When a cell is about to divide, as happens when tissue grows, the DNA in the mother cell has to be doubled, that is, new DNA is synthesized. It is possible to supply the cell with one of the building blocks of the new DNA, and to put a little radioactive tag on that building block.
当一个细胞要开始分裂时，比如在组织生长的时候，母细胞内的DNA必须被复制，就是说，新的DNA是被合成的。这样我们就有可能给细胞加入新DNA的某个积木，而在那个积木上贴上一些放射性的标签。然后，通过扫描大脑，每次当我们发现一个带有标签的细胞，我们就可以知道它是在我们在个DNA添加积木时产生的。