Question

Which of the following is not a feature of plasmids?
(a) Independent replication
(b) Circular structure
(c) Transferable
(d) Single-stranded

Answer 1

Hint: They are present in bacteria and some eukaryotic organisms like yeast. They provide a simple way of producing a large number of DNA from a single DNA molecule or even its fragment. They contain genes which may enhance the survival rate of an organism.

Complete answer:
Plasmids are naturally occurring, circular, double-stranded, extrachromosomal, autonomously or self-replicating DNA. Their size ranges from 1 kb to over 300 kb. They are now being used as ‘cloning vectors’ in the field of recombinant DNA technology which refers to the DNA that acts as a transporting vehicle carrying foreign DNA into a host cell for the purpose of cloning and expression.

Additional Information:
-All cloning vectors have an origin of replication( ori) and thus they can replicate independently within the host cell. The sequences of DNA from where replication begins is known as ’ori’. Thus, ‘the foreign DNA’ carried by the cloning vector also replicates with the vector.
-To ligate or join a target gene in the cloning vector, it first needs to be cleaved. This is achieved by the action of restriction enzymes. But these enzymes recognise only a specific sequence of DNA known as restriction sites. Presence of polylinkers or multiple cloning sites is rich in these restriction sites.
-Genetic markers in the cloning vectors highlight the host cells in which the target gene has been successful.Thus, helping to select the transformed among the non-transformed cells.
-Low molecular weight of cloning vectors is advantageous as they may be readily isolated from host cells. Also, they are usually present as multiple copies.

 So, the correct answer is ‘ Single-stranded’.

Note: An example of a widely used cloning vector is pBR322. It replicates in E.coli. ‘P’ stands for plasmid and BR denotes its developer’s name which is Bolivar and Rodriguez. ‘322’ is a serial number to distinguish it from the other ones manufactured in the same laboratory. The total number of base pairs in pBR322 is 4361. It has been genetically engineered from three different naturally occurring plasmids.