Q.Describe the double-stranded DNA structure proposed by Watson and Crick. OR Describe the Fluid-Mosaic model of the plasma membrane.

Of the four statements, (ii), (iii) and (iv) correctly describe DNA structure; (i) wrongly names the sugar-phosphate linkage.

Evaluating each statement against the Watson-Crick double-helix model of DNA: (i) is FALSE — the bond between the phosphate group and the 3'/5'-hydroxyl groups of adjacent sugars is a PHOSPHODIESTER bond, not a glycosidic bond (a glycosidic bond, by contrast, links the nitrogenous base to the sugar, i.e. an N-glycosidic bond). (ii) is TRUE — alternating sugar and phosphate groups linked by phosphodiester bonds form the continuous backbone of each polynucleotide strand. (iii) is TRUE — the nitrogenous bases are attached to the sugars and project inward, oriented more or less perpendicular to the sugar-phosphate backbone, pairing with a complementary base on the opposite strand (A with T, G with C, via hydrogen bonds). (iv) is TRUE — the two polynucleotide strands of the DNA do …

The two strands of the DNA double helix are a constant 20 A (2 nm) apart, a spacing maintained by the regular geometry of complementary base-pairing.

DNA is a double helix made of two polynucleotide chains coiled around a common axis, running in antiparallel directions and held together by hydrogen bonds between complementary nitrogenous bases (adenine pairs with thymine via two hydrogen bonds; guanine pairs with cytosine via three hydrogen bonds). Because every base pair (whether A-T or G-C) has essentially the same width, the two sugar-phosphate backbones stay a uniform distance apart …

Thymine is the base found in DNA but replaced by uracil in RNA.

Both DNA and RNA are nucleic acids built from nucleotides, and they share three nitrogenous bases: adenine (A), guanine (G), and cytosine (C). The key difference lies in the fourth pyrimidine base: DNA uses THYMINE (T), which pairs with adenine, whereas RNA uses URACIL (U) instead of thymine, which also pairs with adenine. (Additionally, DNA's sugar is deoxyribose while RNA's is ribose, …

In the Watson–Crick B-form DNA double helix, one complete turn measures 3.4 nm and each stacked base pair contributes 0.34 nm of rise, so 3.4 nm ÷ 0.34 nm = 10 base pairs per turn.

Watson and Crick's 1953 double helix model describes DNA as two antiparallel polynucleotide chains coiled around a common axis, held together by hydrogen bonds between complementary bases (A=T, G≡C).

Key helical parameters of B-DNA:

• Diameter of the helix: 2 nm
• Distance between two consecutive base pairs (rise per bp): 0.34 nm …