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CHAPTER >< Isotope Methods 



INTRODUCTION 



There are two kinds of isotopes: radioactive and nonradioactive. Both 

 kinds have important roles in biological studies. They may both be 

 used as tracers in a given substance during its uptake and metabolism 

 by any organism, and also in test-tube syntheses. Radioactive isotopes 

 have two further uses. First, they may be used as indicators of localiza- 

 tion within cells or within organisms without having to take the organism 

 apart. Second, their disintegration may be used to effect an alteration 

 in the functioning or the structure of biological systems. We will give 

 examples of all three uses later in this chapter. 



Some isotopes have a special use because they occur chiefly in one or 

 a few types of biologically important compounds. The following table 

 gives some examples, along with the lifetimes of the radioactive isotopes. 



Isotope Primary occurrence in Lifetime (half-life) 



Carbon- 14 

 Phosphorus-32 

 Tritium-3 

 Iodine-131 

 Sulfur-35 

 Nitrogen- 15 

 Oxygen- 18 

 Deuterium-2 



everything 



nucleic acids 



everything 



thyroid compounds 



proteins 



most compounds 



most compounds 



everything 



5700 years 

 14.3 days 

 12.5 years 

 8.1 days 

 87.1 days 

 stable 

 stable 

 stable 



Even though a number of these isotopes are seen to occur in many 

 biological compounds, they may nevertheless be used in a specific way 

 by synthesizing special compounds containing them. For instance, if 

 thymidine is synthesized with tritium, it can be used to study nucleic 

 acids, because thymidine is known to be incorporated almost entirely 

 by deoxyribonucleic acid. The same isotope could be used to study RNA 

 by using uridine which had been synthesized in the presence of tritium. 



The basic relation for the survival of undecayed radioactive atoms 

 may be obtained by methods readily available to us. If there has been 



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