Polymerase Chain Reaction | Learn With Me 1
listen to the audio version of this post here
Today’s post comes from Preksha Naik
Polymerase chain reaction (PCR) is one of the most important advances in molecular biology. Anytime we have a limited source of DNA (e.g. a single strand of hair on a crime scene suspect or analysis of DNA from ancient times), PCR can be used to generate millions of DNA copies.
To understand PCR, we need to understand the following elements:
- DNA is built from nucleotides (Adenine, Thymidine, Cytosine, and Guanine — ATCG) and consists of two strands of these nucleotide pairings (A always pairs with T, and C always pairs with G). For example, DNA would be the two strands running in parallel (ATTCGT and TAAGCA). The two strands are always complementary to each other.
- The strands are held together by hydrogen bonds, and we can separate them by heating them up (denaturation).
- Primers are short sequences of DNA that can start the chain reaction of binding.
- Polymerases are enzymes (proteins) that can attach nucleotides to primers.
The most complex part of the process is designing the primers. You can liken primers to a starter for making yogurt or bread (something that sets off a chain reaction). The sequence is then: one, denature (split the DNA up into two strands); two, anneal (get the primers to attach to either end of the denatured DNA); and three, extend (get the polymerase to build up the sequences from the primers).
These steps are then repeated between 25 to 35 times. Since the amount of DNA doubles each time, this is roughly 2³⁵ = 34,359,738,370 new strands!
Here is a short video explaining the PCR process start to finish, software, equipment and post-analysis.
