Friday, April 4, 2008

Chemical differences between DNA & RNA

Both RNA and DNA are composed of repeated units. The repeating units of RNA are ribonucleotide monophosphates and of DNA are 2'-deoxyribonucleotide monophosphates.

Both RNA and DNA form long, unbranched polynucleotide chains in which different purine or pyrimidine bases are joined by N-glycosidic bonds to a repeating sugar-phosphate backbone.

The chains have a polarity. The sequence of a nucleic acid is customarily read from 5' to 3'. For example the sequence of the RNA molecule is AUGC and of the DNA molecule is ATGC

The base sequence carries the information, i.e. the sequence ATGC has different information that AGCT even though the same bases are involved.

Consequences of RNA/DNA chemistry

The DNA backbone is more stable, especially to alkaline conditions. The 2' OH on the RNA forms 2'3'phosphodiester intermediates under basic conditions which breaks down to a mix of 2' and 3' nucleoside monophosphates. Therefore, the RNA polynucleotide is unstable.

The 2' deoxyribose allows the sugar to assume a lower energy conformation in the backbone. This helps to increase the stability of DNA polynucleotides. The following link shows 3-D models of the DNA and RNA nucleotides.

Cytidine deamination to Uridine can be detected in DNA but not RNA because deamination of Cytidine in DNA leads to Uridine not Thymidine. Uridine bases in DNA are removed by a specific set of DNA repair enzymes and replaced with cytidine bases.


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