This chapter describes intracellular and extracellular organization of the cell. It discusses the nucleus and its constituents and explains structure and function of deoxyribonucleic acid (DNA). The chapter outlines cell division of meiosis and mitosis and considers DNA replication in cell division. It also describes discuss about molecular and genetic tools of biotechnology. The presence of plasmid DNA in bacteria not only makes bacteria distinct from animals and plants but also makes it an important genetic engineering tool. The cell nucleus in a eukaryotic cell contains the genetic information, where almost all DNA replications and RNA syntheses occur. If the genomic DNA sequence of a gene is known, RT- polymerase chain reaction (PCR) can be used to map the location of exons and introns in the gene. The PCR usually consists of a series of 20–40 repeated temperature changes called cycles.