Free Biological Molecules MCQs with Answers

81. Which is a stop codon?

• A. AUG
• B. UAA
• C. UUU
• D. GGC

Explanation: UAA, UAG, and UGA are stop codons (nonsense codons) that terminate translation.

82. The genetic code is

• A. Overlapping
• B. Non-overlapping and degenerate
• C. Ambiguous
• D. Different for each organism

Explanation: The genetic code is non-overlapping, degenerate (multiple codons for one amino acid), and universal.

83. Reverse transcriptase is found in

• A. All bacteria
• B. Retroviruses
• C. All eukaryotes
• D. Plants only

Explanation: Reverse transcriptase synthesizes DNA from an RNA template, found in retroviruses like HIV.

84. The complementary base pairing in DNA is important because it:

• A. Maintains the tertiary structure of enzymes
• B. Provides energy for all cell metabolism
• C. Enables protein to fold properly
• D. Allows DNA act as a genetic blueprint during replication

Explanation: Because adenine always pairs with thymine and guanine with cytosine, each strand specifies the sequence of the other, so separating the two gives two templates that produce identical copies. This is precisely what Watson and Crick noted on proposing the structure. Protein folding and enzyme structure depend on interactions between amino acid side chains, not on base pairing.

85. The secondary structure of proteins is stabilized by:

• A. Ionic bonds
• B. Hydrogen bonds
• C. Disulfide bridges
• D. Hydrophobic exclusion

Explanation: Regular hydrogen bonding between backbone carbonyl and amide groups coils the chain into an alpha helix or folds it into a beta pleated sheet. Ionic bonds, disulphide bridges and hydrophobic exclusion all act between side chains and belong to the tertiary level instead. Being weak, these hydrogen bonds are the first to break on heating, which is why helices unwind during denaturation.

86. The following is an example of a globular protein:

• A. Keratin
• B. Collagen
• C. Hemoglobin
• D. Histone

Explanation: Haemoglobin folds into a compact soluble ball with hydrophilic residues outside, which is what lets it dissolve in the cytoplasm of a red cell and carry oxygen. Keratin and collagen are fibrous structural proteins, long and insoluble. Histones are globular too, which makes the last option arguable, but haemoglobin is the standard textbook example the paper is looking for.

87. Which property of water helps in moderating Earth's climate and maintaining stable temperature in aquatic environment?

• A. Low viscosity
• B. High specific heat capacity
• C. High surface tension
• D. High polarity

Explanation: A great deal of energy goes into breaking hydrogen bonds before the temperature of water rises, so oceans and lakes absorb and release heat with only small temperature changes and coastal climates stay mild. Polarity is the underlying cause of the hydrogen bonding, but the property that directly buffers temperature is the specific heat capacity. Surface tension matters for insects walking on water rather than for climate.

88. An oligosaccharide is made up of at least:

• A. two saccharide units
• B. ten saccharide units
• C. three to ten saccharide units
• D. more than ten saccharide units

Explanation: This is the option marked correct in the official UHS answer key, taking oligosaccharide in its broad sense of a few sugar units, beginning at two. Many textbooks instead reserve disaccharide for two units and define oligosaccharides as three to ten, which is why option (c) is such a strong distractor. Anything beyond about ten units is a polysaccharide.

89. Which of the following is the correct higher (left) to lower (right) sequence of molecules with respect to their amount in the cell?

• A. DNA to rRNA to tRNA to mRNA
• B. rRNA to tRNA to mRNA to DNA
• C. mRNA to tRNA to rRNA to DNA
• D. DNA to mRNA to tRNA to rRNA

Explanation: This is the order given in the official UHS answer key. Among the three RNAs the ranking is not disputed: ribosomal RNA is by far the most abundant at roughly 80 per cent of cellular RNA, transfer RNA follows, and messenger RNA is the smallest fraction because it is made as needed and degraded quickly. Placing DNA at the head reflects the paper's intent that the genome is the largest single nucleic acid component.

90. Which of the following is a key feature of RNA?

• A. Deoxyribose and thymine
• B. Double stranded and thymine
• C. Ribose and uracil
• D. Ribose and thymine

Explanation: RNA carries ribose, which has a hydroxyl group at carbon 2, and uses uracil in place of thymine, and it is normally single stranded. Deoxyribose and thymine are the DNA features, so options mixing one sugar with the wrong base can be eliminated at once. The extra hydroxyl is also why RNA is chemically less stable than DNA.