Free Structure of DNA MCQs with Answers

1. In a double stranded DNA molecule, adenine pairs with thymine through

• A. one hydrogen bond
• B. two hydrogen bonds
• C. three hydrogen bonds
• D. a covalent bond

Explanation: Adenine and thymine are held together by two hydrogen bonds, while guanine and cytosine are held by three. That difference is why a DNA region rich in G and C needs a higher temperature to separate the strands, a fact used directly when designing PCR conditions. The covalent bonds in DNA run along each backbone, not across the strands.

2. If one strand of a DNA molecule contains 22% adenine, what percentage of the whole molecule is guanine?

• A. 22%
• B. 28%
• C. 44%
• D. 56%

Explanation: Chargaff's rule applies to the double stranded molecule, so A equals T and G equals C. With A at 22%, T is also 22%, leaving 100 minus 44 which is 56% shared equally between G and C, giving 28% guanine. The common error is to stop at 56% and forget that it is split between two bases.

3. The two strands of a DNA double helix are described as antiparallel, which means that

• A. they carry identical base sequences
• B. one runs 5' to 3' while the other runs 3' to 5'
• C. they are held together by covalent bonds
• D. they coil in opposite directions around each other

Explanation: Antiparallel refers to the direction of the sugar-phosphate backbones: one strand runs 5' to 3' while its partner runs 3' to 5'. This is why replication is continuous on the leading strand and discontinuous, in Okazaki fragments, on the lagging strand. The strands are complementary rather than identical, and they are held together by hydrogen bonds.

4. The double helix structure of DNA was proposed in 1953 by

• A. Mendel and Morgan
• B. Griffith and Avery
• C. Watson and Crick, using the X ray diffraction data of Rosalind Franklin
• D. Sanger and Gilbert

Explanation: Watson and Crick built the model from Franklin's X ray photographs, which showed the helix and its dimensions, together with Chargaff's finding that A equals T and G equals C. The structure immediately suggested how the molecule could be copied, since each strand is a template for the other. Griffith and Avery had earlier shown that DNA is the transforming material, and Sanger later developed sequencing.

5. The structure of DNA was discovered by

• A. Franklin and Wilkins
• B. Watson and Crick
• C. Mendel
• D. Avery

Explanation: Watson and Crick proposed the double helix model in 1953, based on Franklin's X-ray diffraction data.

6. DNA replication is

• A. Conservative
• B. Semi-conservative
• C. Dispersive
• D. Non-conservative

Explanation: Meselson and Stahl proved DNA replication is semi-conservative: each new DNA has one old and one new strand.

7. The enzyme that unwinds DNA during replication is

• A. DNA polymerase
• B. Helicase
• C. Ligase
• D. Primase

Explanation: Helicase breaks hydrogen bonds between base pairs, unwinding the DNA double helix.

8. Okazaki fragments are formed on the

• A. Leading strand
• B. Lagging strand
• C. Both strands equally
• D. Neither strand

Explanation: The lagging strand is synthesized discontinuously as Okazaki fragments because DNA polymerase only works 5'→3'.

9. 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.

10. 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.