Question

Plasmids are suitable vectors for genetic cloning as
(a) They are indispensable
(b) They are self-replicating units
(c) They are essential for bacterial reproduction
(d) None of the above

Answer 1

Hint: Vectors are the DNA molecules that act as a transporting vehicle to carry foreign DNA into a host cell for the purpose of cloning and expression. Plasmids are a characteristic feature of many prokaryotes encoding for proteins that elevate their rate of survival.

Complete answer:
A cloning vector is a piece of DNA that is small enough to stably replicate inside an organism after incorporation of a foreign DNA segment. Plasmids are a type of cloning vector that is found in almost all bacterial species. They are normally used to clone DNA segments of small size that may lead up to 10 kilobases. It is an extrachromosomal genetic element of DNA that is capable of replicating independently of the host chromosome. They are generally circular molecules along with some linear too.E.g. pBR322, Ti plasmid.

Additional Information:
- Plasmids are not indispensable for genetic cloning as many other new cloning vectors are now being used which are much more efficient than the plasmids.

- Plasmids are not essential for bacterial reproduction as this function is primarily performed by the circular genome of the bacterium. Their primary functions include providing genes for better survival of the organism like certain antibiotic resistance.
- A cloning vector required to have features like an origin of replication, a selectable marker, and cloning of recognition sites.
- All cloning vectors must have an origin of replication for autonomous replication within the host cells. It is a specific sequence in DNA from where replication begins.
- All cloning vectors must have a recognition site for conveniently inserting foreign DNA into the vector.
- Genetic markers are a set of genes that allows the selection of recombinant cells from non- recombinant ones.
So, the correct answer is ‘(b) They are self-replicating units.’


Note:
- An example of a widely used plasmid as a cloning vector is pBR322. It replicates inside E.coli. P stands for plasmids, BR stands for its developers ‘Bolivar and Rodriguez’ and 322 distinguishes it from others developed in the same laboratory.
- Some other cloning vectors include bacteriophage lambda, cosmids, phasmids, etc.