cytology: the study of the cell 



141 



BASE — SUGAR 



BASE 



SUGAR 



The biochemical approach has shown that the chromosome is composed 

 of two principal classes of substance, nucleoproteins and a globular type of 

 protein. Nucleoproteins are compounds composed of nucleic acids, associated 

 with protein. The protein associated with nucleic acid seems to be of a 

 relatively simple type — mostly histone. Nucleic acids exist in the form of 

 complicated molecules, each composed of units known as nucleotides: each 

 nucleotide consists of a purine or 

 pyrimidine base attached to a sugar, 

 which is in turn attached to a mole- 

 cule of phosphoric acid. The nucleo- 

 tides are arranged in parallel, some- 

 what like the rungs of a ladder, which 

 are attached to each other, but only 

 at one end. The rung consists of the 

 base and sugar; the part which at- 

 taches the rungs together at one end 

 is the phosphoric acid. The latter also 

 attaches the nucleic acid to the asso- 

 ciated protein (fig. 6). 



Considerable progress has been 

 made in recent years in the field of 

 nucleic acid chemistry. With the help 

 of X-ray diffraction the three-dimen- 

 sional structure is being elucidated. 

 The work of Watson and Crick 

 (1953a, b), for instance, suggests that 

 nucleic acid molecules exist in pairs, 

 the two molecules wound around one 

 another, with the nucleotides extend- 

 ing horizontally inward (fig. 7). Each 



BASE — SUGAR 



BASE — SUGAR 



PHOSPHORIC ACID- 



PHOSPHORIC ACID 



PHOSPHORIC ACID- 



PHOSPHORIC ACID 



Fig. 6. Diagram to show relation of 

 various chemical entities making up nu- 

 cleic acid. The bonds to the right attach 

 the phosphoric acid to protein, usually 

 histone. Each horizontal chain of base- 

 sugar-acid constitutes a nucleotide. A 

 nucleic acid molecule consists of a long 

 chain of nucleotides, of which there are 

 several kinds. The order in which the 

 different kinds of nucleotides are ar- 

 ranged and their relative frequency 

 determine the specific chemical charac- 

 teristics of the nucleic acid molecule. 

 Nucleic acid can exist in myriad forms. 



nucleotide is attached to a nucleotide 



of the other nucleic acid molecule by its base, in a very precise manner, the 

 two nucleic acid molecules together thus resembling a twisted ladder. It used 

 to be thought that the nucleotides containing the two purine bases adenine and 

 guanine and the two pyrimidine bases thymine (or uracil) and cytosine were 

 present in a nucleic acid molecule in equal numbers and arranged in a regu- 

 lar sequence. This is no longer found to be true. Not only are the various nu- 

 cleotides present in varying proportions, but there are also more than four 

 kinds of nucleotides now recognized. Far from there being but a single pat- 

 tern of arrangement of four nucleotides, a very great variety of arrange- 

 ments of an unknown number of different nucleotides is indicated. 



In addition to the nucleoproteins, certain more complicated proteins are 

 present also in the chromosomes, proteins of a globulin type. They consti- 



