In DNA there are four bases. These bases have carbon-nitrogen ring structure, because of the nitrogen atoms they are called nitrogenous bases. There are two types of ring structure. Adenine and Guanine are purines each having two joined carbon nitrogen rings but with different side chains. Thymine and cytosine are pyrimidines, each has only one carbon nitrogen ring and again they differ in their side chains.
In DNA, the nucleosides have ribose as the sugar component and so are ribonucleosides. In DNA the sugar is deoxyribose (i.e. the 2�-OH group in ribose is replaced by a hydrogen atom, hence deoxy) and so the nucleosides are deoxynucleosides. For DNA these are deoxyadenosine, deoxyguanosine, deoxythymidine and deoxycytidine. In each case the C1 of the sugar is joined to the base via one of its nitrogen atoms. If the base is a pyrimidine, the nitrogen at the 1 position (N1) is involved in bonding to the sugar. If the base is a purine, the bonding is to the N9 position of the base.
A nucleotide is phosphate ester of a nucleoside. It consists of a phosphate group joined to nucleoside at the hydroxyl group attached to C5 of the sugar i.e., it is a nucleoside 5-phosphate or 5�nucleotide. The primed number denotes the atom of the sugar to which the phosphate is bonded. In DNA the nucleotides have deoxyribose as the sugar and hence are called deoxynucleotides. Deoxynucleotides may have single phosphate group, two phosphate groups or three phosphate groups. Deoxynucleoside triphosphates are the precursor for DNA synthesis.These are deoxyadenosine triphosphate, deoxyguanosine triphosphate, deoxycytidine triphosphate, deoxythymidine triphosphate.
In a DNA molecule, the different nucleotides are covalently joined to form a long polymer chain by covalent bonding between the phosphates and sugars. For any one nucleotide, the phosphate attached to the hydroxyl group at the 5� position of the sugar is in turn bonded to the hydroxyl group on the 3� carbon of the sugar o the next nucleotide. The first nucleotides has adroxyl. T 5�phosphate not bonded to any other molecule and the last nucleotide has a free 3� hydroxyl. Thus, each DNA chain has polarity, it has 5; end and 3� end.
Each nucleotide can be thought of as a single letter in an alphabet that has only four letters, A, G, C, T. Different genes have different sequences of these four nucleotides and so code for different biological messages. Since the deoxynucleotides in DNA differ only in the bases they carry, this sequence of deoxynucleotides can be recorded simply as base sequence. The base sequence is written in the 5� to 3� direction.
In 1953, Watson and Crick worked out the three dimensional structure of DNA starting from X-ray diffraction photographs taken by Franklin and Wilkins. They deduced that DNA is composed of two strands wound around each other to form a double helix, with the bases on the inside and the sugar phosphate backbones on the outside. In the double helix, the two DNA strands are organized in an antiparallel arrangement. The bases of the two strands form hydrogen bombs to each other. A pairs with T and G pairs with C. This is called complementary base pairiong.
