How can isotopes be used in biological research?
They are used to track compounds through biological processes.Here is an illustration from early genetics research.
Alfred Hershey and Martha Chase used radioactive isotopes of sulfur (S-35) and phosphorus (P-32) as markers in the protein (sulfur marker) and DNA (phosphorus marker) of bacteriophage viruses.
They knew such viruses inject material into bacteria; but what do they inject? By using these markers Hershey and Chase could figure that out, because the sulfur would be found in proteins but not in DNA, while the phosphorus would be found in DNA but not in protein. So whichever marker showed up inside the bacteria would tell them what the viruses injected into it.
Using this strategy they determined it was DNA that was being injected into the bacteria.
Such tracking is possible because every radioactive isotope has a unique radiation "signature." Even if different atoms give off the same type of radiation (e.g., alpha, beta, or gamma) each isotope's radiation will have a unique energy associated with it. Therefore any isotope can be identified and tracked. There are many, many, examples of isotopes being used in similar ways.
Radioactive carbon (C-14) is also used to date material that used to be living. The level of C-14 remains consistent while an organism is alive, but decreases after death occurs. The amount of radioactivity in an organism's remains can be used to tell when it was alive. This isotope is good up to about 50 000 years after death.
For things dating further back, such as fossils, other elements can be used.