Standard Polymerase Chain Reaction (PCR)

Basic Conditions using standard Taq polymerase

PCR reactions involve template, forward and reverse primers, buffer, dNTPs, DNA polymerase and water. A typical reaction has a final volume of 30 μl, a template concentration of 0.1ng/μl, and primer concentrations of 500nM each. This chart shows the volumes of various ingredients that should be used.

These PCR conditions are suitable for products ranging up to 3 kb in length. PCRs that result in longer products may require optimization of the dNTP and primer concentrations and the use of special DNA polymerase kits.

Template 10然 UF primer 10然 DR primer 10x buffer 2mM dNTPs 5U/無 taq ddH2O Total
X 1.5 1.5 3 3 0.3 20.7 - X 30 μl

10x dNTPs are 2 mM in each dNTP.

If the stock concentration of DNA is 1ng/μl, 3 μl would give the desired concentration of template. 1.5 μl of a 10μM primer gives a final concentration of 500nM. Water is added as needed to create a final volume of 30 μl.

When more than a few reactions are being made (5+), it is advisable to use a mastermix. A mastermix is a batch of all of the ingredients common to all the reactions, i.e., water, buffer, polymerase, dNTPs. To make a mastermix, take the amount of each of the individual ingredients needed (ex: 3 μl 10x dNTPs) and multiply it by (number of reactions + 1). So if 8 reactions are being run, the mastermix would contain 27 μl 10x dNTPs. When many PCR reactions are being run (15+) it often becomes necessary to add more than just 1 extra batch for the final volume.

Mastermixes not only save time and materials, but they also allow for much greater precision. Measuring out 0.3 μl of 100x Taq polymerase with a pipette is very inaccurate, however, 2.7 μl of the same Taq can go into the MM, from which volumes of higher precision can be withdrawn.

In the thermocycler, make sure that the program "PCR" contains the appropriate temperatures and times. The most important step to check, is that the 72蚓 elongation step should be roughly 1 minute for every 1000bp of the longest PCR product. If the longest expected product is 2500bp, then 2.5 minutes at 72蚓 should be appropriate. For most PCRs, 30-40 cycles should be appropriate.

Further Reading

Basic conditions using NEB Phusion HF Polymerase

When using Phusion polymerase, use the NEB calculator to approximate your annealing temperatures.

For a 20 ul reaction

Template 10然 UF primer 10然 DR primer 5x Phusion buffer 10mM dNTPs Phusion poly ddH2O Total
X 1.0 1.0 4 0.4 0.2 up to 20ul 20 μl

For a 50 ul reaction

Template 10然 UF primer 10然 DR primer 5x Phusion buffer 10mM dNTPs Phusion poly ddH2O Total
X 2.5 2.5 10 1 0.5 up to 50痞 50 μl

Final Concentrations

Template For primer Rev primer 5x Phusion buffer 10mM dNTPs Phusion poly ddH2O
<250 ng 0.5然 0.5然 1x 200然 1.0 units/50ul PCR -

NEB recommends a template amount (for 50痞 reaction) of 50 ng - 250 ng for genomic DNA and 1 pg - 10 ng for plasmid or viral DNA.

Although the recommended Phusion polymerase concentration is 1.0 units/50 ul rxn (20 units/ml) this concentration may have to be changed based on amplicon length and difficulty. The concentration may vary from (0.5-2 units/50 ul rxn) but should not exceed 2 units/50 ul rxn, especially for amplicons longer than 5 kb.

Thermal cycler conditions:

Denaturation: An initial denaturation of 30s at 98 C is sufficient for most amplicons from pure DNA templates; however, longer denaturation times can be used (up to 3 m) for templates that may require it. During thermocycling, this step should be kept to a minimum; typically, NEB recommends a 5-10 s denaturation at 98 C for most templates.

Annealing: Note that the annealing temperatures required for use with Phusion tend to be higher than with other polymerases. Use the NEB calculator to approximate your annealing temperatures. Typically, primers greater than 20 nucleotides in length anneal for 10-30 seconds at 3 C above the Tm of the lower Tm primer.

Extension: Times are dependent on amplicon length and complexity. Generally, an extension time of 15 seconds per kb can be used. For complex amplicons, such as genomic DNA, an extension time of 30 seconds per kb is recommended.

Please note that PCR products generated using Phusion poly have blunt ends.

Agarose Gel Electrophoresis

In order to analyze PCR results, the products are run on an agarose gel and the resulting gel is observed in UV light.

First, the gel has to be made. A standard 1% agarose gel uses 1g of agarose for every 100 ml of buffer. A different percentage may be used, and gels with less than 1% agarose may be used to clearly distinguish products of very similar sizes. For a standard 50 ml gel, add .5g agarose and 50 ml TAE (1X) buffer to a 125ml flask and heat for 1:30 minutes. Meanwhile, assemble a gel rig and find a comb with an appropriate number of wells, then place the comb into the rig. After heating add 2.5 μl SYBR Safe (5μl SYBR Safe for every 100mL gel) and swirl to mix. Pour the liquid from the flask into the rig and wait about 30 minutes for it to solidify.

Once the gel has solidified, gather all PCR products that are to be run, an appropriately sized ladder, and 6x loading dye. The latter two may be found in the 4蚓 fridge in the computer room adjacent to the gel area. First, load 6-7 μl of ladder into the first well. The easiest way to combine dye and DNA is to cut out a 4x4 sheet of parafilm, make a drop of 1 μl of dye onto the parafilm for each sample to be run. Next, add 5 μl of PCR product to the dye and pipette up and down to homogenize. Once all samples are combined with dye, load them into the gel, making note of what sample goes into what lane.

Whole-Cell PCR

  • Grow fresh overnight cultures of the cells to be used as template in LB.
    • Important: Do NOT use E. coli grown in Davis Minimal (DM) media as template for whole-cell PCR. Ingredients in DM interfere with reliable amplification. (Probably the high concentration of citrate added as a chelating agent).
    • Amplification from frozen cultures is also not as reliable as from freshly grown cells.
  • Dilute cells 1000x fold into ddH2O (NOT saline) in a 1.7 ml Eppendorf tube or 96-well microplate.
    • Too many cells or too much residual media is a common source of failure for whole-cell PCR.
  • Add the diluted cell mixture as 1/10th the final volume of the PCR reaction.
  • Add an initial denaturing step of 10 minutes to your PCR program at 94蚓 to lyse the cells.
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