Which of the following is not a characteristic feature of plasmids?

The correct answer is Option (3) → Single-stranded

A plasmid is an autonomously replicating, circular, extra-chromosomal DNA found in bacteria. It is double-stranded DNA, like the bacterial chromosome. Restriction enzymes cut both of its strands, and DNA ligase can join foreign DNA into it, which is why plasmids serve as cloning vectors.

Option (1) Independent replication — A feature of plasmids. A plasmid copies itself on its own, independently of the chromosomal DNA. Its origin of replication (ori) also controls its copy number.

Option (2) Circular structure — A feature of plasmids. Plasmid DNA is a closed circle, as the drawing of pBR322 shows.

Option (3) Single-stranded — Not a feature of plasmids. Plasmid DNA is double-stranded. A restriction enzyme cuts each of the two strands of its double helix, which leaves the sticky ends used in cloning.

Option (4) Double-stranded — A feature of plasmids. Like other DNA used in cloning, plasmid DNA is a double helix of two strands.

NCERT Extract:
Principles of Biotechnology – “… The construction of the first recombinant DNA emerged from the possibility of linking a gene encoding antibiotic resistance with a native plasmid (autonomously replicating circular extra-chromosomal DNA) of Salmonella typhimurium. Stanley Cohen and Herbert Boyer accomplished this in 1972 by isolating the antibiotic resistance gene by cutting out a piece of DNA from a plasmid which was responsible for conferring antibiotic resistance.”
Restriction Enzymes – “… Once it finds its specific recognition sequence, it will bind to the DNA and cut each of the two strands of the double helix at specific points in their sugar-phosphate backbones (Figure 9.1).”