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Vol. 96, Issue 8, 4396-4401, April 13, 1999

Biochemistry
 Prebiotic cytosine synthesis: A critical analysis and implications for the origin of life

Robert Shapiro*

Department of Chemistry, New York University, 100 Washington Square East, New York, NY 10003

Communicated by Leslie Orgel, The Salk Institute for Biological Studies, San Diego, CA, January 25, 1999 (received for review November 19, 1998)

A number of theories propose that RNA, or an RNA-like substance, played a role in the origin of life. Usually, such hypotheses presume that the Watson-Crick bases were readily available on prebiotic Earth, for spontaneous incorporation into a replicator. Cytosine, however, has not been reported in analyses of meteorites nor is it among the products of electric spark discharge experiments. The reported prebiotic syntheses of cytosine involve the reaction of cyanoacetylene (or its hydrolysis product, cyanoacetaldehyde), with cyanate, cyanogen, or urea. These substances undergo side reactions with common nucleophiles that appear to proceed more rapidly than cytosine formation. To favor cytosine formation, reactant concentrations are required that are implausible in a natural setting. Furthermore, cytosine is consumed by deamination (the half-life for deamination at 25°C is approx 340 yr) and other reactions. No reactions have been described thus far that would produce cytosine, even in a specialized local setting, at a rate sufficient to compensate for its decomposition. On the basis of this evidence, it appears quite unlikely that cytosine played a role in the origin of life. Theories that involve replicators that function without the Watson-Crick pairs, or no replicator at all, remain as viable alternatives.

*   To whom reprint requests should be addressed. e-mail: rs2{at}is2.nyu.edu.

Copyright © 1999 by The National Academy of Sciences  0027-8424/99/964396-6$2.00/0
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