The Origin of Life 17 



this regular chain is one of five bases: either a purine (adenine, 

 guanine) or a pyrimidine (cytosine, thymine in DNA, or uracil in 

 RNA). The configuration of the DNA molecule appears to be a 

 double helix of the sugar-phosphate backbones held together with 

 cross-linkages of paired bases (Fig. 1.4). Because of their chemical 

 properties, these bases line up the following pairs: adenine with 

 thymine and cytosine with guanine. This helical structure best ex- 

 plains the results of X-ray diffraction and other studies to determine 

 the physical properties of the molecule. 



When chromosome duplication takes place in preparation for cell 

 division, the complementary helices probably "unwind," and each 

 helix forms a chemical template on which DNA precursors attach 

 to re-form the complementary strand and reestablish the double 

 helix. This is the mechanism by which the genetic code is passed 

 from cell to cell and (through the gametes) from organism to 

 organism. The genetic code has been shown to be the actual se- 

 quence of nucleotides in the DNA strand. 



READING THE CODE 



Protein synthesis takes place largely in the ribosomes ( microsomes ) 

 of the cell— cvtoplasmic structures physically separated from the 

 nuclear DNA. The ribosomes contain the vast majority of the RNA 

 in the cell. DNA serves as a template against which can be as- 

 sembled another identical strand of DNA or a strand of RNA. Thus 

 the code may be transferred to "messenger" RNA molecules which 

 presumably carry it to the ribosomes where protein formation 

 occurs. (It is not known how the cell "tells" the DNA whether to 

 make more DNA or RNA. ) 



There are several possible explanations of how the DNA master 

 blueprint and the RNA messengers control the assembly of various 

 proteins. It is necessary for the code, transferred from the DN.\ 

 to the RNA, to be able unambiguously to control the sequential 

 positions of 20 common amino acids which may go into the com- 

 position of proteins. One suggestion was based on the mathematical 

 demonstration that 20, and only 20, different sequences that will not 

 be subject to the confusion of overlapping can be constructed by 

 taking the four nucleotide units three at a time. For example, if the 

 nucleotides are numbered 1, 2, 3, and 4, the sequences 131 and 312 

 would be overlapping; if they were placed adjacent to each other, 

 the sequence might be 141131312, in which 131 occurs in two oxer- 

 lapping positions. The nonoverlapping triplets would be 112, 212. 



