PCR

In molecular biology, the **polymerase chain reaction** (**PCR**) is a technique to elongate a section or more of a DNA strand across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence. The method relies on __**thermal cycling**__.

//Thermal Cycling//: Heating and cooling to alter the DNA strand that is being enlarged.
 * In 1993 Mullis was awarded the Nobel Prize in Chemistry for his work on PCR.
 * This process is used to diagnose hereditary diseases, identify genetic figerprints mostly for paternity testing and forensic sciencetists, cloning of DNA for sequencing, etc.

__Needed in the process:__ This process is usually done by a thermal cycler.
 * DNA template that contains the DNA region that is being elongated
 * Two primers that are complementary to the 3' (three prime) ends
 * DNA polymerase with a temperature optimum at around 70 °C
 * Deoxynucleoside triphosphates- the building blocks from which the DNA polymerases synthesize a new DNA strand
 * Buffer solution- provides a good chemical environment for stability of the DNA polymerase

//Thermal Cycler:// The thermal cycler heats and cools the reaction tubes to achieve the temperatures required at each step of the reaction. Many modern thermal cyclers make use of the Peltier effect which permits both heating and cooling of the block holding the PCR tubes simply by reversing the electric current.





__There are 3 main steps to the procedure:__ __Procedure:__
 * //Denaturation step:// This step is the first regular cycling step and consists of heating the reaction which makes the DNA from the DNA template melt by disrupting the hydrogen bonds that hold together the base pairs, making a single strand of DNA.
 * //Annealing step:// This process cools the single strand of DNA and the polymerase binds to the primer-template hybrid and begins DNA synthesis.
 * //Elongation step:// Finally the DNA polymerase synthesizes a new DNA strand complementary to the DNA template strand. The extension time depends both on the DNA polymerase used and on the length of the DNA fragment to be amplified.



To check whether the PCR generated the anticipated DNA fragment, agarose gel electrophoresis is used for size separation of the PCR products. The sizes of PCR products are determined by comparing a DNA ladder, which contains DNA fragments of known size, run on the gel alongside the PCR products.

__The PCR process can be divided into three stages:__


 * 1) //Exponential amplification:// At every cycle, the amount of product is doubled if done 100% correctly.
 * 2) //Levelling off stage//: The reaction slows as the DNA polymerase loses activity and as consumption of reagents such as **dNTPs** and primers causes them to become limiting.
 * **dNTP**- It is a deoxyribonucleotide, which is the monomer, of DNA.

3. //Plateau:// No more product accumulates due to exhaustion of reagents and enzyme.