Book : Molecular Biology
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Chemical Differences between DNA & RNA

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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.

Chemical consequences of RNA/DNA

► 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. 

► 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. 

► The role of DNA is long-term information storage. Thus DNA can be looked upon as a chemical information storage medium. All such media have certain common properties. 

► The molecule must be able to carry information: 

► The molecule must be able to hold information, without this property it is useless. 

► The molecule must be readable: 

► The information in the medium must be able to be used for some purpose. It is no use putting information into a storage medium if the information cannot be retrieved. 

► The molecule must be stable and secure. 

► The information storage medium must be passed from generation to generation. Thus the molecule must be able to remain essentially unchanged for many generations. 

► The role of RNA is three-fold: as a structural molecule, as an information transfer molecule, as an information decoding molecule 

► RNA molecules read and interpret the information in DNA. RNA molecules are key players in the reactions that turn information into useful work.

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