Chapter 4 



PROTEINS 



THE PROTEINS OF THE NORMAL CELL 



G 



iell proteins fall into 2 main categories: 



1. The nucleoproteins. 



2. Proteins located mainly in the cytoplasm and cytoplasmic 

 particles. The former were originally thought to be specific for the 

 nucleus, but are now known to be important components of the 

 cytoplasm. They consist of nucleic acids combined with other pro- 

 teins as nucleoproteins. Nucleic acids are built up by condensation 

 of nucleotides which in turn consist of a pyrimidine derivative such 

 as cytosine, linked to a pentose sugar, e.g. ribose, which is linked 

 to phosphoric acid. Successive nucleotides are joined together 

 through the phosphoric acid residues to form nucleic acids as de- 

 picted in the figure. 



The nucleic acids found in nature are classified as: 



1 . Ribonucleic acid (RNA) , which contains d-ribose as its sugar. 



CH 2 OH 



OH 



OH 



OH 



Much of the cytoplasmic nucleic acid is RNA which is stored in 

 regions of the cell that are intensely basophilic to staining. The 

 Nissl bodies of nerve cells and the ergastoplasm are such sites and 

 microsomes in general are heavily loaded with RNA. The nucleolus 

 is also a reservoir for nuclear RNA. In all such locations the RNA is 

 probably combined with protein. 



2. Deoxyribonucleic acid (DNA) which contains d-2-deoxyri- 

 bose as its sugar. 



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