Structure of Purines and Pyrimidines

STRUCTURE OF PURINES AND PYRIMIDINES
  • Pyrimidines have a six member ring. Purines have fused five and six member ring
  • Each nucleic acid is synthesized from only four types of bases. The two types purines, Adenine and Guanine are present in both DNA and RNA. The two pyrimidines in DNA are Cytosine and Thymine, in RNA , Uracil is found instead of Thymine.
  • The only difference between Uracil and Thymine is the presence of methyl substituent at C5. The bases are usually referred by their initial letters. So DNA contains A,G,C,T, while RNA contains A,G,C,U.
  • In nucleic acids, the bases are covalently attached to the 1’-position of a pentose sugar ring, to form a nucleoside. The point of attachment of the base to sugar is the 1-position (N-1) of the pyrimidines and the 9-position of purines. The bond between the bases and the sugar is the glycosylic (or glycosidic) bond. If the sugar is ribose (RNA), the nucleosides are adenosine, guanosine, cytidine and uridine. If the sugar is deoxyribose (DNA), the nucleosides are deoxyadenosine, deoxyguanosine, deoxycytidine and deoxythymidine.
  • A nucleotide is a nucleoside with one or more phosphate groups bound covalently to the 3’-, 5’-or the 2’ position (in RNA only). If the sugar is dexoyribose, then the compounds are termed as deoxynucleotides. The phosphate group attached to the 5’-carbon of the sugar and upto three phosphate groups can attach at that position.
  • During the synthesis of DNA or RNA, two phosphates are split off as pyrophosphate to leave one phosphate per nucleotide incorporated into the nucleic acid chain. The repeat unit of a DNA or RNA chain is hence a nucleotide.
  • Nucleotides are linked together into a polynucleotide chain by a back bone consisting of an alternating series of sugar and phosphate residues. The 5’ position of one pentose ring is connected to the 3’ position of next pentose ring via phosphate group. Thus the phosphodiester sugar back bone is said to consists of 5’---- 3’ linkages. The terminal nucleotide at one end of the chain has a 5’ group, the terminal nucleotide at the other end has a free 3’ group. It is conventional to write nucleotide sequences from the 5’ direction towards the 3’ terminus. A base sugar moiety is a nucleoside; a base sugar-phosphate moiety is nucleotide.
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Last modified: Tuesday, 15 May 2012, 4:57 AM