Isotopes of Phosphorus

There are many isotopes of phosphorus, only one of which is stable (³¹P). The rest of the isotopes are radioactive with generally very short half-lives, which vary between a few nanoseconds to a few seconds. Two of the radioactive phosphorus isotopes have longer half-lives. ³²P has a half-life of 14 days and ³³P has a half-life of 25 days. These half-lives are long enough to be useful for analysis and for this reason the isotopes can be used to mark DNA.

³²P played an important role in the 1952 Hershey-Chase Experiment. In this experiment, Alfred Hershey and Martha Chase used radioactive isotopes of phosphorus and sulfur to determine that DNA was genetic material and not proteins. Sulfur can be found in proteins but not DNA, and phosphorus can be found in DNA but not proteins. This made Phosphorus and Sulfur effective markers of DNA and protein, respectively. The experiment was set up as follows: Hershey and Chase grew one sample of a virus in the presence of radioactive ³⁵S and another sample of a virus in the presence of ³²P. Then, they allowed both samples to infect bacteria. They blended the ³⁵S and the ³²P samples separately and centrifuged the two samples. Centrifuging separated the genetic material from the non-genetic material.  The genetic material penetrated the solid that contained the bacterial cells at the bottom of the tube while the non-genetic material remained in the liquid. By analyzing their radioactive markers, Hershey and Chase found that the ³²P remained with the bacteria, and the ³⁵S remained in the supernatant liquid. These results were confirmed by further tests involving the radioactive Phosphorus.