(a) Restriction Enzymes : 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. Restriction endonucleases acts as an important tool in genetic engineering. It is commonly used to make a cut in the sequence to obtain DNA fragments with sticky ends. Sticky ends are single stranded portions of DNA which can form hydrogen bonds with their complementary cut DNA segments. These ends are later on joined by enzyme ligase. 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.(b) Plasmid : Plasmid is an extra-chromosomal DNA molecule in bacteria that is capable of replicating, independent of chromosomal DNA. They carry extra genes which are not essential ' for bacterial survival but provide their hosts with extra advantages such as antibiotic resistance. Plasmids, which are used in genetic engineering are called vectors. They serve as a vehicle to carry a foreign DNA sequence into a given host cell. An example of such vector is pBR322.