Q.(a) Explain the structure of DNA. OR

The DNA backbone is a sugar–phosphate chain; the correct option is (a).

Each strand of the DNA double helix is a polynucleotide. Its backbone (the two side 'pillars' of the ladder) is formed by alternating deoxyribose sugar and phosphate groups joined by phosphodiester bonds. The nitrogenous bases project inward from this sugar–phos …

Nucleotides in a DNA strand are linked by covalent (phosphodiester) bonds.

Within a single polynucleotide strand, one nucleotide is joined to the next by a phosphodiester linkage: the phosphate group at the 5' carbon of one sugar is covalently bonded to the 3'-OH of the sugar of the adjacent nucleotide. These are strong covalent bonds that form the sugar-phosphate backbone. (Hydrogen bonds, by contrast, only hold the t …

Maurice Wilkins and Rosalind Franklin provided the X-ray diffraction photographs of DNA.

The discovery of the DNA double helix drew on several lines of evidence:

• Maurice Wilkins and Rosalind Franklin obtained the X-RAY DIFFRACTION patterns of DNA fibres, which revealed its helical, regularly repeating structure. …

The classic Watson–Crick double helix, the most common and biologically dominant conformation of DNA, is designated the B-form.

In 1953, James Watson and Francis Crick proposed a model for the structure of DNA, drawing on Rosalind Franklin and Maurice Wilkins's X-ray diffraction images. Their model describes DNA as:

• A right-handed double helix of two antiparallel polynucleotide strands.
• A sugar–phosphate backbone on the outside, with nitrogenous bases paired on the inside (A with T via 2 hydrogen bonds, G with C via 3 hydrogen bonds), stacked like the rungs of a twisted ladder.
• A helix diameter of about 20 Å, with about 10 base pairs per complete turn and a rise of about 3.4 Å per base pair (about 34 Å per turn). …

James Watson and Francis Crick proposed the double helix model of DNA in 1953.

Watson and Crick, using X-ray diffraction data (notably Rosalind Franklin's Photo 51) along with Chargaff's base-pairing ratios, built a model showing that DNA consists of two polynucleotide strands wound around each other in a right-handed double helix. The two strands run antiparallel to each other, with the sugar-phosphate backbone on the outside and the nitrogenous bases (adenine pairing with thymine via two hydrogen bonds, and guanine pairing with cytosine via three hydrogen bonds) facing inward. This 1953 model earned Watson, Crick and Wilkins the Nobel Prize in 1962.

…

Watson and Crick (along with Maurice Wilkins) received the Nobel Prize in Physiology or Medicine in 1962 for discovering the double-helical structure of DNA, published originally in 1953.

Watson and Crick proposed the double helix model of DNA structure in 1953, building on Rosalind Franklin's and Maurice Wilkins's X-ray diffraction data (notably "Photo 51"). Nine years later, in 1962, the Nobel Prize in Physiology or Medicine was awarded jointly to James Watson, Francis Crick, and Maurice Wilkins "for their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material." Rosalind F …

The DNA double helix has a constant diameter of 20 Å between its two sugar-phosphate strands, because a purine always pairs with a pyrimidine.

According to the Watson-Crick model of DNA, the molecule is a right-handed double helix made of two antiparallel polynucleotide chains coiled around a common axis. The distance between the two strands (the diameter of the helix) is uniform at 20 Å throughout the molecule - this uniformity is possible only because a two-ringed purine (A or G) always pairs with a one-ringed pyrimidine (T or C) via hydrogen bonds …