Question

In bacteria, plasmid is
A. Extrachromosomal material
B. Main DNA
C. Non-functional DNA
D. Repetitive genes

Answer 1

Hint: Plasmids are used in the technique of gene-manipulation as cloning vectors. Low molecular weight, ability to confer readily with selectable phenotypic traits and several sites for large numbers of restriction enzymes are the characteristics of an ideal plasmid.

Complete answer:
They are defined as circular, double-stranded DNA molecules that occur in an extrachromosomal state and are self-replicating in nature. Some plasmids may have one or two copies per cell.
Plasmids may be present in greater amounts also like 15-100 plasmids per cell. However some multi-copy plasmids are widely distributed throughout prokaryotes varying in size from less than 1X106 Daltons to greater than 200x106 Daltons and these are generally dispensable.
The plasmids have a replication control system that helps them in their maintenance in bacteria at a characteristic level. There are two types of plasmids- single copy plasmids and multi copy plasmids.
The plasmids that are maintained at one plasmid per host genome are called single copy plasmids whereas multi copy plasmids are under relaxed replication control which means that they accumulate in very large amounts per cell when the bacteria stop growing.
Three widely studied bacterial plasmids are:
-F-plasmids- They are responsible for conjugation.
-R-plasmids- They bear genes for resistance to antibiotics.
-Col-plasmids- Such plasmids code for colicins, the proteins that kill sensitive E.coli cells. They bear genes which provide immunity to colicin.
So, the correct answer is A. Extrachromosomal material.

Note: The plasmid vector is isolated from the bacterial cell and cleaved at one site by restriction endonuclease which converts the circular plasmid into a linear molecule. The two ends of the linear plasmid are then joined to the ends of the foreign DNA to be inserted with the help of enzyme DNA ligase. This regenerates recombinant plasmid.