds with C
A forms two hydrogen bonds with T

Note: 1)Angle of interaction between base pairs result in major and minor grooves.
2)Helix diameter is 20 Å
3)Helix rise per base pair is 3.32 Å
4)Helix pitch (distance along the axis per 360 degree turn) is 33.2 Å
5)10.4 base pairs per helical turn
6)Base pairs are in the inside of the molecule stacked close to each other.