Question

Double helix model of DNA which was proposed by Watson and Crick was of
A. C – DNA
B. B – DNA
C. D – DNA
D. Z - DNA

Answer 1

The DNA double helix is probably the most vital molecule present in all living organisms. The double-helical structure of DNA was uncovered through the work of James Watson, Francis Crick, Rosalind Franklin and other researchers as well.

Complete answer:
The genetic material is composed primarily of subunits called nucleotides. Nucleotide comprises sugar (deoxyribose), a phosphate group, one of four nitrogenous bases: adenine (A), thymine (T ), guanine (G) or cytosine ( C). C and T bases, which have only one ring, are considered pyrimidines, while A and G bases, that have two rings, were termed purines. It's often believed that three DNA conformations are found in nature, A-DNA, B-DNA and Z-DNA. The B form specified by James Watson and Francis Crick is presumed to predominate in cells. It is 23.7 Å wide and 34 Å per 10 bp of sequence. The double helix makes a complete rotation of its axis every 10.4–10.5 base pair in solution. This rate of twisting (named helical pitch) largely depends on the piling forces which each base exerts over its neighbours throughout the chain. Adenine is always paired with Thymine and Guanine is paired with Cytosine, and the base pairing of opposite strands is stereo-chemically selective. For the A-T and G-C base pairs, respectively, two and three hydrogen bonds are formed.
So, the correct option B.

Additional information: Although grooves in B-DNA are of similar depth, the major groove is substantially broader than the minor groove. A-DNA has a major groove that is cavernous and a tiny groove that is very deep. With a shallow minor groove and the main groove into which bases are extruded, Z-DNA does have the opposite configuration.

Note: B-DNA is the most frequent, as well as the most variable in the structure of the three families of DNA helices. The coupling of bases through glycosidic linkages to the backbone sugars is asymmetrical. This results in the formation on opposite sides of the base pairs, the minor and major grooves, of two different grooves.