Nucleic Acids are extremely complex molecules produced by living cells and viruses and are responsible for storing and transferring genetic information. They are enormous molecules made up of long strands of subunits, called bases that are arranged in a precise sequence. These are read by other components of the cell and used as a guide in making proteins. The genetic information is stored in packages, on genes, where one gene holds the information for the production of one polypeptide sequence. Gene is a unit of inheritance, a piece of the genetic material that determines the inheritance of a particular characteristic, or group of characteristics. Genes are carried by chromosomes in the cell nucleus and are arranged in a line along each chromosome. Every gene occupies a place, or locus, on the chromosome. Thus a gene might be defined as a polynucleotide, tRNA, or rRNA. The functional genes are responsible for encoding RNA molecule, encoding proteins. The genes of structure and regulation determine the structure of a particular protein (e.g., an enzyme) and regulation. The present book is an effort to put recently acquired information at one place.