• Each restriction endonuclease recognizes a specific palondromic nucleotide sequence in the DNA. Once it finds its specific recognition sequence it bind to DNA and cuts each of the two strands of DNA. • During post transcriptional modification in eukaryotes, poly(A) tail (200–300 adenylate residues) are added at 3' end of hnRNA. • During DNA replication Okazaki fragments are synthesized discontinuously and joined by DNA ligase. • A PCR primer sequence is termed degenerate if some of its position have several possible bases.