[1.5]
a) Restriction enzymes are molecular scissors used in molecular biology for cutting DNA sequences from a specific site. It plays an important role in gene manipulation. The enzyme recognizes a specific 4-8 base pair sequence known as the recognition sequence and cut the sequence at a specific site.
For example, the enzyme EcoRI cuts the DNA between G and A only when the sequence GAATTC is present in the DNA.
↓
5' - GAATTC - 3'
3' - CTTAAG - 5'
↑
Hence, structures called overhangs or sticky ends are generated on each strand. 5'-G and AATTC 3' 3' - CTTAA G5' These sticky ends form hydrogen bonds with their complementary counterparts with the help of DNA ligase.
[1.5]
b) Plasmid is an extra-chromosomal DNA molecule in bacteria that is capable of replicating, independent of chromosomal DNA.They serve as a vehicle to carry a foreign DNA sequence into a given host cell.
An example of such vector is pBR322.