“PCR primers are short single-stranded DNA sequences which help in the amplification of DNA during PCR reaction.”
PCR technique is one of the most anticipated technique is the genetic science, as it facilitates replication of DNA in vitro. Each and every component of PCR reaction are equally important.
dNTPs, PCR buffer, primer, water, Taq DNA polymerase and template DNA are the major ingredient for PCR reaction.
We have covered an entire article on the role of MgCl2 in PCR please read the article for more detail
PCR reaction is complete in three steps (initiation, elongation and termination). Initiation begins with the binding of a primer, therefor primer is the primary ingredient of PCR reaction.
Generally, PCR primers are DNA primers. As we all know that in in vivoreplication short RNA primers are used instead of DNA primer while in PCR we are using DNA primer. There are several assumptions that favour the use of DNA primer in PCR :
- DNA primers are more temperature stable than RNA primers.
- The process of DNA polymerization in PCR is unidirectional so there is no chance of removal of short RNA primer after the polymerization is completed.
- Additionally, DNA polymerase I help in removing of short RNA primer in vivoreplication which is not present in PCR.
Ultimately we are interested in studying DNA not RNA that is the reason we want only DNA primer for PCR.
Before we get started with the primers, we have to understand several terminologies.
The first and foremost is melting temperature and annealing temperature of DNA.
Melting temperature is a temperature at which the half on the DNA (template DNA) is broken opens. The melting temperature of DNA depends on the GC and AT content of DNA. We know why, right.
For more detail on the DNA replication process, read the article: The general process of DNA Replication
Primer binds to the sequences which have the same melting temperature as the primer. Otherwise, if the temperature is not appropriate than it binds other than its target site.
The annealing temperature is a temperature which required to anneal primer to its complementary strand. The annealing temperature varies from primer to primer.
The primer used in PCR:
Each enzyme required a co-factor and a substrate for completion of the enzymatic reaction. Taq DNA polymerase required free 3’OH end for starting the polymerization. The primer provides a free 3’ OH end for polymerase and it is work as a substrate for the enzyme to work.
Criteria for primer used in PCR:
A temperature at which primer can bind to its complementary sequence is called as annealing temperature.
It is a very important parameter in designing a primer. Annealing temperature should be 5ºC lower than the melting temperature. Melting temperature of the primer is calculated using the formula below,
Tm= 4 (G + C) + 2 (A + T)
An ideal annealing temperature of the primer is ranging between 56ºC to 65ºC. Variation in this range hinders PCR amplification.
If the annealing temperature is too low, the primer can bind to any of the complementary sequences and gives a non-specific result. The primer cannot bind if the annealing temperature is too high.
Length of the primer
Primers are short sequences, generally, 18 to 23 nucleotide long primer always give the best result in PCR. Shorter primers (>18bp) do not have the affinity to amplify properly in each cycle.
If the primer is too short, annealing temperature becomes lower and it reduces amplification capacity.
Long primers are also not recommended because the annealing temperature of the long primer is too high, it leads to non-specific binding. However long primer is highly specific in long-range PCR.
GC content in primer
Another important factor in the designing primer is GC content. GC content between 40% to 60% is acceptable. The annealing temperature of the primer between 55ºC to 65ºC with 50% GC is best in performance.
If GC content is too high, at given annealing temperature primer will mismatch with other sequences. Additionally, GC reach sequences are highly non-specific. The chance of mismatch in GC rich region is very high as compared to the AT-rich region.
It is critical to understand that if 8 to 10 bases of primer will match with other sequences and the annealing temperature is too low, it definitely amplifies the DNA but gives the false result.
Complementation in forward and reverse primers of PCR
While designing primer, keep in mind that both forward and reverse primer do not match with each other or are not complementary with each other. Otherwise, instead of binding with target sequences, both primer will bind with each other and creates a dimer.
More than 4 complementary bases and lower annealing temperature induces dimer formation. When primers are binds with each other instead of binding with the target sequence, it creates a dimer. Dimers can easily be amplified in PCR because it is shorter sequences of up to 50 to 60 bp.
Read the article Prokaryotic DNA replication
Repeated bases can bind within the primer and makes primer non-active. If some of the dinucleotides or trinucleotide are repeated in primer, it binds within the primer and creates hairpin.
More specifically, if repeated bases are present on the terminal end of 3’ end it will create a serious problem in PCR.
My ultimate guide to designing a PCR primer
During my research works my topics are majorly PCR centred. I want to share my experience on how we can successfully design a primer.
Firstly, Identify you template sequence.
It is very important to identify which gene or DNA fragment we want to amplify. Identify that sequence and obtain it from NCBI.
Now go to the primer 3 software which is open access and freely available primer designing tool and it is widely accepted. However, each primer designing companies have their own primer design software.
You can go to primer 3 from here: http://bioinfo.ut.ee/primer3-0.4.0/
Actually, I think you should try it side by side in another tab. I will Give you one sequence,
This is a beta-globin gene sequence (copy and paste it in a box of primer 3)
Now select the options for forward primer and reverse primer shown as red arrows. Never select the option given in the middle (labelled as black) because we want to run the simple PCR hence we do not need a probe.
In the next step just for understanding read the information given on primer 3 page, read the specification but do not click on any of the boxes because all the information is automatically or by default set by the software.
In the next step as shown in the figure click on the “Pick primer” button and wait for the result.
The primer 3 output is shown in the figure (above). Analyze first the result window. You can see that the parameters like the length of the primer, GC content, annealing temperature and hairpin formation all are under the standard criteria.
Now take a look at the red line. The primer gives you 231bp fragment so when you run the PCR based on the criteria of this primer, your product should be 231.
The arrows (>>>>>> and <<<<<<<) shows the annealing site of primer to your sequence. Additionally, the software gives you other pairs of possible primers in the bottom as shown in the figure (Below).
Your primer is ready for the order. In the next step find out the company which gives service in your area. Send them the detail or fill the online form of primer detail. While filling the detail, keep backcrossing your sequence.
If you made a mistake in a single base, you will not get the PCR result or false result.
You will receive the primers in precipitated form with one primer specification paper as shown in the figure.
The specification paper has all the information regarding the primer. It contains the yield at 260nm OD, a sequence of primer, the yield of primer in microgram, the yield of primer in nano mol and other specification as shown in the figure (above).
Our primer is in the form of the solid precipitate. We have to revive it for further use. Recall the PCR procedure, we need an approximately 10pmol primer for our PCR reaction.
We have covered an article on DNA precipitation please read the article for a detailed understanding of DNA precipitation. read the article here: Role of alcohol in DNA extraction
Suppose the given concentration of or primer is 29.1nM. When we add PCR grade water of 291µl to the primer tube, the final concentration of our tube become 100pM/µl.
Do all the procedure in a sterile area now gently try to dissolve the primer in water. This concentration is our stock concentration of PCR primer.
To achieve 10pmol final concentration for PCR reaction, take 1microliter from the stock primer and add 9µl of water (again PCR grade) to it. Now our primer with 10pM/µl concentration is read. We can use 1µl from this working.
For long-term use of primer revive all the primer tubes in TE buffer and make different aliquots of the tubes. Store all tubes in -20C.
10pM/µl concentration is sufficient for the 25µl PCR reaction, excess concentration of primer results in dimers and non-specific binding.
Article written by: Tushar ChauhanArticle reviewed by: Tushar kachhadiya