81. A bacterial population has a plasmid with copy number ‘n’. It was observed that on an average in one out of 2(n-1) cell divisions. there was spontaneous plasmid curing. It was inferred from the observation that:
A. Each cell division does not have equal probability of plasmid curing.
B. There is no evidence for any mechanism of plasmid segregation in the two daughter cells.
C. Plasmid distribution to daughter cells random.
D. Each plasmid has an equal chance of being in either of the two daughter cells.

Which of the combinations of above statements is true?

(1) A and B
(2) B and D
(3) only A
(4) B, C and D

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Background

In bacterial populations, plasmids are extrachromosomal DNA elements that replicate independently. The copy number ‘n’ of a plasmid refers to the average number of plasmid copies per cell. During cell division, plasmids must be distributed to daughter cells to maintain plasmid inheritance. Plasmid curing refers to the loss of plasmids from cells, which can occur spontaneously.

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Interpretation of the Observation

• The observed spontaneous plasmid curing frequency is approximately 1 in 2(n-1) cell divisions.

• This frequency suggests that plasmid segregation to daughter cells occurs randomly rather than by an active partitioning mechanism.

• If plasmids were actively segregated, curing rates would be much lower due to precise distribution.

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Analysis of Statements

A. Each cell division does not have equal probability of plasmid curing.

• This is not supported by the observation; spontaneous curing is a probabilistic event consistent across divisions.

B. There is no evidence for any mechanism of plasmid segregation in the two daughter cells.

• True. The curing frequency suggests lack of active segregation, implying random partitioning.

C. Plasmid distribution to daughter cells is random.

• True. Random segregation leads to expected curing rates matching the observed frequency.

D. Each plasmid has an equal chance of being in either of the two daughter cells.

• True. This assumption underlies the random segregation model.

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Supporting Evidence from Literature

• Many low-copy-number plasmids encode partitioning (par) systems that actively segregate plasmids using ATPase-driven mechanisms and centromere-like sites to ensure faithful inheritance.

• High-copy-number plasmids often rely on random distribution due to their abundance, minimizing plasmid loss probability.

• The observed curing frequency aligns with a random segregation model, where each plasmid independently segregates to daughter cells with equal probability.

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Correct Combination of Statements

(4) B, C and D

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Summary Table

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Conclusion

The spontaneous plasmid curing frequency observed indicates that plasmid segregation occurs randomly without an active partitioning mechanism. Each plasmid copy has an equal chance of being inherited by either daughter cell, leading to the calculated curing rate. Therefore, statements B, C, and D are true, while statement A is incorrect.

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Keywords

plasmid curing, plasmid segregation, bacterial plasmids, random plasmid distribution, partitioning systems, plasmid copy number, bacterial cell division, plasmid inheritance, par systems, plasmid loss frequency

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Final answer:
(4) B, C and D