Manipulation of DNA in genetic engineering became possible due to discovery of

Manipulation of DNA reuires small DNA segments of an organism that constitute specific genes. There is a upper limit to the size of DNA segments that can be cloned in any of the available vectors. This poses a need of digestion of large DNA into fragments that can be inserted into the vector DNA. Restriction endonucleases recognize specific 4- to 8-bp sequences, called restriction sites and then cleave both DNA strands at this site. Restriction sites commonly are short palindromic sequences which are read same on each DNA strand in the 5'-->3' direction. This creates DNA fragments carrying desired genes with sticky ends that are then easily inserted in vector DNA by DNA ligase enzyme. Transcriptase enzyme catalyze the transcription of RNA molecules and, hence, are of no use in DNA manipulation. Primase enzymes forms small DNA/RNA segments that serve as primer for polymerase enzyme which can not sythesize the DNA/RNA de novo.