DO YOU KNOW ? DNA and RNA differ in several characteristics: DNA nucleotides lack an oxygen on the #2 carbon of ribose when compared to the ribose of RNA. For this reason, DNA nucleotides are said to be composed of deoxyribose. DNA includes a nitrogenous base called Thymine while RNA replaces it with Uracil. DNA in its natural state in organisms exists as a double stranded molecule, while RNA is normally a single stranded molecule. In most organisms, DNA stores all the cell's genetic information, while RNA is a temporary messenger molecule. Complementarity in DNA The double stranded structure of DNA is the result of the formation of hydrogen bonds between the nitrogenous bases on one strand of DNA with the nitrogenous bases on a different strand of DNA. There are four nitrogenous bases in DNA: thymine, a pyrimidine. adenine, a purine. cytosine, a pyrimidine. guanine, a purine. These names are abbreviated by the letters T, A, C, and G. The key to DNA's remarkable ability to replicate itself lies in the fact that each nitrogenous base can only form the proper hydrogen bonds with one other nitrogenous base. Thymine can only hydrogen bond with adenine. Cytosine can only hydrogen bond with guanine. This property of the nitrogenous bases is called complementarity because each base has its specific complement. It is very important to remember these base-pairing rules, as they are the single most important feature of nucleic acid structure. Summarized, the base-pairing rules are: Adenine pairs with thymine: A-T. Guanine pairs with cytosine: G-C. In RNA, where thymine has been replaced by the nitrogenous base uracil, the base-pairing rules do not change. Uracil also pairs with adenine. Complementarity allows DNA to replicate itself, and also to be rewritten in the form of RNA. The RNA can then be translated into an amino acid sequence, also through the help of complementarity. The sequence of nitrogenous bases is retained throughout replication, transcription, and translation because of these base-pairing rules. It may seem strange to think that a spiraling, double-stranded molecule of DNA, composed of simple sugars, phosphates, and four different nitrogenous bases, could somehow lead to the production of all proteins necessary for the functions of the cell. The process is really beautifully simple: DNA is transcribed, or rewritten, into the form of RNA; the RNA molecule is translated into an amino acid sequence; and the amino acid sequence folds to produce a three-dimensional protein. (FROM= homepages.ius.edu)