30 : 6/ Tracer Techniques 567 



and form templates for protein synthesis. Phosphorus is used by living 

 cells in the energy-storing compound ATP (adenosine triphosphate). 

 It plays an important part in many syntheses and oxidations. Calcium 

 phosphate is a major constituent of bone. Naturally occurring phos- 

 phorus is all P 31 . 



Phosphorus has one biologically important radioactive isotope, P 32 . 

 It is made from sulfur by the reaction 



S32 + n > _^ p32 + H ' 



For economic reasons, the reaction is carried out using neutrons from a 

 nuclear pile. Phosphorus isotope P 32 can be separated from the other 

 reaction products by chemical methods. 



The isotope P 32 has a half-life of 14.3 days. Just as is true of I 131 , it is 

 convenient to use because its half-life is long enough to permit experi- 

 ments but short enough to produce an easily detectable rate of dis- 

 integration. Both C 14 and P 32 emit only negative beta particles. Those 

 from C 14 are difficult to detect because their maximum energy is only 

 about 0.15 Mev; particles from P 32 are easy to detect because they have 

 a maximum energy of 1.7 Mev, more than a factor of 10 greater than 

 that of C 14 . 



The tracer isotope P 32 has been used to study the rate of renewal of 

 deoxyribose-nucleic acid (DNA) and ribose-nucleic acid (RNA). 3 In 

 brain tissue, less than 1 per cent of the DNA is renewed per day. In 

 liver tissue, it approaches 1 per cent, whereas in the mucosa of the small 

 intestines it may be as high as 15 per cent. This illustrates that in cells 

 that are not multiplying rapidly, there is comparatively little turnover 

 of DNA. In embryonic tissue and cancer tissue, the rate of DNA 

 removal (or new synthesis) is even greater than in the mucosa of the 

 small intestines. This evidence is one of the lines indicating that DNA 

 is associated with genetic information. 



RNA is synthesized much more rapidly than DNA in all tissues except 

 the most rapidly growing ones. Although the rate of synthesis of RNA 

 is somewhat higher in the mucosa of the small intestines than in the 

 liver, the difference is only a factor of two or three, as opposed to a 

 factor of 15 between the DNA rates. This evidence is in accord with 

 the idea that RNA is not directly responsible for the transmission of 

 genetic information. 



6. Stable Isotopes 



In addition to radioactive isotopes, stable ones may also be used as 

 tracers. However, there is no comparably simple way of detecting the 

 3 See Chapter 15 for a discussion of nucleic acids. 



