Free Biological Molecules MCQs with Answers

61. In double stranded DNA, the total amount of purines always equals the total amount of pyrimidines because

• A. the sugars bond only to purines
• B. phosphate groups pair with pyrimidines
• C. hydrogen bonds form only between bases of equal size
• D. each base pair contains one purine and one pyrimidine

Explanation: Adenine and guanine are the purines, thymine and cytosine the pyrimidines, and every pair across the helix matches one of each. This is why Chargaff found A plus G equals T plus C, and it keeps the two strands a constant distance apart.

62. 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.

63. 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.

64. 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.

65. The bond formed between two amino acids is called

• A. Glycosidic bond
• B. Peptide bond
• C. Ester bond
• D. Phosphodiester bond

Explanation: A peptide bond (amide bond) forms between the carboxyl group of one amino acid and the amino group of another, with loss of water.

66. Which test is used to detect proteins?

• A. Benedict's test
• B. Iodine test
• C. Biuret test
• D. Emulsion test

Explanation: Biuret test uses NaOH and CuSO4 to detect peptide bonds, giving a purple/violet color with proteins.

67. The secondary structure of protein includes

• A. α-helix and β-pleated sheet
• B. Quaternary folding
• C. Amino acid sequence
• D. Disulfide bridges

Explanation: Secondary structure involves local folding into α-helix or β-pleated sheet, stabilized by hydrogen bonds between backbone atoms.

68. Which lipid is a major component of cell membranes?

• A. Triglyceride
• B. Phospholipid
• C. Steroid
• D. Wax

Explanation: Phospholipids form the lipid bilayer of cell membranes due to their amphipathic nature (hydrophilic head, hydrophobic tails).

69. Starch and glycogen are polymers of

• A. β-glucose
• B. α-glucose
• C. Fructose
• D. Galactose

Explanation: Both starch and glycogen are polymers of α-glucose, linked by α-1,4 and α-1,6 glycosidic bonds.

70. Which vitamin is fat-soluble?

• A. Vitamin C
• B. Vitamin B12
• C. Vitamin D
• D. Vitamin B6

Explanation: Vitamins A, D, E, and K are fat-soluble. Vitamin C and B vitamins are water-soluble.