GATE 2024 BT Q.21: solution and answer

Official answer: ASource: Official answer key, GATE 2024 BT (IISc Bengaluru)

1. The question asks which phage has a genome of single-stranded, circular DNA, so recall the genome type of each option.

2. (A) $\varphi$X174 is a small icosahedral phage of E. coli. Its genome is a circular single-stranded DNA of about 5.4 kb. It copies itself through a double-stranded replicative form, but the DNA packaged in the virion is single-stranded and circular. This is correct.

3. (B) $\lambda$ has a linear double-stranded DNA genome of about 48.5 kb in the virion. Its two ends carry short single-stranded cohesive (cos) overhangs, which pair and circularise the molecule inside the host. The genome itself is double-stranded. Wrong.

4. (C) T5 is a T-series phage with a linear double-stranded DNA genome of about 120 kb. Wrong.

5. (D) P1 is a temperate phage with a linear double-stranded DNA genome of about 94 kb. It circularises in the host by recombination, but it is double-stranded. Wrong.

6. Only $\varphi$X174 has a single-stranded circular DNA genome, so the answer is (A).