Chemical composition of DNA: DNA extract from many living organisms has revealed it to be a complex macromolecule. It is a long chain polymer composed of monomers (units) called nucleotides. Each nucleotide inturn is composed of three sub units. These are – a) nitrogen base, b) sugar and c) phosphate.
Nitrogen bases: The nitrogen bases are of two kinds viz., pyrimidines and purines. Both pyrimidines and purines are heterocyclic compounds, with a basic six membered ring in which one or the other carbon position is re-placed by nitrogen.
The pyrimidines have a basic six membered ring of benzene in which, position 1′ and 3′ are replaced by nitrogen. There are two kinds of pyrimidines-cytosine and thymine. Structurally cytosine has a single six membered ring with a hydroxyl group at the 2′ position and an amino group at the 6′ position (2 hydorxy, 6 amino pyrimidine). Thymine also has a single six membered ring with hydorxyl groups at the 2′ and 6′ position and a methyl group attached to the 5′ position (2,6 hydroxy,5 methyl pyrimidine).
The purines have a double ringed structure. Attached to the six membered benzene ring is a 5 membered imidazole ring. The imidazole ring is joined to the hexagonal ring at 4′ and 5′ positions. The benzene ring has nitrogen in the 1′ and 3′ position (replacing Carbon).
There are two types of purines viz. Adenine and guanine. Adenine has an amino group at the 6′ position (6 aminopurine), while guanine has an aminogroup at the 2′ position and a hydroxyl group at the 6′ position (2 amino, 6 hydroxy purine).
Sugar: The sugar molecule in DNA is basically a ribose sugar. The ribose sugar is a pentose sugar. It is a 5 membered ring. The ring consists of four carbon atoms while the 5’ C atom is outside the ring. Positions 1’, 3′ and 5’ & in the ring have OH groups while the 2’-carbon position has only H. There is no O; hence the sugar is said to be Deoxyribose (no oxygen in the second, carbon position).
Phosphate: It has one central phosphorus with four oxygen atoms.
Nucleosides: A nitrogen base coupled with sugars forms a nucleoside. DNA has four nuleosides – a) Deoxycytidine b) Deoxythymidine, c) Deoxyadenosine and d) Dexoyguanosine. The sugar molecule is attached to the nitrogen base to the 3′ position in the benzene ring in the case of pyrimidines and to the 9′ position of the imidazole ring in the case of purines.
Nucleotides: A nucleotide is derived from the nucleoside by the addition of phosphoric acid molecule to the latter. The phosphate group is usually linked to the two sugar moleucles above and below. The linkage is with the 3′ C atom of one deoxyribose sugar and with the 5′ C atom of another. DNA has four nucleotides- Deoxycytidilic acid, Deoxy thymidilic acid, deoxy adenylic acid and Deoxyguanilic acid.
Polynucleotide: DNA is said to be a polynucleotide chain with a number of nucleotides joining together with the help of DNA polymerase.