Uracil formation from cytosine

Deoxyuridine and thymidine are substrates for pyrimidine nucleoside phosphorylases, but deoxycytidine (and cytidine) is generally regarded as being inert to phosphorolysis (7) Tarris demonstration of deoxycytidine formation from cytosine in extracts of fish milt is an exception to this generalization (8). Catabolism of C3rtosine nucleosides is initiated by deamination to form uracil nucleosides which can be phosphorolyzed. 

Carbamoyl phosphate synthetase formation in liver taken from tadpoles treated with thyroxine is enhanced by the addition of orotic acid, uracil or uridine (cytosine and adenosine had no effect). The synthesis of this enzyme is not affected by these pyrimidines in untreated animals. This indicates that there is a relative pyrimidine deficiency during thyroxine-induced metamorphosis [140]. 

A common source of DNA damage is the spontaneous loss of the amine group on cytosine and the formation of an amide. This occurs at a rate of about 100 times a day. Fortunately, the body produces enzymes able to detect and repair such degraded cytosines. Given this information, suggest why DNA differs from RNA in possessing the nucleotide thymine rather than uracil. 

Base pairing specific hydrogen bonding between adenine and uracil, adenine and thymine, and cytosine and guanine in a double stranded molecule of RNA or DNA, or in the interaction between DNA and RNA. B.p. is the basis of the formation of the double helix of DNA from two eomplementary single strands (see Deoxyribonucleic acid). 

Prior to the definitive experiments of Friedkin and Komberg (see below), early tracer studies with orotate and with nucleoside derivatives of uracil and cytosine had demonstrated that the pyrimidine ring of DNA thymine could be derived from these compounds. In addition, it had become apparent that the methyl group of thymine was derived from one-carbon units at the oxidation level of formaldehyde rather than at the oxidation level of formate formaldehyde and the hydroxymethyl group... 

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