This chapter seeks to familiarize the readers with organisation and structure of DNA. Watson-Crick and Wilkins to propose a model of a DNA in 1953 based on the X-ray diffraction photographs of DNA taken by Rosalind Franklin. Salient features of Watson-Crick Model of DNA are right-handed double stranded DNA helix, base pairing rule, two strands are antiparallel, hydrogen bonding and grooves of the DNA. Six types of DNA are A-DNA, B-DNA, C-DNA, D-DNA, E-DNA, and Z-DNA which are discussed briefly in this chapter. Triple-stranded DNA is generated by the hydrogen bonding of a third strand into the major groove of B-DNA. This chapter also covers the supercoiling of DNA and the central dogma of molecular biology. At the end of the chapter, multiple choice questions are also given along with their explanatory answers.