Hint: The Double helical structure of DNA was proposed by Watson and Crick. DNA is composed of two straight chains which run opposite to each other and are twisted in the form of Helix. The chain is made up of nucleotides. Both the chains are bound together by the hydrogen bond between the nucleotide bases.
To solve this question, we must know about the structure of DNA.
The structure of DNA looks like a twisted staircase. The DNA helix is made up of phosphate, sugar and nucleotide bases Adenine, guanine, cytosine, thymine. According to Watson and Crick model, the structure of DNA is as follows-
1.The DNA Helix is right handed.
2.The two strands of DNA are held together by hydrogen bonds between the nucleotides- A=T, C forms three hydrogen bonds with guanine.
3.The width of each turn is around 34 Angstrom and has around 10 base pairs per turn.
4.The frequency of twist depends on the stacking forces. Each successive turn in the stacking base pair is around 36 ͦ in a clockwise direction.
Hence, the correct answer is option (A)
Note: There are mainly three forms of DNA- A-form, B-form, Z-form. The double stranded right handed helix exists in the B-form which is the most common form of DNA. Z-DNA is a left handed helix. The main function of DNA is to carry the genetic material.