What is a plasmid?

The correct answer is Option (3) → Autonomously replicating extrachromosomal DNA.

A plasmid is a small, circular piece of DNA found in many bacteria outside the chromosome. It has its own origin of replication, so it can copy itself independently of the chromosomal DNA. Because of this, plasmids are used as vectors to carry alien DNA into a host.

Option (1) An alien piece of DNA. — Incorrect. The alien piece of DNA is the foreign DNA that we want to clone. The plasmid is the vehicle that carries it into the host.

Option (2) Single-stranded nuclear DNA. — Incorrect. A plasmid is double stranded and lies outside the chromosome, not in the nucleus. Bacteria in fact have no nucleus.

Option (3) Autonomously replicating extrachromosomal DNA. — Correct. A plasmid replicates on its own (autonomously) and is extra-chromosomal, that is, separate from the chromosome.

Option (4) Autonomously replicating nuclear DNA. — Incorrect. It does replicate on its own, but it is not nuclear DNA. It is extra-chromosomal DNA in the cytoplasm of the bacterium.

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.”
Cloning Vectors – “You know that plasmids and bacteriophages have the ability to replicate within bacterial cells independent of the control of chromosomal DNA.”