Identify the characteristic of plasmid.

The correct answer is Option (4) → It can replicate within bacterial cell independent of the control of chromosomal DNA

Option (1) It can replicate under the control of bacterial chromosomal DNA — Incorrect. Plasmids are autonomously replicating circular DNA molecules. They replicate independently of the bacterial chromosomal DNA, which is why they are useful as vectors.

Option (2) It is made of ds RNA — Incorrect. Plasmids are made of double-stranded DNA, not dsRNA. They are extra-chromosomal circular DNA molecules in bacteria.

Option (3) It shows the property of reverse transcription — Incorrect. Reverse transcription, making DNA from RNA, is a feature of retroviruses such as HIV. Plasmids are DNA and copy themselves as DNA; they make no DNA from RNA.

Option (4) It can replicate within bacterial cell independent of the control of chromosomal DNA — Correct. A plasmid is a small circle of DNA outside the bacterial chromosome. It has its own origin of replication, so it replicates independently of the chromosomal DNA; this is why it is used as a cloning vector.

NCERT Extract:
Cloning Vectors – “… You know that plasmids and bacteriophages have the ability to replicate within bacterial cells independent of the control of chromosomal DNA. Bacteriophages because of their high number per cell, have very high copy numbers of their genome within the bacterial cells. Some plasmids may have only one or two copies per cell whereas others may have 15-100 copies per cell.”