What Is The Structure Of DNA?

           In 1953, James Watson and Francis Crick gave the model for the structure of DNA. They proposed that DNA molecule is made up of two polynucleotide strands, which are twisted around each other in the form of a double helix. The double helix looks like a spiral stair case of uniform diameter. The railings of this stair case are composed of deoxyribose sugar and phosphates, while the steps are made of pairs of nitrogenous bases. Each step consists of a purine paired with a pyrimidine. Their pairing is highly specific. Adenine always paired with thymine, and guanine always pairs with cytosine. Two hydrogen bonds are formed between Adenine and Thymine, while three hydrogen bonds are formed between guanine and cytosine. Many of these four types of nucleotides join together to form polynucleotide chain of DNA molecule.

           Nitrogen base pairing between adenine and cytosine or between guanine and thymine is not possible because such hydrogen bonds can not be formed. The polynucleotide strands are held together in their helical configuration by these hydrogen bonds between bases in opposing strands. The base pairs in DNA are 0.34nm apart, and each turn of the double helix has 10 base pairs. The sugar phosphate backbones of the two complementary strands are anti parallel due to opposite chemical polarity.