Free Classification of Biological Molecules MCQs with Answers

31. The backbone of a single strand of DNA consists of

• A. alternating bases and phosphate groups
• B. bases only, stacked one above another
• C. alternating deoxyribose sugars and phosphate groups
• D. alternating sugars and bases

Explanation: Phosphodiester bonds link the sugar of one nucleotide to the phosphate of the next, making a repeating sugar phosphate rail with the bases hanging off it as side groups. The bases face inward to pair with the opposite strand rather than forming the backbone.

32. DNA replication is described as semi conservative because each daughter molecule

• A. contains one old strand and one newly synthesised strand
• B. is built entirely from new nucleotides
• C. conserves only half of its original bases
• D. is rebuilt only in every second cell cycle

Explanation: When the two parental strands separate, each serves as a template for a new partner, so both daughter helices retain one original strand. This was proved by Meselson and Stahl using nitrogen isotopes, ruling out models in which old DNA was chopped up or fully conserved.

33. Chargaff's rules state that in any DNA molecule

• A. adenine equals guanine and cytosine equals thymine
• B. the amount of adenine is always greater than that of thymine
• C. the amount of adenine equals thymine and guanine equals cytosine
• D. purines are always more numerous than pyrimidines

Explanation: Because A pairs with T and G pairs with C across the helix, their amounts must match in any double stranded DNA. Pairing A with G instead would ignore the actual hydrogen bonding partners, and purines equal rather than outnumber pyrimidines.