In this video, we're going to talk more about the second step of each PCR cycle, which is annealing. In this second step, the DNA primers are actually going to anneal to the heat-denatured single-stranded DNA. The temperature is going to be lowered to about 55 degrees Celsius. If you recall in step number 1, the temperatures were as high as 95 degrees Celsius. This lowering of the temperature allows for complementary base pairing to occur between the DNA primers and the single-stranded DNA itself.

Taq polymerase remains inactive at these cooler temperatures because the temperatures are just too cool for it to synthesize DNA. When we take a look at our image on the left-hand side, at the second step, annealing, you can see that the cooler temperatures allow the DNA primers to anneal to the DNA. After the first step of PCR, which is denaturation, we know that we've generated single-stranded DNA, this heat-denatured DNA. In the second step, the temperatures are cold so that the DNA primers can anneal to the DNA, as we see here. Notice that they are annealed on different DNA strands and are annealed facing towards each other in terms of their 5' and 3' ends. This one goes from left to right, and this one goes from right to left, facing towards each other.

Now that these DNA primers have been annealed, in the third step, they can be extended. However, it cannot be extended here in the second step because these temperatures of 55 degrees Celsius are just too cold for the Taq polymerase to work. Notice the Taq polymerase is saying, "brrr, I can't work like this." He's all cold, and he can only work when we increase those temperatures just a little bit to the right temperatures. This here concludes our brief introduction to the second step of PCR, annealing of the DNA primers. We'll be able to talk about the third and final step of PCR in our next video.