THE PROBLEMS 



cules. Moreover, nature has produced an immense variety of cate- 

 gories of different organisms. Yet, when the living world is con- 

 sidered at the cellular level, one discovers unity. Uvity of pla?i: 

 each cell possesses a nucleus imbedded in protoplasm. Ufiity of 

 functiojT. the metabolism is essentially the same in each cell. Unity 

 of coviposition: the main macromolecules of all living beings are 

 composed of the same small molecules. For, in order to build the 

 immense diversity of the living systems, nature has made use of a 

 strictly limited number of building blocks. The problem of diversity 

 of structures and functions, the problem of heredity, and the prob- 

 lem of diversification of species have been solved by the elegant use 

 of a small number of building blocks organized into specific macro- 

 molecules. 



What specificity is will be learned later. For the time being, it is 

 only necessary to know that the most important macromolecules 

 are nucleic acids and proteins. Nucleic acids are made of four nucleic 



Figure 1. The Components of Nucleic Acid. 



(a) Adenosine-3-phosphate (nucleotide): adenine, ribose, phosphoric acid. 

 {h) Fragment of a nucleic acid chain. The pentoses of the nucleotides are 

 united by phosphodiester bonds. 



[H] 



