Hershey and Chase experiment in 1952 proved that the DNA is a genetic material; the finding revolutionized the field of biology. In 1953, the DNA double helical structure was elucidated by Watson and Crick based on the X-ray diffraction image by Rosalind Franklin. Subsequently the genetic code was elucidated by Nirenberg and Khorana. Further Maniatis and coworkers cloned the globin gene. All these discoveries opened a large arena for manipulating the animal genome and ask the question whether we can treat any heritable disease by using molecular biology tool. The field of gene therapy emerged as a new area for the researchers around the world.

A hereditary unit that consists of sequence of DNA which occupies a specific location on a chromosome and determines a particular characteristic of a living organism is called as Gene . Fluorescence in situ hybridization (FISH) is a method by which a gene is localized with the help of a fluorescent dye in a chromosome. The gene can be visualized for its correct position and location in the chromosome. Any deviation from the normal can be suspected for a potential genetic disease.

All the living cells in the universe are divided into either a prokaryotic (primitive nucleus) or a eukaryotic cell (true nucleus). Although they are different from each other in many aspects but both contain genes that code for specific polypeptides required for their perpetuation in the environment.

Table 1.1 Comparison of a eukaryotic cells with a prokaryotic cell: