In 1953 Watson and Crick postulated a three-dimensional model of DNA structure that accounted for both the X-ray data and the characteristic base pairing in DNA. It is the hereditary material in humans and almost all other organisms. It is the central information storage structural system of most animals and plants, and even some viruses. The name is derived from its structure, which is a sugar and phosphate backbone which have bases sticking out from it--so-called bases. So that "deoxyribo" refers to the sugar and the nucleic acid refers to the phosphate and the bases. It consists of two helical polynucleotide chains. Two polynucleotide chains coil around the same axis to form a right-handed double helix. In the helix, two chains or strands are anti-parallel ie; have an opposite polarity. Backbone of each chain which consist of alternative sugar-phosphate residues (hydrophilic) are on the outside of the double helix, facing the surrounding. The purine and pyrimidine bases of each strand faces inward towards each other. The bases are stacked perpendicular to the long axis of the double helix. DNA exists in a variety of sizes and shapes. In the nucleus of human somatic cells, each of the 46 chromosomes consists of a single DNA duplex molecule about 4 cm long. If the chromosomes in each somatic cell were placed end to end, the DNA would stretch/elongate almost a couple of metre long. The helix has two external grooves, the narrow groove is called as minor groove, while the wide groove is called major groove. The major groove is the site for DNA binding proteins. The minor grooves often are the site for binding small molecules. The pair of bases are always between a purine and pyrimidine, specifically the pairs A-T and G-C which are the base pair found by Chargaff. There is hydrogen bond between the bases. 2H- bonds between A & T and 3H- bonds between G & C. Two chains do not have the same composition, but two chains are complementary to each other, such chains are called as Complementary chains. 1st chain……ATACGCAC……3A,1T,3C,1G 2nd chain…..TATGCGTC…….1A,3T,1C,3G An important property of DNA is that it can replicate, or make copies of itself. Each strand of DNA in the double helix can serve as a pattern for duplicating the sequence of bases. This is crucial when cells divide because each novice cell needs to have an exact copy of the DNA present in the parent cell. Functions • The nucleic acid is concerned with the storage and transfer of genetic information. • Nucleotides are precursors of the nucleic acid, deoxyribonucleic acid (DNA) ribonucleic acid (RNA). • DNA must be stable. • There are coding and non-coding regions found on DNA. • Coding region code for genes (proteins). • Non-coding regions can be either DNA junk or help regulate protein synthesis.