Which of the following statements about plasmids is incorrect ?
Which of the following statements about plasmids is incorrect ?
Options
Correct option: B — 'Their replication is controlled by chromosomal DNA.'
This is the incorrect statement. Plasmids possess their own origin of replication (ori) and replicate autonomously, independent of the bacterial chromosome.
- The other three statements (A, C, D) are all true of plasmids.
Marking Scheme
- 11 mark: correct option B (replication controlled by chromosomal DNA is the incorrect statement).
- 2No marks if a true statement (A/C/D) is selected as 'incorrect'.
Hint
The question asks which is FALSE — plasmids replicate on their own, not under chromosomal control.
Quick Oral Answer
Plasmids replicate autonomously using their own origin of replication, so the statement that their replication is controlled by chromosomal DNA is incorrect — the answer is B.
Analysis & Explanation
Concept
A plasmid is a small, circular, double-stranded, extra-chromosomal DNA molecule found in bacteria that replicates independently of the main chromosome. This autonomy is exactly what makes plasmids useful cloning vectors.
Why B is the incorrect statement (the answer)
Plasmid replication is directed by the plasmid's own origin of replication, not controlled by chromosomal DNA. Their copy number can even be regulated independently of the chromosome.
Why the other statements are correct
- A — True: plasmids do replicate within the bacterial cell.
- C — True: they are autonomously replicating, circular, extra-chromosomal DNA.
- D — True: many carry antibiotic-resistance genes, used as selectable markers in genetic engineering.
Exam trap
The question asks for the incorrect statement — read carefully. Statement C and statement B seem to contradict each other; C ('autonomously replicating') is the true one, so B must be the false one.
Common Mistakes
- 1Overlooking the word 'incorrect' and choosing a true statement such as C or D.
- 2Believing plasmid replication depends on the bacterial chromosome — plasmids have their own ori and replicate autonomously.
Interesting Facts
The plasmid pBR322 was one of the first artificial cloning vectors and carries two antibiotic-resistance genes (ampR and tetR) used as selectable markers.
Bacteria can transfer antibiotic-resistance plasmids to other bacteria by conjugation, a major cause of spreading drug resistance.
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Frequently Asked Questions
Why are plasmids used as cloning vectors?
Plasmids replicate autonomously inside bacteria, are small and circular, can be easily cut and joined with restriction enzymes and ligase, and often carry selectable markers like antibiotic-resistance genes — letting scientists insert a gene of interest and multiply it along with the plasmid.
How do plasmids replicate independently of the chromosome?
Each plasmid contains its own origin of replication (ori), a sequence that recruits the cell's replication machinery. This lets the plasmid copy itself and control its copy number independently of the bacterial chromosome.