This book is about genes, their propagation, expression, the controls under which they work, the way they are organized in the prokaryotes and in the eukaryotes, the structure of their underlying molecule DNA, the way they are packaged in a small space called chromosome, the way we cut and paste them from one organism to another, the way we manipulate tem otherwise in the laboratory, and about the diseases that are caused if their expression or the controls on their expression fall by the wayside. The book has an experimental bas and all major points are illustrated with experiments. History provides perspective. Therefore, wherever necessary, historical developments have been recounted to put the subject in proper orientation. Finally, a lot of references have been cited at the end of each chapter for further reading. Keeping in view the wealth of information available on the worldwide web and the tendency of the modern student to look at the material available on the internet first and to touch the printed material only if nothing is available on the net, a great deal of useful WebPages have also been cited. Since molecular biology is now a core subject in all life science course, the book is a must for students of Biotechnology, Biochemistry, Microbiology, Biophysics, Botany, Zoology, Pharmacy other medical sciences and allied fields.