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GATE 2022 BT Q.18: solution and answer

GATE 2022 · BTQ.18 · 1 mark · MCQ

The type of nucleic acid present in λ–phage is

  1. (A)

    Double stranded DNA

  2. (B)

    Single stranded circular DNA

  3. (C)

    Single stranded DNA

  4. (D)

    Single stranded RNA

See the question in the paper
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Answer and solution

ANSWER

The nucleic acid of -phage is double stranded DNA, option (A).

Official answer: ASource: Official answer key, GATE 2022 BT (IIT Kharagpur)

  1. Phage is a temperate bacteriophage of : after infection it can either lyse the host or integrate as a prophage (lysogeny).

  2. The virion carries a linear, double-stranded DNA genome of about 48.5 kb. Each 5' end has a 12-nucleotide single-stranded overhang (the cos ends), and the two overhangs are complementary.

  3. These short overhangs anneal after entry into the host, so the genome circularises inside the cell. The overhangs are only a small part of the molecule, so the genome is double stranded.

  4. Option (A), double stranded DNA: correct.

  5. Option (B), single stranded circular DNA: wrong. This is the genome of phages such as M13 and X174, not .

  6. Option (C), single stranded DNA: wrong. Only the 12-nucleotide cos ends are single stranded, not the genome.

  7. Option (D), single stranded RNA: wrong. This is the genome of phages such as MS2 and Q.

  8. So the answer is (A).

KEY IDEA

Phage has a linear dsDNA genome (about 48.5 kb) whose 12-nucleotide complementary single-stranded cos ends let it circularise in the host. Genome type is what the question asks, and that is dsDNA.

WATCH OUT FOR

Picking single stranded DNA because of the single-stranded cos overhangs, or confusing with M13 or X174 (single stranded DNA phages) or MS2 (RNA phage).

From the GATE 2022 BT paper. Every question of this paper · Download the full paper · All 11 Pathogens & antibiotics questions, with notes

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