278 



X-ray Analyses of Proteins and Nucleic Acids / 15 : 3 



elimination of a water molecule, with a phosphate group. A nucleoside 

 in turn is the condensation product of a five-carbon sugar (pentose) plus 

 an organic base derived from a purine or pyrimidine ring. Symbolically, 

 this may be represented as 



h,o 



Base + Sugar — *-> Nucleoside 



Nucleoside + H 2 PO, 



■OH- 



Nucleotide 



rc-Nucleotide 



-(n-l)H.O 



Nucleic Acid 



The number n is very large because the molecular weight of nucleic acid 

 molecules usually runs in the millions. 



There are two types of sugar molecules included in nucleic acids. 

 Throughout any one nucleic acid molecule, all the sugar residues are 

 the same. One sugar found in nucleic acids is called D-ribose, in which 

 case the polymer is ribose nucleic acid (RNA) ; the other sugar possible 

 is Z)-2-deoxyribose, in which case the polymer is deoxyribose nucleic 

 acid (DNA). The structures of ribose and deoxyribose are shown 

 below, in a ring form. The ring with carbon atoms at four of the 

 corners is supposed, in this illustration, to appear to lie in one plane 

 with the hydrogen and hydroxyl groups on bonds at right angles to the 

 plane. Although this type of structure is often drawn, it is believed 

 that the ring is not restricted to one plane but rather pleated, up and 

 down, about the plane, as illustrated in the chair model in Figure 3. 



CH 2 OH OH 



CH 2 OH OH 



.D-ribose 



Z)-2-deoxyribose 



The organic bases referred to above are derived from purine and 

 pyrimidine rings. These rings have structures which can be shown as 



H 

 N^H 



H' 



r/ 



H 



pyrimidine 



purine 



