Chemistry Notes

Positions in Biotechnology

Nucleic acids

1. Which of the following is a key structural difference between DNA and RNA?
A) DNA contains thymine, while RNA contains uracil.
B) RNA has a double-helix structure, while DNA is single-stranded.
C) DNA contains ribose sugar, while RNA contains deoxyribose sugar.
D) RNA is more stable under alkaline conditions than DNA.

Answer: A
Explanation: The primary structural difference is the presence of thymine in DNA and uracil in RNA. Option C is incorrect because DNA contains deoxyribose (not ribose), and RNA contains ribose. Option B is reversed (DNA is double-stranded). Option D is false, as RNA is less stable under alkaline conditions.

2. During DNA replication, the enzyme DNA polymerase synthesizes new strands:
A) Only in the 3' to 5' direction.
B) Only in the 5' to 3' direction.
C) Bidirectionally, in both directions.
D) Without requiring a primer.

Answer: B
Explanation: DNA polymerase adds nucleotides only in the 5'→3' direction because it requires a free 3'-OH group to extend the chain. Option C refers to replication forks moving bidirectionally, but synthesis by polymerase itself is unidirectional. Option D is false, as primers are essential for DNA polymerase to initiate synthesis.

3. The melting temperature (Tm) of a DNA duplex is most directly influenced by:
A) The length of the DNA strand.
B) The presence of histones.
C) The ratio of purines to pyrimidines.
D) The G-C content of the DNA.

Answer: D
Explanation: G-C base pairs (with three hydrogen bonds) require more energy to denature than A-T pairs (two bonds), so higher G-C content increases Tm. While length (A) also affects Tm, G-C content is the most direct factor. Option C is incorrect because purine/pyrimidine ratios do not directly determine Tm.

4. Which statement about RNA is true?
A) All RNA molecules are translated into proteins.
B) rRNA is a structural component of ribosomes.
C) RNA is always single-stranded.
D) RNA cannot form hydrogen bonds with DNA.

Answer: B
Explanation: rRNA forms the scaffold of ribosomes. Option A is false (e.g., tRNA and rRNA are not translated). Option C is incorrect (some RNA, like tRNA, has double-stranded regions). Option D is false (e.g., during transcription, RNA pairs with DNA).

5. In the PCR (Polymerase Chain Reaction) technique, why is a heat-stable DNA polymerase used?
A) To prevent DNA denaturation at high temperatures.
B) To withstand the heat required for primer annealing.
C) To survive the high temperatures during the denaturation step.
D) To optimize primer binding during extension.

Answer: C
Explanation: Heat-stable polymerases (e.g., Taq polymerase) are essential because PCR involves repeated high-temperature steps (e.g., denaturation at 95°C), which would destroy ordinary enzymes. Option A is incorrect because heat is used to denature DNA, not prevent it.