Mutants of bacteriophage that carry deletions can be used to rapidly locate mutational sites of newly obtained mutants. The mapping is based on whether wild type recombinants can be recovered when the deletion mutant and the novel mutant are brought together.

Four independent deletions (1 to 4) of a region were used to map 4 novel mutations (A to D). The deletions (starting from a fixed site) are shown below (the lines denote the region of deletion):

The results of mapping are summarized in the table, where ‘+’ denotes the recovery of wild-type recombinants and ‘–’ denotes the inability to do so.

Further, it was observed that

out of the 4 novel mutants, no revertant was observed for mutant A

Mutant B and C do not complement each other.

The following conclusions were made:

• A. Mutation A lies within the region of deletion 1.
• B. Mutations can be ordered as A-D-B-C.
• C. Mutant A could be a deletion.
• D. Mutants B and C are located on 2 independent cistrons.

Which one of the following options represents a combination of all correct statements?

Correct option is A

The experiment involved studying four mutants (A, B, C, D) of a bacteriophage, where various deletion regions were tested to see if wild-type recombinants could be recovered. The key points noted in the study include:

• When mutant 1 is tested, it fails to recover wild-type for deletion 1.
• Mutant 2 recovers wild-type with deletion 1 and fails with deletion 2.
• Mutant 3 fails with deletion 1 but recovers with deletion 2.
• Mutant 4 does not recover with any deletions.

Statement Analysis:

• Statement A: Mutation A lies within the region of deletion 1.

  • This statement is True. Since mutation A does not recover with deletion 1, it implies that mutation A must lie within this deletion. If it were outside, some recovery would be possible.

• Statement B: Mutations can be ordered as A-D-B-C.

  • This statement is True. If we order based on the inability to recover with deletion 1 (which includes A), and given the recovery patterns, the order must precede D. Since B and C do not complement each other, they can follow D. This ordering fits the evidence.

• Statement C: Mutant A could be a deletion.

  • This statement is True. The inability to recover wild-type suggests that mutant A might be a deletion in the region tested.

• Statement D: Mutants B and C are located on two independent cistrons.

  • This statement is False. Since B and C do not complement each other, it indicates they affect the same gene or functional unit, meaning they are on the same cistron.

Therefore, the correct statements are A, B, and C.