509 



Pyrimidines 



CH3 



NH2 



CH3 



NH, 



CH2OH 



) H 



Ai 



Thymine 



(2,6-dioxy-5- 



methylpyrimidine) 



O H 



5-Methyl 



cytosine 



(2-oxy-5-methyl- 



6-aminopyrimidine) 



N II N 



An 



O H 



5-Hydroxymethyl 



cytosine 



(2-oxy-5-hydroxy- 



methyl-6-amino- 



pyrimidine) 



In some Escherichia coli bacteriophages the 5-methyl- 

 cytosine is replaced by 5-hydroxymethylcytosine. A sub- 

 stance believed to be 5-ribosyluracil has been isolated in 

 considerable quantities from yeast RNA. 



The united pyrimidine and ribose moieties are called 

 nucleosides, and the phosphorylated nucleosides are called 

 nucleotides. 



NH2 



O 



T 



HOCH2 /^\ HO— P— O— CH2 / 



I ^ 



OH 



OH OH OH 



Cytidine Cytidyiic Acid 



(a nucleoside) (a nucleotide) 



The nucleic acids are, then, polymeric nucleotides, a free 

 phosphoric acid function being esterified by a free pen- 

 tose alcohol group. 



In neither RNA nor DNA are the four main heterocy- 

 chc components present in equimolar quantities, and, 

 moreover, there is of course species variation. For ex- 

 ample, yeast DNA contains more adenine and thymine 

 than guanosine and cytosine, while the reverse is true 

 for some bacteria. The molar sum of the purines gener- 

 ally equals that of the pyrimidines, and, more specifically, 

 the number of moles of adenine present equals the num- 

 ber of moles of thymine, and the cytosine (and methyl- 

 cytosine) equals the guanine. 



There is good evidence now that most DNA is composed 



