PCR is an enzyme-mediated reaction, and as with any enzyme, the reaction must occur at the enzyme's ideal operating temperature. The enzymes that are used for the PCR are DNA-dependent DNA polymerases (DDDP) derived from thermophilic (heat-loving) bacteria. As such, the enzymes function at higher temperatures than the enzymes we commonly use in the laboratory or have working in our bodies. These DNA polymerases operate at 60-75°C, and can even survive at temperatures above 90°C. This is important because a part of the PCR requires that the reaction reaches ~95°C as we shall see.
Apart from the DNA polymerase, PCR needs a DNA template to copy, and a pair of short DNA sequences called oligonucleotides or "primers" (described here) to get the DNA polymerase started.
Broadly speaking, there are 3 steps identified by incubating at different temperatures. The 3 steps make up a PCR "cycle".
- Double-stranded DNA separation or denaturation (D in Figure 1)
- Primer annealing to template DNA (A in Figure 1)
- Primer extension (E in Figure 1)
|Figure 1.A PCR cycle.|
At temperatures above 90°C, double-stranded DNA denatures or "melts". That means the weak hydrogen bonds that usually hold the two complementary strands together at normal temperatures are disrupted resulting in two single stranded DNA strands (shown below in an idealised form).
At the annealing temperature (TA), primers that collide with their complementary sequence can hybrdise or "bind" to it. The chance of such an encounter happening is increased because we use a vast excess of each primer in the reaction mixture compared to the number of template molecules present.
The assay in the example below has been designed to amplify a region of the template spanned by and including, the primer sequences.
At the extension temperature (TE), the DNA polymerase binds to the hybridized primer and begins to add complementary nucleotides (i.e. every time the polymerase reads a "G" on the template strand, its adds a "C"; an "A" for a "T"; a "G" for a "C" and a "T" for an"A"), chemically binding each new addition to the last to form a growing chain. The process only occurs in one direction. In our example, the green primer is binding to its complementary template sequence and is facing toward the right (this is called the 5' (five-prime) to 3' (three prime) direction. Extension occurs in the direction that the primer faces. The result is a new double-stranded PCR product we usually call an "amplicon". An amplicon can be defined as an amplified molecule of a single type, in this case, an exact replicate of the original template.
Exponential Template Duplication..
The process is then repeated by cycling through the temperatures over and over again (35 to 55 times). Each cycle results in a new DNA duplex, each strand acting as a potential template for one or other primer.
Some interesting things stand out from the figure below.
The original template strands (blue and red) continue to act as templates because the PCR process is not destructive. However, each cycle produces a greater number of the shorter amplicon molecules. These are shorter in our example because the primers shown, bind within the template sequence. Eventually the majority of the amplicon in the reaction vessel will be the expected length, i.e. just the region spanned by, and including the primer sequences.
It is possible to mathematically predict the pattern of amplicon accumulation. In our example, we have started with two strands. In a perfect PCR reaction (which rarely occurs!), we have two new strands making a total of four. After the second cycle we have eight strands, then 16, 32 and so on. The reaction is doubling the number of strands each cycle or to make that an equation, we have 2n
Note: to make the process easier to understand, I have drawn the DNA strands as straight lines - in reality, DNA does not exist in as simple a form as this.
- PCR primers...a primer!
- Reverse transcription polymerase chain reaction (RT-PCR)...a primer
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