150 



BIG MOLECULES 



will exist, one for each daughter cell after mitosis. There is now some evi- 

 dence that the condensation reaction is enzyme-controlled, and, current with 

 the times, someone has humorously suggested that an enzyme called "un- 

 twisterase^ controls the uncoupling of the two DNA strands. The reaction 

 is quite sensitive to salts and to pH, which usually indicates that strong elec- 

 trical forces along the structure are important. There is also some evidence 

 that RNA is formed by condensation around the two-stranded DNA, as a 

 third party. DNA itself is not only synthesized by an enzyme, but is also 

 degraded by one called DNAse. 



8A small 



base-access 

 grooves 



large 



sugar-phosphate 

 outside ring 



ISA- 



Figure 6-11. Schematic Drawing of Twin- 

 Coiled DNA Molecule. (Refer to Table 6-3 

 for detailed structure.) 



Much has been learned within the past eight years about .these important 

 molecules. However, more than what has been said is beyond our scope 

 here. It is currently a very active and popular phase of the study of big 

 molecules. They are big, too: molecular weight 5 to 125 million! If uncoiled, 

 the DNA of a human cell would stretch out to a full length of about 1 mm. 



Coding Theory 



The manner in which DNA and RNA molecules can carry genetic in- 

 formation and control the sizes, shapes, and functioning of all the parts of 

 the complete living system is still a mystery, although some progress has 

 been made in understanding how this is done. 



The coding problem is simply enough stated as follows: Since there are 

 only four different pyridine and pyrimidine bases in the nucleic acid mole- 

 cule, and vet there are 20 or more amino acids which must be arranged in 



