PROTEINS 



121 



although others may be found. In addition, glucose, galactose, and 

 other sugars occur in special nucleotides of metabolic importance 

 but have not been isolated from nucleic acids. 



The bases belong to two series, derivatives of pyrimidinc and 

 purine. These are: 



N-^^^^ 



■N^ 



-N 



,^ 



-NH 



liyriiiiidine 



punnp 



The four pvrimidincs most abiuulant in nucleic acids arc: 



H.,N 



,1^ 



N. 



',0 



O.- 



H 



'.O 



NH 



.NH 



CHg 



H 



,N. 



.-^ 



O 



N„ 



■:y 



NH 



cytosine 



uracil 



thymine 



H.,Nr^^-V^" 



5 - methylcytosine 



One other, hydroxymethylcytosine, has been isolated in the nucleic 

 acids from certain bacterial viruses. The most abundant purines are: 



NH.. 



^ .N. 



NH 



NH 



adenine 



guanine 



with others known from metabolic processes. These bases are con- 

 densed by way of the semiacetal forms of D-ribose and D-deoxyribose to 

 form ribonucleosides and deoxyribonucleosides, respectively. Not all 

 possible combinations are known. Cytosine, uracil, adenine, and 

 guanine are found attached to ribose and cytosine, methylcytosine, 

 hydroxymethylcytosine, thymine, adenine, and guanine with the deoxy 

 sugar. The linkages are similar for all pyrimidines on the one hand 

 and for the purines on the other. Thus, the structures of the nucleo- 

 sides may be illustrated by: 



,N. 



H.,N 



^ 



jO 



OH OH 



I /I l\l 



N— C^ H H ..C 



NH, 



H 



0' 



CH,,OH 



CHvOH 



cytidine 



adenosine 



