Because all we have to do is create a polyceetail. Because we know if we have the A's on this side, they will base pair with the T's on the other side. So we basically create a DNA primer complementary to the three end of the modified mRNA molecule and recall that all eukaryotic mRNA have a polyatail on the three prime ends. So we simply create a DNA primer with the polyt sequence and now we mix the primer with our mRNA molecule in a solution at the right temperature and the kneeling process will take place and the DNA primer will hybridize with this polyatail on the three prime end of that mRNA molecule. Now in the next step, in step two we want to add that reverse transcriptase. That reverse transcriptase will bind onto that primer and in the presence of, of the four types of deoxy nucleus cytrifosphates, it will begin to synthesize that complementary strain, the DNA strand that is complementary to that mRNA.