Mention the characteristics of plasmids which make it a good vector.

Answer: A plasmid is an extrachromosomal, circular, double-stranded DNA of bacteria (e.g. pBR322). It is a good cloning vector because:

1. It is small (generally 1 to 500 kbp) and easy to isolate and to introduce into a host cell.
2. It has an origin of replication (ori), so it replicates independently of the host chromosome and gives many copies.
3. It carries selectable marker genes (e.g. antibiotic resistance genes amp^R, tet^R) that help to pick out transformed cells.
4. It has unique recognition sites for restriction enzymes where foreign DNA can be inserted (cloning sites).
5. It is not harmful to the host and can be transferred to other cells.