Dideoxynucleoside triphosphates can be incorporated into a growing DNA chain by DNA polymerases. Suppose you are carrying out a DNA replication experiment in vitro (in a test tube not in a living cell).
You add to your reaction mixture a 2 kb single stranded linear template DNA and a 15 base primer that is complementary to the first 15 bases at the 3' end of the template, all 4 dNTPs, and DNA polymerase. After the reaction has gone to completion you electrophorese the products in agarose and stain. The dNTPs you add contain a-32P so you can identify all reaction products by radioautography by exposing the gel to a photographic film. What do expect the autoradiogram to look like? You carry out the same reaction but, in addition to all of the components above, some ddATP is also added. What do you expect the gel to look like in this case?