Understanding of DNA Sequences

The study of viruses revealed that their genomes are very small. SV40 virus of monkey has 5,243 base pairs in which there are five genes. Now, it is possible to study the gene structure with this small genome. However, still 5000-odd base pairs can be a great challenge 

The restriction enzymes are present in most bacterial cells to restrict the ability of foreign DNA to take over the protein-synthetic machinery of cells. Their own DNA is protected by the restriction enzyme due to the presence of cellular enzymes that methylate specific nucleotides, so that the nucleotides cannot be recognized by the restriction enzymes. 

Every restriction enzyme recognizes and cuts a specific DNA sequence which usually consists of four or six nucleotide pairs. For example, a restriction endonuclease Eco RI obtained from Escherichia coli cuts wherever it encounters the nucleotide sequence GAATTC, whereas Bal 1 from 

Cuts on the line of symmetry to generate flush-end molecules. 

Cuts on the positions symmetrically placed around the line of symmetry to generate cohesive-end (or sticky-end) molecules. 

---