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Laboratory for Environmental Pathogens Research Department of Environmental Sciences University of Toledo
Polymerase Chain Reaction (PCR)
Background information The polymerase chain reaction (PCR) is an enzymatic process that allows for the detection of specific genes within an environmental DNA sample. PCR utilizes short, user defined DNA sequences called oligonucleotide primers, the sequence of which are complementary to target regions of genes known to encode for specific microbial functions (e.g. contaminant degradation). In brief, the DNA sample is denatured to produce single stranded DNA, called template DNA, to which the oligonucleotide primers can bind. The enzyme DNA polymerase then adds nucleotide bases to the end of each primer, using the template DNA as a guide to extend the primer thereby producing new double stranded DNA. This process is repeated for a number of cycles to enrich the DNA sample for the desired genes targeted by the oligonucleotide primers. Since each cycle of PCR involves creating two new double stranded DNAs from each DNA molecule present, the amount of DNA theoretically doubles with every cycle of PCR. Therefore, after two cycles the concentration of DNA increases by 2^2 - fold, after 3 cycles a 2 3
- fold increase, etc. After N cycles, PCR generates a 2 N - fold increase in the target DNA. From: http://allserv.rug.ac.be/~avierstr/principles/pcr.html
Materials Template DNA (genomic, plasmid, bacterial colony, etc.) Primers (resuspended in sterile water or TE to a concentration of 100 mM ) Buffer (usually 10X , usually sold with Taq polymerase or you can make your own) MgCl 2 (available in 25mM or 50 mM stocks) Bovine serum albumin (BSA, 30 mg ml
- 1 stock) Taq DNA polymerase dNTPs ( 2.5 mM working solution) Note: a 2.5 mM working solution of dNTPs means that the final concentration of each dNTP (dATP, dCTP, dGTP, and dTTP) is 2.5 mM, not that all dNTPs together make 2.5 mM. dNTPs come as 100 mM stocks. Therefore, to make the working solution thaw and add 20 μL of each dNTP to 720 μL of nuclease-free water, mix thoroughly and aliquot in 100 μl volumes. Store at - 20°C. Sterile, nuclease-free water Gloves PCR thermalcycler Pipettes (1- 10 μl, 5- 50 μl, 20- 200 μl, and 100- 1000 μl) and aerosol barrier pipette tips PCR tubes (0.2 ml or 0.5 ml) Master mix tubes (1.5 ml microcentrifuge tubes) PCR allows the production of more than 10 million copies of a target DNA sequence from only a few molecules. Therefore, PCR is very sensitive to contamination from non-target DNA. Several steps should be taken to reduce the chance for contamination, including:
- Fresh gloves should be worn for DNA purification and each reaction set-up.
- Using aerosol tips (tips with a wad of cotton at the top)
- Limit the amount of close contact with sample tubes and reagents, i.e. don’t spit on your workspace.
- DNA sample preparation, reaction mixture assembly and the PCR process, in addition to the subsequent reaction product analysis, should be performed in separate areas.
- The reagents for PCR should be prepared separately and used solely for this purpose.
- Only nuclease-free water should be used in the preparation and suspension of PCR reagents.
- Unless the solution is purchased sterile, autoclaving of all solutions, except dNTPs, primers and Taq DNA polymerase is recommended.
- Aliquoted solutions in small portions and store them in designated PCR areas. Aliquots of PCR reagents should be stored separately from DNA samples.
buffers are often available in 10X concentration and are sometimes Taq formulation-specific. Although most protocols recommend a final buffer concentration of 1X, increasing the concentration to 1.5X might result in increased PCR product yield. PCR primers PCR primers are short fragments of single stranded DNA (15-30 nucleotides in length) that are complementary to DNA sequences that flank the target region of interest. The purpose of PCR primers is to provide a “free” 3’-OH group to which the DNA polymerase can add dNTPs. The C and G nucleotides should be distributed uniformly throughout of the primer and comprise approximately 40-60% of the bases. More than three G or C nucleotides at the 3'-end of the primer should be avoided, as nonspecific priming may occur. The primer should not be self-complementary or complementary to any other primer in the reaction mixture, in order to avoid primer-dimer and hairpin formation. All possible sites of complementarity between primers and the template DNA should be noted. If primers are degenerate, at least 3 conservative nucleotides must be located at the primer's 3'-end. The melting temperature of flanking primers should not differ by more than 5°C. Therefore, the GC content and length must be chosen accordingly. If the primer is shorter than 25 nucleotides, the approx. melting temperature (Tm) is calculated using the following formula: , where G , C , A , and T , are the number of respective nucleotides in the primer. If the primer is longer than 25 nucleotides, the melting temperature should be calculated using specialized computer programs where the interactions of adjacent bases, the influence of salt concentration, etc. are evaluated. The PCR annealing temperature (TA) should be approximately 5°C lower than the primer melting temperature.
MgCl 2 The concentration of MgCl 2 influences the stringency of the interaction between the primers and the template DNA. The range of MgCl 2 usually tested is from 0.
- 4 mM in 0.5 mM increments, while the default starting point is often is 1.5 mM. Low MgCl 2 concentrations can help to eliminate non-specific priming and background PCR products and are desirable when fidelity of DNA synthesis is critical. At the same time however, too few Mg 2+ ions can result in a low yield of PCR product. High MgCl 2 concentrations can help to stabilize interaction of the primers with their intended template if it is not being amplified, but can also result in nonspecific binding and erroneous PCR product formation. Be aware that some PCR buffers (often sold in 10X stocks) already contain some MgCl 2. Therefore the amount of additional MgCl 2 must be carefully monitored. Deoxynucleotide triphosphates (dNTPs) DNTPs are the nucleotide bases added to the growing DNA strand by the DNA polymerase. The concentration of each dNTP in the reaction mixture is usually 200 μM. It is very important to have equal concentrations of each dNTP (dATP, dCTP, dGTP, dTTP), as inaccuracy in the concentration of even a single dNTP dramatically increases the misincorporation level. Taq DNA polymerase This DNA polymerase is isolated from the bacterium Thermus aquaticus , which lives in hot environments and requires biomolecules that are heat stable. Therefore, Taq DNA polymerase can efficiently synthesize DNA under the heat- intensive conditions of the PCR reaction. Usually 0.02 units of Taq DNA polymerase are used per μl of reaction mix. Higher Taq DNA Polymerase concentrations may cause synthesis of nonspecific products. However, if inhibitors are present in the reaction mix (e.g., if the template DNA used is not highly purified), higher amounts of Taq DNA Polymerase (0.04-0.06 units) may be necessary to obtain a better yield of amplification products. Bovine serum albumin (BSA, optional) BSA is a PCR additive that in some cases can enhance the yield of PCR products. It is thought that the BSA sequesters inhibitors that can result in inefficient reactions, however the exact mechanism is unknown. From experience, BSA might or might not result in any yield increase, but usually will not inhibit the reaction itself.
melting temperature of primer-template DNA duplex. In most cases the TA is between 50 and 65o^ C. However, if nonspecific PCR products are obtained in addition to the expected product, the annealing temperature should be optimized by increasing it stepwise by 1- 2 o C. Extension The processivity of Taq DNA polymerase is approximately 150 nucleotides sec
- 1 . Based on this estimate, the extension time necessary to synthesize copies of the 16S rRNA gene (1500 bp) is 10 seconds. However, reaction conditions, template quality, and the presence of inhibitors require that the extension time be increased, usually to between 45-60 seconds for 1500 bp. When larger DNA fragments are amplified, the extending time is usually increased by 1min for each 1000 bp. The extending step is commonly performed at 70-75°C, as the rate of DNA synthesis by Taq DNA polymerase is highest at this temperature. Cycle number The number of PCR cycles depends on the amount of template DNA in the reaction mix and on the expected yield of the PCR product. For less than 10 copies of template DNA, 40 cycles should be performed. If the initial quantity of template DNA is higher, 25-35 cycles are usually sufficient. Final extension After the last cycle, the samples are usually incubated at 72°C for 5-15 minutes to fill-in the protruding ends of newly synthesized PCR products. Also, during this step, the terminal transferase activity of Taq DNA Polymerase adds extra “A” nucleotides to the 3'-ends of PCR products. Therefore, if PCR fragments are to be cloned into T/A vectors, this step can be prolonged to up to 30 minutes. After the final extension the products may be electrophoresed or stored at – 20 o^ C. C. Setting up a PCR reaction to amplify a portion of the 16S rRNA gene of bacteria Below is a portion of the “ BAC2 ”spreadsheet that is used in our laboratory to calculate the volume of PCR reagents needed to amplify a portion of the bacterial 16S rRNA gene. Click on the figure to access the actual BAC2 spreadsheet.
In general we do not mix the reagents for each individual reaction. Rather, a master mix is made in which enough of each reagent to satisfy all reactions is combined into one tube, and then dispensed among the individual reaction tubes. Template DNA is then added to each tube. This limits pipetting errors as well as the potential for contamination, as the reagent tubes are opened and dispensed from only one time per reaction set-up. Notice also that the number of reactions (“rxn #”, in green) calculated by this spreadsheet is one more than the actual number to be run (six vs. seven). This accounts for any pipetting errors and ensures that enough master mix is made to satisfy all of the reactions. In this example, PCR is being performed on 0.5 μl of DNA (final concentration = 1 ng μl
- 1 ) from six samples including one negative control with no DNA added. The PCR volume is 25 μl. The spreadsheet calculates, based on the stock- and desired- concentrations of each reagent, the volume of reagents needed for each reaction. The spreadsheet then calculates the volume necessary for all the reactions. This is the amount to be added to the master mix. The total number of ml of master mix to be aliquoted into each reaction tube as well as the DNA volume is shown in the right-hand table (24.5 and 0.5 μl, respectively). This spreadsheet provides information for setting up reactions to be run with primers “Bac 968 F-GC” and “Bac 1401 R” with the BAC2 program of the thermalcycler. Reaction set-up for other primer sets will require a different spreadsheet. To set up the above PCR reaction, proceed as follows:
- Wear gloves.
- Remove the reagents and DNA from the freezer and allow them to thaw on ice. You can let them thaw at room temperature, but be sure to immediately put them back on ice once thawed.
