Question

Which of the following statements is not correct about cloning vectors?

• A. ‘Ori’ is a sequence responsible for controlling the copy number of the linked DNA
• B. The ligation of alien DNA is carried out at a restriction site present in one of the two antibiotic resistance genes
• C. Selectable marker selectively permitting the growth of the non-transformants
• D. In order to link the alien DNA, the vector needs to have single recognition site for the commonly used restriction enzymes

Answer 1

The correct option is C Selectable marker selectively permitting the growth of the non-transformants

Vector
Vectors are the DNA molecules which can carry the foreign DNA fragment into the host cells for cloning. Examples include plasmids.

Features of an ideal vector
Common features of an ideal vector include the following:

• Presence of origin of replication (ori)
• Selectable markers
• Sites for the action of restriction endonucleases (restriction sites)
• Small sizes
• Copy number

Origin of replication
The ori site or origin of the replication site is a specific nucleotide sequence on the vector DNA that initiates replication, without being dependent on genomic DNA for replication. Any piece of DNA when linked to this sequence can be made to replicate within the host cells. The copy number of the desired gene depends upon the efficiency of the ori. Therefore the statement in option ‘A’ is correct.

Selectable markers
Selectable markers are specific nucleotide sequences like an antibiotic resistance gene present on the vector. The vector containing these selectable markers would carry the gene of interest into the host cell conferring antibiotic resistance to them. The selection of the transformants can be done by growing these cells in media containing the antibiotic. Therefore the given statement in option ‘B’ is incorrect.

Recognition site
Recognition sites are specific nucleotide sequences, present on the plasmid DNA, that can be recognised and cleaved by restriction endonucleases for incorporation of the gene of interest. A vector should have a single recognition site for a particular restriction enzyme to avoid the multiple cuts on the vector. Therefore the given statement in option ‘C’ is correct.

Insertion of alien DNA
The ligation of alien DNA is carried out at a restriction site present in one of the two antibiotic resistance genes. This causes insertional inactivation of that antibiotic gene. It is done to recognise the transformant cells when grown on the medium containing that particular antibiotic. Therefore the given statement in option ‘D’ is correct.

Final answer:
b. Selectable marker selectively permitting the growth of the non-transformants