Basic Structure
The structure of DNA can be examined at different levels.
DNA is comprised of two polynucleotide chains. These two chains wind around each other to form a double helix. In turn the nucleotides forming these chains are comprised of three parts each. A base, a sugar and a phosphate group.
DNA is made up of nucleotides, each of which contains one of four different nitrogen containing bases (guanine, thymine, cytosine and adenine), one sugar (deoxyribose) and one phosphate group.
RNA also contains another nucleotide base (uracil) and a different sugar (ribose)
These bases are all aromatic. Two contain one ring (pyrimidine) and two contain two rings (purine)
The purine bases are adenine(A) and guanine (G);
The pyrimidine bases are cytosine (C) and thymine (T);
Chargaff's rules state that DNA from any source contains the bases A and T in a 1:1 ratio, and also the bases C and G in a 1:1 ratio.
Ie the number of Purine bases = the mumber of pyrimidine bases.
This is because in a double helix, base pairing occurs.
When two DNA molecules come together to form a double helix, the bases on one strand bond to the bases in the other. in order to do this , the two DNA strands must be antiparallel ie one (DNA strands link covalently by phosphodiester bonds between the 3'-hydroxyl group of one sugar and the 5'-phosphate of the next) runs from 3' to 5' and the other in the other direction.
The bases always pair A to T and C to G. This is why the ratios between the different bases are as they are. This complementary base-pairing allows the bases to form the most energetically favourable position in the double helix
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Author: Katherine Brown, University of Sheffield,
England. (document modification date: 28th may 2004)
