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The following statements were made about the rolling circle mechanism of replication:

(A) It occurs unidirectionally, with only one replicating fork.
(B) The E. coli ΦX174 phage uses this mechanism to replicate its double-stranded circular genome.
(C) E. coli utilizes this mechanism to replicate its double-stranded DNA genome.
(D) In λ phage, the progeny DNA may range several genomes long before it is packaged.
(E) The lagging strand is not formed in the rolling circle mechanism of replication.

Which one of the following options shows the correct combination of all correct statements?

Correct option is B

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Statement A: "It occurs unidirectionally, with only one replicating fork."

• Correct.
• Rolling circle replication is a unidirectional process, meaning replication proceeds in one direction around the circular DNA.
• A single replication fork is formed, and the leading strand is continuously synthesized while the other strand is displaced.

Statement B: "The E. coli ΦX174 phage uses this mechanism to replicate its double-stranded circular genome."

• Incorrect. ΦX174 phage has a single-stranded DNA genome and uses a theta replication mechanism instead of rolling circle replication for its genome synthesis.

Statement C: "E. coli utilizes this mechanism to replicate its double-stranded DNA genome."

• Incorrect.
• E. coli uses bidirectional (theta) replication for its chromosomal DNA, not rolling circle replication.
• The rolling circle mechanism is mainly used by bacteriophages (such as λ phage and M13), plasmids, and some viruses.

Statement D: "In λ phage, the progeny DNA may range several genomes long before it is packaged."

• Correct.
• In bacteriophage λ, rolling circle replication produces concatemeric DNA (long continuous DNA containing multiple genomes).
• This concatemer is then cut into genome-sized pieces before packaging into phage heads.

Statement E: "The lagging strand is not formed in the rolling circle mechanism of replication."

• Incorrect.
• In rolling circle replication, only the leading strand is continuously synthesized at the replication fork, while the displaced strand remains single-stranded temporarily.
• However, this displaced strand can later serve as a template for lagging strand synthesis
• Thus, lagging strand synthesis does occur, but it happens separately from leading strand synthesis.

Information Booster:

1. Rolling circle replication occurs in bacteriophages (e.g., λ phage, M13), plasmids, and some viruses.
2. Concatemers (long DNA molecules containing multiple genome copies) are formed during replication before being processed.
3. ΦX174 phage does NOT use rolling circle replication, as it has a single-stranded genome that undergoes theta replication.
4. E. coli does NOT use rolling circle replication; it employs theta replication for its chromosome.
5. The rolling circle mechanism generates only a leading strand, with the displaced strand either remaining single-stranded or replicated separately.

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