In this video, we're going to talk about the 3rd and final step of each PCR cycle, which is extension. And so the final step of PCR, which is this third step, is the extension of the new DNA strand by the thermostable Taq polymerase. And so the temperature in the third step is going to be changed and increased from 55 degrees Celsius in the second step to about 72 degrees Celsius here in the third step. And this 72 degrees Celsius is an ideal temperature for the activity of the Taq polymerase. And so the Taq polymerase is going to incorporate deoxyribonucleotides and use them to extend the DNA primers on each strand of the DNA, making 2 identical copies of the DNA. And so, if we take a look at our image down below at this third step here of the PCR cycle extension, what you'll notice is that over here on the left, we're starting off with what we ended with in the second step, which is the annealing of those DNA primers. And so here in the second step, the temperature is going to be increased to the ideal temperature for Taq polymerase. And so the Taq polymerase is able to incorporate the DNA nucleotides, the free DNA nucleotides, and use those free DNA nucleotides to extend each of the primers. And so you can see that the extension of the primers is always going to be in the 5 prime to 3 prime direction. And so the Taq polymerase will extend this in this direction, and the Taq polymerase here will extend this primer in this direction from right to left. And so the Taq polymerase extends the new strands of DNA until we have the template DNA which has been replicated.

So the template DNA has been replicated here. And so you can see that originally we now have 2 identical copies of the original template DNA. And so that is going to conclude this 3rd and final step of each PCR cycle. And so after the third step has been completed, then it can go back into a new cycle of PCR, where each of these products from the 1st cycle can be used as reactants in the second cycle and so on. And so this here concludes our introduction to the 3rd step of PCR extension. And we'll be able to get some practice applying all of the concepts that we've talked about as we move forward in our course. So I'll see you all in our next video.