Q.21 Which one of the following bacteriophages has a genome composed of single
stranded circular DNA?
(A) ØX174
(B) λ
(C) T5
(D) P1

The Single-Stranded Circular DNA Virus (Correct Answer Revealed)

ØX174 (phi X 174) is the bacteriophage with a genome made of single-stranded circular DNA, distinguishing it from others like lambda or T5 that use double-stranded forms. This feature makes it a classic model in molecular biology for studying ssDNA replication and the first fully sequenced DNA genome in 1977. Understanding these differences helps in exams on virology and genetics.​

Correct Answer

(A) ØX174 stands out as the only option with a single-stranded, circular DNA genome of about 5,386 nucleotides, positive-sense, infecting E. coli. Its icosahedral structure lacks a tail, and replication involves rolling-circle mechanism after forming a double-stranded replicative form.​

Option Breakdown

• (A) ØX174: Features circular ssDNA (+ strand), confirmed across studies for its unique packaging and gene overlap.​

• (B) λ (Lambda): Contains linear double-stranded DNA (dsDNA) of ~48.5 kb that circularizes inside the host for lysogenic or lytic cycles.​

• (C) T5: Has linear dsDNA (~121 kb) with terminal redundancies, using multiple host enzymes for head injection.​

• (D) P1: Possessess circular dsDNA prophage form (~90 kb), known for generalized transduction in E. coli.​