Which of the following feature(s) facilitate cloning into a vector?

1. Origin of replication

2. Selectable marker

3. Cloning sites

The correct answer is Option 4, i.e. 1, 2 and 3.

Key Points

• Vectors used at present, are engineered in such a way that they help easy linking of foreign DNA and selection of recombinants from non-recombinants.
• The following are the features that are required to facilitate cloning into a vector.

1. Origin of replication (ori):
   • This is a sequence from where replication starts and any piece of DNA when linked to this sequence can be made to replicate within the host cells.
   • This sequence is also responsible for controlling the copy number of the linked DNA.
   • So, if one wants to recover many copies of the target DNA it should be cloned in a vector whose origin support high copy number.
2. Selectable marker:
   • In addition to ‘ori’, the vector requires a selectable marker, which helps in identifying and eliminating non-transformants and selectively permitting the growth of the transformants.
3. Cloning sites:
   • In order to link the alien DNA, the vector needs to have very few, preferably single, recognition sites for the commonly used restriction enzymes.
   • The ligation of alien DNA is carried out at a restriction site present in one of the two antibiotic resistance genes.

Explanation:

• For example, the above figure is of plasmid pBR322, which is a cloning vector.
• It contains an origin of replication (ori).
• It has several cloning sites like PstI, PvuI, EcoRI, BamHI and SalI.
• The selectable markers here are ampicillin resistance (amp^R) and tetracycline resistance (tet^R).