Introduction to Polymerase Chain Reaction - Video Tutorials & Practice Problems
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Introduction to Polymerase Chain Reaction
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In this video, we're going to begin our introduction to a technique that's called polymerase chain reaction. And so when a researcher studies the DNA sequence of a specific gene in a genome, they actually need to make many copies of that gene in order to be able to study it. And so this technique, polymerase chain reaction, which is commonly abbreviated as just PCR, is a technique that's used to rapidly isolate and amplify a specific sequence of DNA. Now, the term amplify is just referring to the process of making more copies of that DNA. And so, this is kind of similar to DNA cloning in a way because DNA cloning is also going to make more copies of the DNA. However, unlike DNA cloning, which is going to be using living cells to make more copies of the DNA. Polymerase chain reaction or PCR is going to take place in a test tube. And so, of course, the test tube is not going to include living cells. And so, we'll be able to talk more about this process of polymerase chain reaction and PCR as we continue to move forward in our course. But for now, this here concludes our brief introduction to polymerase chain reaction or PCR, and I'll see you all in our next video.
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Why do we use PCR?
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So now that we've briefly introduced polymerase chain reaction or PCR in our previous lesson video, it's fair to ask why do we use PCR? Well, it turns out that scientists will use PCR for many, many different reasons. And that's because PCR is a quick and efficient process for generating many identical copies of DNA in a test tube. Now DNA cloning, which occurs inside of living cells and does not occur in test tubes, is going to be more accurate, but it's actually a less efficient process. And that's because, recall, DNA cloning occurs inside of cells, and although there are less mutations inside of cells, it's going to take a lot longer to amplify the DNA inside of cells. And that's because a lot of times cells take 24 hours or more to be able to grow, and then after you grow the cells, you have to isolate the DNA, and that's a whole another step within itself. And so PCR is much more quick and much more efficient. And so in our example, down below, we're gonna take a look at, just in general, how PCR, or what PCR can do. And so, it says here that PCR can be used to amplify the amount of DNA taken from a crime scene so that a detective can actually investigate the DNA. And so down below over here, notice we have this scientist that has a test tube here with some DNA that perhaps was found at a crime scene, but notice that there's not a whole lot of DNA within this sample. And so notice the scientist is saying, I wish I had more of this particular DNA of interest, so that he has enough DNA to be able to run tests on it. And so this is where PCR can come into play because the process of PCR, polymerase chain reaction, can amplify the amount of DNA to make many, many identical copies of the DNA as we see over here. And so the scientists are saying, wow, that amplified very, very quickly. And the process of PCR can be done in a relatively short period of time, maybe something like 2 hours, 1 and a half, 2 hours, whereas DNA cloning, again, is gonna take much, much longer, which it could take well over 24 hours in many cases. And so, why do we use PCR? To make many identical copies of DNA in a test tube very, very quickly and efficiently. And so again, as we move forward in our course, we're going to continue to talk more and more about PCR and the steps of PCR. So I'll see you all in our next video.
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Problem
Problem
PCR is used to _____.
A
Amplify a single gene or small sequence of DNA.
B
Create DNA without introns.
C
Insert foreign DNA into a host cell where it can be replicated.
D
Cut segments of DNA creating complimentary sticky ends.
E
None of these.
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Components of a Polymerase Chain Reaction
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5m
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So before we can talk about the details of each of the steps in polymerase chain reaction or PCR, it's first helpful to talk about the components of Polymerase Chain Reaction. And so recall from our previous lesson video that PCR differs from DNA cloning in the location that the DNA is replicated. And so PCR occurs within a test tube, whereas cloning is going to occur within a cell. And so the components that go into the test tube during PCR include, these components that we have listed down below. And so the components of the PCR mixture include, a here is going to be a template DNA. And so the template DNA is going to contain the sequence of interest for the study that the scientist is interested in amplifying and creating more copies of it. And so down below in this table on the left hand side, you can see that the components of the PCR mixture in this table, corresponds with the components of the PCR mixture that we have up above in the text. And so the template DNA, you can see, is right here in this image down below. Now for the second component, B, there are 2 primers that are required, 2 DNA primers, And these 2 primers are going to be complementary to the opposite strands of DNA and are going to be oriented towards each other and serve as the starting point for the amplification. And so down below, what you can see is part b here are the DNA primers, which are gonna be small little DNA molecules, that are oriented opposite from one another. So notice that one goes from 5 prime to 3 prime left to right, and the other goes 5 prime to 3 prime right to left. And we'll talk more about these DNA primers and exactly how they're used as we move forward in our course. Now the third component that's going to be needed is a thermostable DNA polymerase. And the thermostable DNA polymerase is going to be the main enzyme that's used to synthesize the sequence of interest to amplify and make more copies of the DNA of interest. And so down below here in the 3rd component, c, you can see that we need a thermostable DNA polymerase, which is really, commonly going to be a DNA polymerase called Taq polymerase, which we'll get to talk more about Taq polymerase as we move forward in our course. Now the 4th and final component here that's going to be needed, in the PCR mixture is, d here, which is going to be all 4 deoxyribonucleotides or all 4 DNA nucleotides that are used to synthesize DNA. And so down below here in this image, you can see that d is going to include all 4 of those DNA nucleotides, which you can see over here, which include, these nucleotides that have the nitrogenous bases t, a, c, and g. And so these are the main components that are needed for a PCR mixture. Now over here on the right hand side, what we're showing you is a little glimpse of the process of amplifying a gene with PCR. And so, of course, you're gonna start off with your template DNA, which we have here in this first, column. And it turns out that PCR, polymerase chain reaction, actually occurs in a series of cycles. And so, the first cycle you can see is going to amplify the DNA. The second cycle will amplify the DNA even more, and the third cycle will amplify the DNA even more. And continuous cycles will amplify the DNA more and more and more. And so there's actually a PCR formula that can be used to determine the number of new copies of template DNA that are made at each cycle. And the formula is relatively simple. It's just 2 raised to the power of n where n is gonna act as an exponent here. And n is just a variable that represents the number of PCR cycles. And so for example, if we wanted to calculate the number of new copies of template DNA in the first cycle of PCR, all we need to do is take 2 raised to the power of 1 since one is representing, cycle number 1. And so 2 raised to the 1 is equal to 2. And so notice that after the first cycle, there are 2 new copies of DNA. Now after the second cycle, if we wanted to calculate how many new copies of template DNA there would be, you just take 2 raised to the power of 2. And 2 raised to the power of 2 is equal to 4. And so you can see that there are 4 new copies of the DNA after cycle number 2 of PCR. And the same goes for cycle number 3. If we wanna calculate the number of new copies of template DNA, you just take 2 and raise it to the power of 3, which is, the number of cycles, cycle 3. And 2 raised to the 3 is equal to 8. And so you can see that there are 8 copies of the DNA, after cycle number 3. And so, we'll be able to get some practice applying these concepts that we've learned as we move forward in our course. And then later, we'll get to talk about the actual steps that are involved in polymerase chain reaction or PCR. So I'll see you all in our next video.
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Problem
Problem
The polymerase chain reaction:
A
Doubles the template sequence of DNA with each cycle.
B
Makes a single copy of the template sequence of DNA with each cycle.
C
Is used to determine the sequence of a region of DNA.