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How The Concentration Of The Template Affect Pcr Product

Written By Thursday, 14 April 2022 Add Comment Edit
  • Journal Listing
  • J Vis Exp
  • (63); 2012
  • PMC4846334

J Vis Exp. 2012; (63): 3998.

Polymerase Chain Reaction: Basic Protocol Plus Troubleshooting and Optimization Strategies

Todd C. Lorenz

oneMicrobiology, Immunology, and Molecular Genetics, University of California, Los Angeles

Abstract

In the biological sciences in that location accept been technological advances that catapult the discipline into golden ages of discovery. For case, the field of microbiology was transformed with the advent of Anton van Leeuwenhoek'south microscope, which allowed scientists to visualize prokaryotes for the first fourth dimension. The development of the polymerase chain reaction (PCR) is one of those innovations that inverse the course of molecular science with its touch on spanning countless subdisciplines in biological science. The theoretical process was outlined past Keppe and coworkers in 1971; nonetheless, it was another 14 years until the complete PCR procedure was described and experimentally practical past Kary Mullis while at Cetus Corporation in 1985. Automation and refinement of this technique progressed with the introduction of a thermal stable Deoxyribonucleic acid polymerase from the bacterium Thermus aquaticus, consequently the name Taq Deoxyribonucleic acid polymerase.

PCR is a powerful distension technique that tin can generate an ample supply of a specific segment of Deoxyribonucleic acid (i.due east., an amplicon) from only a small amount of starting material (i.eastward., Deoxyribonucleic acid template or target sequence). While straightforward and generally trouble-free, at that place are pitfalls that complicate the reaction producing spurious results. When PCR fails it can atomic number 82 to many non-specific Dna products of varying sizes that appear as a ladder or smear of bands on agarose gels. Sometimes no products form at all. Another potential problem occurs when mutations are unintentionally introduced in the amplicons, resulting in a heterogeneous population of PCR products. PCR failures can become frustrating unless patience and careful troubleshooting are employed to sort out and solve the trouble(s). This protocol outlines the basic principles of PCR, provides a methodology that volition result in amplification of almost target sequences, and presents strategies for optimizing a reaction. By post-obit this PCR guide, students should exist able to: ● Ready reactions and thermal cycling conditions for a conventional PCR experiment ● Understand the role of various reaction components and their overall effect on a PCR experiment ● Design and optimize a PCR experiment for whatsoever DNA template ● Troubleshoot failed PCR experiments

Keywords: Basic Protocols, Issue 63, PCR, optimization, primer blueprint, melting temperature, Tm, troubleshooting, additives, enhancers, template DNA quantification, thermal cycler, molecular biology, genetics

Protocol

one. Designing Primers

Designing advisable primers is essential to the successful result of a PCR experiment. When designing a set of primers to a specific region of Dna desired for amplification, one primer should amalgamate to the plus strand, which by convention is oriented in the v' → 3' direction (also known as the sense or nontemplate strand) and the other primer should complement the minus strand, which is oriented in the 3' → v' direction (antisense or template strand). There are a few common bug that arise when designing primers: 1) self-annealing of primers resulting in formation of secondary structures such equally hairpin loops (Figure 1a); ii) primer annealing to each other, rather then the DNA template, creating primer dimers (Figure 1b); iii) drastically different melting temperatures (Tthousand) for each primer, making information technology hard to select an annealing temperature that will permit both primers to efficiently bind to their target sequence during themal cycling (Figure 1c) (Meet the sections Calculating MELTING TEMPERATURE (Tm) and MODIFICATIONS TO CYCLING Weather for more information on Tgrands).

  1. Below is a list of characteristics that should be considered when designing primers.

    1. Primer length should be 15-30 nucleotide residues (bases).

    2. Optimal Thou-C content should range betwixt xl-60%.

    3. The 3' end of primers should contain a One thousand or C in social club to clamp the primer and prevent "animate" of ends, increasing priming efficiency. DNA "breathing" occurs when ends do not stay annealed but fray or split autonomously. The three hydrogen bonds in GC pairs help prevent breathing but besides increment the melting temperature of the primers.

    4. The 3' ends of a primer set, which includes a plus strand primer and a minus strand primer, should not be complementary to each other, nor can the 3' end of a single primer be complementary to other sequences in the primer. These ii scenarios issue in formation of primer dimers and hairpin loop structures, respectively.

    5. Optimal melting temperatures (T1000) for primers range between 52-58 °C, although the range can be expanded to 45-65 °C. The final Tm for both primers should differ by no more than 5 °C.

    6. Di-nucleotide repeats (eastward.g., GCGCGCGCGC or ATATATATAT) or unmarried base runs (e.g., AAAAA or CCCCC) should exist avoided as they can cause slipping along the primed segment of Deoxyribonucleic acid and or hairpin loop structures to form. If unavoidable due to nature of the Dna template, so only include repeats or single base runs with a maximum of 4 bases.

Notes:

  1. In that location are many computer programs designed to aid in designing primer pairs. NCBI Primer design tool http://www.ncbi.nlm.nih.gov/tools/primer-blast/ and Primer3 http://frodo.wi.mit.edu/primer3/ are recommended websites for this purpose.

  2. In order to avoid amplification of related pseudogenes or homologs it could be useful to run a blast on NCBI to check for the target specificity of the primers.

2. Materials and Reagents

  1. When setting upward a PCR experiment, it is important to be prepared. Clothing gloves to avert contaminating the reaction mixture or reagents. Include a negative command, and if possible a positive command.

  2. Arrange all reagents needed for the PCR experiment in a freshly filled ice bucket, and let them thaw completely earlier setting up a reaction (Figure 2). Go on the reagents on water ice throughout the experiment.

    • Standard PCR reagents include a set of appropriate primers for the desired target gene or DNA segment to be amplified, Deoxyribonucleic acid polymerase, a buffer for the specific Dna polymerase, deoxynucleotides (dNTPs), DNA template, and sterile water.

    • Additional reagents may include Magnesium salt Mg2+ (at a final concentration of 0.v to 5.0 mM), Potassium table salt K+ (at a final concentration of 35 to 100 mM), dimethylsulfoxide (DMSO; at a final concentration of 1-x%), formamide (at a concluding concentration of 1.25-10%), bovine serum albumin (at a last concentration of ten-100 μg/ml), and Betaine (at a final concentration of 0.5 M to 2.five M). Additives are discussed further in the trouble shooting section.

  3. Organize laboratory equipment on the workbench.

    • Materials include PCR tubes and caps, a PCR tube rack, an ethanol-resistant marker, and a set up of micropipettors that dispense between 1 - 10 μl (P10), 2 - xx μl (P20), xx - 200 μl (P200) and 200 - 1000 μl (P1000), too as a thermal cycler.

    • When setting upward several PCR experiments that all use the aforementioned reagents, they can be scaled appropriately and combined together in a master mixture (Chief Mix). This step can be done in a sterile 1.eight ml microcentrifuge tube (run into Notes).

    • To analyze the amplicons resulting from a PCR experiment, reagents and equipment for agarose gel electrophoresis is required. To estimate the size of a PCR product, an appropriate, commercially available molecular weight size standard is needed.

3. Setting up a Reaction Mixture

  1. Starting time by making a tabular array of reagents that volition be added to the reaction mixture (see Table ane).

  2. Next, label PCR tube(southward) with the ethanol-resistant marker.

  3. Reaction volumes volition vary depending on the concentrations of the stock reagents. The final concentrations (CF) for a typical 50 μl reaction are as follows.

    • X buffer (usually supplied by the manufacturer of the Dna polymerase; may contain fifteen mM MgCltwo). Add 5 μl of 10X buffer per reaction.

    • 200 μM dNTPs (50 μM of each of the four nucleotides). Add together ane μl of 10 mM dNTPs per reaction (dATP, dCTP, dTTP and dGTP are at 2.v mM each).

    • 1.five mM Mg2+. Add together only if it is not present in the 10X buffer or as needed for PCR optimization. For instance, to obtain the 4.0 mM Mgii+ required for optimal amplicon production of a conserved 566 bp DNA segment constitute in an uncharacterized Mycobacteriophage add eight μl of 25 mM MgCl2 to the reaction (Effigy iii).

    • 20 to l pmol of each primer. Add together i μl of each twenty μM primer.

    • Add together 10iv to 107 molecules (or about 1 to 1000 ng) DNA template. Add 0.5 μl of 2ng/μl genomic Mycobacteriophage Deoxyribonucleic acid.

    • Add together 0.5 to 2.five units of Dna polymerase per l μl reaction (Meet manufacturers recommendations) For example, add 0.5 μl of Sigma 0.5 Units/μl Taq Dna polymerase.

    • Add together Q.S. sterile distilled water to obtain a 50 μl last volume per reaction as pre-determined in the tabular array of reagents (Q.Southward. is a Latin abbreviation for quantum satis meaning the corporeality that is needed). Thus, 33 μl per reaction is required to bring the volume up to 50 μl. However, information technology should be noted that water is added offset but requires initially making a table of reagents and determining the volumes of all other reagents added to the reaction.

four. Basic PCR Protocol

  1. Place a 96 well plate into the water ice saucepan as a holder for the 0.2 ml thin walled PCR tubes. Allowing PCR reagents to exist added into cold 0.ii ml thin walled PCR tubes will help prevent nuclease activity and nonspecific priming.

  2. Pipette the post-obit PCR reagents in the following society into a 0.2 ml thin walled PCR tube (Figure 4): Sterile H2o, 10X PCR buffer, dNTPs, MgCltwo, primers, and template Deoxyribonucleic acid (See Table 1). Since experiments should take at least a negative command, and perchance a positive command, it is beneficial to set upward a Master Mix in a one.8 ml microcentrifuge tube (See explanation in Notes).

  3. In a divide 0.2 ml thin walled PCR tubes (Effigy iv) add all the reagents with the exception of template Deoxyribonucleic acid for a negative command (increment the water to compensate for the missing book). In addition, another reaction (if reagents are bachelor) should contain a positive control using template Deoxyribonucleic acid and or primers previously known to amplify nether the same conditions as the experimental PCR tubes.

  4. Taq DNA polymerase is typically stored in a l% glycerol solution and for consummate dispersal in the reaction mix requires gentle mixing of the PCR reagents past pipetting upwards and downwards at to the lowest degree xx times. The micropipettor should exist set to about half the reaction volume of the master mix when mixing, and care should be taken to avoid introducing bubbles.

  5. Put caps on the 0.2 ml thin walled PCR tubes and place them into the thermal cycler (Figure 5). Once the lid to the thermal cycler is firmly airtight beginning the programme (see Table 2).

  6. When the program has finished, the 0.2 ml sparse walled PCR tubes may be removed and stored at iv °C. PCR products can exist detected by loading aliquots of each reaction into wells of an agarose gel then staining DNA that has migrated into the gel following electrophoresis with ethidium bromide. If a PCR product is present, the ethidium bromide will intercalate betwixt the bases of the DNA strands, allowing bands to be visualized with a UV illuminator.

Notes:

  1. When setting upwardly multiple PCR experiments, information technology is advantageous to get together a mixture of reagents common to all reactions (i.eastward., Main Mix). Usually the cocktail contains a solution of Dna polymerase, dNTPs, reaction buffer, and water assembled into a i.8 ml microcentrifuge tube. The amount of each reagent added to the Chief Mix is equivalent to the full number of reactions plus 10% rounded up to the nearest whole reaction. For example, if there are x x 0.1 = ane reaction, then (ten + 1) x 5 μl 10X buffer equals 55 μl of 10X buffer for the Master Mix. The reagents in the Master Mix are mixed thoroughly past gently pumping the plunger of a micropipettor up and down virtually 20 times as described to a higher place. Each PCR tube receives an aliquot of the Principal Mix to which the Deoxyribonucleic acid template, any required primers, and experiment-specific reagents are then added (see Tables ane and 7).

  2. The post-obit website offers a figurer for determining the number of copies of a template Dna (http://world wide web.uri.edu/research/gsc/resources/cndna.html). The total number of copies of double stranded Deoxyribonucleic acid may be calculated using the following equation: Number of copies of Dna = (DNA amount (ng) x half-dozen.022x1023) / (length of Dna x 1x10ix ng/ml x 650 Daltons) Calculating the number of copies of DNA is used to make up one's mind how much template is needed per reaction.

  3. Fake positives may occur as a consequence of behave-over from some other PCR reaction which would be visualized every bit multiple undesired products on an agarose gel after electrophoresis. Therefore, it is prudent to use proper technique, include a negative command (and positive control when possible).

  4. While ethidium bromide is the most common stain for nucleic acids there are several safer and less toxic alternatives. The following website describes several of the alternatives including Methylene Bluish, Crystal Violet, SYBR Safe, and Gel Red along with descriptions of how to use and detect the final production (http://bitesizebio.com/articles/ethidium-bromide-the-alternatives/).

  5. While most mod PCR machines use 0.2 ml tubes, some models may crave reactions in 0.five ml tubes. See your thermal cyclers manual to determine the advisable size tube.

half-dozen. Setting Up Thermal Cycling Conditions

  1. PCR thermal cyclers speedily heat and absurd the reaction mixture, assuasive for heat-induced denaturation of duplex Dna (strand separation), annealing of primers to the plus and minus strands of the DNA template, and elongation of the PCR product. Cycling times are calculated based on the size of the template and the GC content of the DNA. The general formula starts with an initial denaturation stride at 94 °C to 98 °C depending on the optimal temperature for Dna polymerase activity and G-C content of the template Dna. A typical reaction will offset with a 1 minute denaturation at 94 °C. Any longer than 3 minutes may inactivate the DNA polymerase, destroying its enzymatic activity. One method, known as hot-start PCR, drastically extends the initial denaturation time from 3 minutes up to 9 minutes. With hot-kickoff PCR, the DNA polymerase is added after the initial exaggerated denaturation pace is finished. This protocol modification avoids likely inactivation of the Dna polymerase enzyme. Refer to the Troubleshooting section of this protocol for more information about hot first PCR and other alternative methods.

  2. The side by side stride is to prepare the thermal cycler to initiate the get-go of 25 to 35 rounds of a 3-step temperature cycle (Tabular array 2). While increasing the number of cycles higher up 35 will event in a greater quantity of PCR products, likewise many rounds often results in the enrichment of undesirable secondary products. The three temperature steps in a single bicycle reach iii tasks: the offset step denatures the template (and in after cycles, the amplicons as well), the second step allows optimal annealing of primers, and the 3rd stride permits the DNA polymerase to bind to the DNA template and synthesize the PCR product. The duration and temperature of each stride inside a cycle may exist altered to optimize production of the desired amplicon. The time for the denaturation footstep is kept every bit short as possible. Normally ten to lx seconds is sufficient for most Deoxyribonucleic acid templates. The denaturation time and temperature may vary depending on the G-C content of the template Deoxyribonucleic acid, besides as the ramp rate, which is the time it takes the thermal cycler to change from one temperature to the adjacent. The temperature for this stride is usually the same every bit that used for the initial denaturation phase (step #i above; eastward.g., 94 °C). A thirty second annealing step follows within the bike at a temperature set most v °C below the credible Tm of the primers (ideally between 52 °C to 58 °C). The cycle concludes with an elongation step. The temperature depends on the Deoxyribonucleic acid polymerase selected for the experiment. For instance, Taq DNA polymerase has an optimal elongation temperature of 70 °C to 80 °C and requires ane minute to elongate the offset 2 kb, and then requires an extra minute for each boosted ane kb amplified. Pfu Deoxyribonucleic acid Polymerase is another thermostable enzyme that has an optimal elongation temperature of 75 °C. Pfu Dna Polymerase is recommended for apply in PCR and primer extension reactions that require high fidelity and requires 2 minutes for every i kb to exist amplified. Meet manufacturer recommendations for exact elongation temperatures and elongation time indicated for each specific Deoxyribonucleic acid polymerase.

  3. The final phase of thermal cycling incorporates an extended elongation period of 5 minutes or longer. This final step allows synthesis of many uncompleted amplicons to finish and, in the case of Taq Deoxyribonucleic acid polymerase, permits the addition of an adenine residuum to the 3' ends of all PCR products. This modification is mediated by the concluding transferase action of Taq Dna polymerase and is useful for subsequent molecular cloning procedures that require a 3'-overhang.

  4. Termination of the reaction is accomplished past spooky the mixture to 4 °C and/or by the add-on of EDTA to a final concentration of ten mM.

seven. Important Considerations When Troubleshooting PCR

If standard PCR weather condition do not yield the desired amplicon, PCR optimization is necessary to attain amend results. The stringency of a reaction may be modulated such that the specificity is adjusted past altering variables (eastward.1000., reagent concentrations, cycling atmospheric condition) that impact the outcome of the amplicon contour. For example, if the reaction is not stringent enough, many spurious amplicons will exist generated with variable lengths. If the reaction is too stringent, no product will be produced. Troubleshooting PCR reactions may be a frustrating endeavor at times. Withal, careful analysis and a good agreement of the reagents used in a PCR experiment can reduce the amount of time and trials needed to obtain the desired results. Of all the considerations that touch on PCR stringency, titration of Mgii+ and/or manipulating annealing temperatures likely will solve nigh problems. Notwithstanding, before changing anything, exist sure that an erroneous event was not due to human error. Outset past confirming all reagents were added to a given reaction and that the reagents were not contaminated. Also have note of the erroneous result, and ask the following questions: Are primer dimers visible on the gel after electrophoresis (these run as small bands <100 b well-nigh the lesser of the lane)? Are there not-specific products (bands that migrate at a different size than the desired product)? Was there a lack of any product? Is the target Deoxyribonucleic acid on a plasmid or in a genomic Deoxyribonucleic acid extract? Too, it is wise to clarify the G-C content of the desired amplicon.

  1. First make up one's mind if whatever of the PCR reagents are catastrophic to your reaction. This tin can be achieved by preparing new reagents (e.g., fresh working stocks, new dilutions), and and then systematically adding one new reagent at a fourth dimension to reaction mixtures. This process will determine which reagent was the culprit for the failed PCR experiment. In the case of very one-time Deoxyribonucleic acid, which frequently accumulates inhibitors, information technology has been demonstrated that addition of bovine serum albumin may aid alleviate the trouble.

  2. Primer dimers can form when primers preferentially self anneal or anneal to the other primer in the reaction. If this occurs, a small-scale product of less than 100 bp will appear on the agarose gel. Start past altering the ratio of template to primer; if the primer concentration is in extreme backlog over the template concentration, then the primers will exist more than likely to anneal to themselves or each other over the DNA template. Calculation DMSO and or using a hot start thermal cycling method may resolve the problem. In the end it may exist necessary to design new primers.

  3. Not-specific products are produced when PCR stringency is excessively low resulting in non-specific PCR bands with variable lengths. This produces a ladder outcome on an agarose gel. It then is appropriate to choose PCR conditions that increment stringency. A smear of various sizes may also consequence from primers designed to highly repetitive sequences when amplifying genomic Deoxyribonucleic acid. However, the same primers may amplify a target sequence on a plasmid without encountering the aforementioned trouble.

  4. Lack of PCR products is likely due to reaction weather that are likewise stringent. Primer dimers and hairpin loop structures that grade with the primers or in the denatured template Dna may also prevent distension of PCR products because these molecules may no longer base pair with the desired DNA analogue.

  5. If the Thousand-C content has not been analyzed, it is fourth dimension to do so. PCR of Thousand-C rich regions (GC content >60%) pose some of the greatest challenges to PCR. However, at that place are many additives that take been used to help alleviate the challenges.

8. Manipulating PCR Reagents

Understanding the function of reagents used on conventional PCR is critical when first deciding how best to alter reaction conditions to obtain the desired product. Success simply may rely on irresolute the concentration of MgCl2, KCl, dNTPs, primers, template Deoxyribonucleic acid, or Deoxyribonucleic acid polymerase. Withal, the wrong concentration of such reagents may lead to spurious results, decreasing the stringency of the reaction. When troubleshooting PCR, only 1 reagent should be manipulated at a fourth dimension. Yet, it may exist prudent to titrate the manipulated reagent.

  1. Magnesium common salt Mg2+ (final reaction concentration of 0.5 to v.0 mM) Thermostable DNA polymerases crave the presence of magnesium to act as a cofactor during the reaction process. Changing the magnesium concentration is one of the easiest reagents to manipulate with perhaps the greatest impact on the stringency of PCR. In general, the PCR product yield will increase with the improver of greater concentrations of Mg2+. However, increased concentrations of Mg2+ volition as well subtract the specificity and allegiance of the DNA polymerase. Most manufacturers include a solution of Magnesium chloride (MgCl2) along with the DNA polymerase and a 10X PCR buffer solution. The 10 10 PCR buffer solutions may comprise 15 mM MgCl2, which is enough for a typical PCR reaction, or it may be added separately at a concentration optimized for a particular reaction. Mgtwo+ is not actually consumed in the reaction, merely the reaction cannot proceed without it existence present. When there is also much Mg2+, it may prevent complete denaturation of the Deoxyribonucleic acid template by stabilizing the duplex strand. Too much Mgii+ likewise tin stabilize spurious annealing of primers to incorrect template sites and decrease specificity resulting in undesired PCR products. When there is not enough Mg2+, the reaction will not proceed, resulting in no PCR product.

  2. Potassium salt K+ (final reaction concentration of 35 to 100 mM) Longer PCR products (10 to forty kb) do good from reducing potassium table salt (KCl) from its normal 50 mM reaction concentration, oftentimes in conjunction with the addition of DMSO and/or glycerol. If the desired amplicon is below 1000 bp and long non-specific products are forming, specificity may be improved by titrating KCl, increasing the concentration in 10 mM increments upward to 100 mM. Increasing the salt concentration permits shorter Dna molecules to denature preferentially to longer DNA molecules.

  3. Deoxynucleotide five'-triphosphates (final reaction concentration of 20 and 200 μM each) Deoxynucleotide v'-triphosphates (dNTPs) can crusade issues for PCR if they are not at the appropriate equivalent concentrations (i.east., [A] = [T] = [C] = [G]) and/ or due to their instability from repeated freezing and thawing. The usual dNTP concentration is 50 μM of EACH of the four dNTPs. However, PCR tin tolerate concentrations betwixt 20 and 200 μM each. Lower concentrations of dNTPs may increase both the specificity and fidelity of the reaction while excessive dNTP concentrations tin actually inhibit PCR. Nevertheless, for longer PCR-fragments, a higher dNTP concentration may be required. A large change in the dNTP concentration may necessitate a corresponding modify in the concentration of Mg2+.

  4. Thermal stable Dna polymerases PCR enzymes and buffers associated with those enzymes have come a long way since the initial Taq Dna polymerase was first employed. Thus, choosing an advisable enzyme can be helpful for obtaining desired amplicon products. For instance the use of Taq DNA polymerase may be preferred over Pfu Dna polymerase if processivity and/or the addition of an adenine residue to the 3' ends of the PCR product is desired. The addition of a three' adenine has become a useful strategy for cloning PCR products into TA vectors whit iii' thymine overhangs. Withal, if fidelity is more than important an enzyme such as Pfu may exist a better choice. Several manufactures have an assortment of specific DNA polymerases designed for specialized needs. Accept a look at the reaction conditions and characteristics of the desired amplicon, and and then match the PCR experiment with the appropriate Dna polymerase. About manufactures have tables that aid Dna polymerase choice by listing characteristics such every bit fidelity, yield, speed, optimal target lengths, and whether it is useful for G-C rich amplification or hot start PCR.

  5. Template DNA DNA quality and purity will have a substantial consequence on the likelihood of a successful PCR experiment. DNA and RNA concentrations can be adamant using their optical density measurements at 260 nm (OD260). The mass of purified nucleic acids in solution is calculated at fifty μg/ml of double stranded Deoxyribonucleic acid or forty μg/ml for either RNA or single stranded DNA at an OD260 =1.0. Deoxyribonucleic acid extraction contaminants are common inhibitors in PCR and should exist carefully avoided. Common DNA extraction inhibitors of PCR include protein, RNA, organic solvents, and detergents. Using the maximum absorption of nucleic acids OD260 compared to that of proteins OD280 (OD260/280), it is possible to determine an estimate of the purity of extracted Deoxyribonucleic acid. Ideally, the ratio of OD260/280 is between 1.viii and 2.0. Lower OD260/280 is indicative of poly peptide and/ or solvent contagion which, in all probability, will exist problematic for PCR. In addition to the quality of template Deoxyribonucleic acid, optimization of the quantity of DNA may profoundly benefit the outcome of a PCR experiment. Although it is convenient to determine the quantity in ng/μl, which is often the output for modern nanospectrophotometers, the relevant unit for a successful PCR experiment is the number of molecules. That is, how many copies of DNA template contain a sequence complementary to the PCR primers? Optimal target molecules are between 10four to xvii molecules and may be calculated as was described in the notes above.

ix. Additive Reagents

Additive reagents may yield results when all else fails. Understanding the reagents and what they are used for is critical in determining which reagents may be about effective in the acquisition of the desired PCR product. Calculation reagents to the reaction is complicated past the fact that manipulation of one reagent may bear upon the usable concentration of another reagent. In addition to the reagents listed below, proprietary commercially bachelor additives are available from many biotechnology companies.

ten. Additives That Do good G-C Rich Templates

  1. Dimethylsulfoxide (last reaction concentration of 1-10% DMSO) In PCR experiments in which the template DNA is particularly G-C rich (GC content >60%), adding DMSO may enhance the reaction by disrupting base pairing and effectively lowering the Tyard. Some T1000 calculators include a variable entry for adding the concentration of DMSO desired in the PCR experiment. However, calculation more than two% DMSO may crave adding more than DNA polymerase as it has been demonstrated to inhibit Taq DNA polymerase.

  2. Formamide (final reaction concentration of 1.25-10%) Similar DMSO, formamide too disrupts base of operations pairing while increasing the stringency of primer annealing, which results in less non-specific priming and increased amplification efficiency. Formamide also has been shown to be an enhancer for K-C rich templates.

  3. 7-deaza-ii'-deoxyguanosine v'-triphosphate (final reaction concentration of dc7GTP; 3 dc7GTP:1 dGTP 50 μM) Using 3 parts, or 37.5 μM, of the guanosine base analog dc7GTP in conjunction with i role, or 12.v μM, dGTP will destabilize formation of secondary structures in the product. Equally the amplicon or template DNA is denatured, information technology will oft form secondary structures such as hairpin loops. Incorporation of dc7GTP into the DNA amplicon will prohibit formation of these aberrant structures.

Note:

dc7GTP attenuates the signal of ethidium bromide staining which is why it is used in a 3:1 ratio with dGTP.

  1. Betaine (concluding reaction concentration of 0.5M to 2.5M) Betaine (N,North,N-trimethylglycine) is a zwitterionic amino acid analog that reduces and may even eliminate the Deoxyribonucleic acid melting temperature dependence on nucleotide composition. It is used as an additive to help PCR distension of G-C rich targets. Betaine is often employed in combination with DMSO and can greatly enhance the chances of amplifying target Deoxyribonucleic acid with high Thousand-C content.

11. Additives That Help PCR in the Presence of Inhibitors

  1. Non ionic detergents function to suppress secondary structure formation and help stabilize the Deoxyribonucleic acid polymerase. Not ionic detergents such every bit Triton Ten-100, Tween 20, or NP-forty may be used at reaction concentrations of 0.1 to one% in social club to increase amplicon production. However, concentrations above 1% may exist inhibitory to PCR. The presence of non ionic detergents decreases PCR stringency, potentially leading to spurious product germination. Nonetheless, their use will as well neutralize the inhibitory affects of SDS, an occasional contaminant of Deoxyribonucleic acid extraction protocols.

  2. Addition of specific proteins such equally Bovine serum albumin (BSA) used at 400 ng/μl and/ or T4 gene 32 protein at 150 ng/μl aid PCR in the presence of inhibitors such as FeCl3, hemin, fulvic acid, humic acid, tannic acids, or extracts from feces, fresh water, and marine water. Yet, some PCR inhibitors, including bile salts, bilirubin, EDTA, NaCl, SDS, or Triton Ten-100, are not alleviated by addition of either BSA or T4 gene 32 protein.

12. Modifications to Cycling Atmospheric condition

  1. Optimizing the annealing temperature will heighten whatever PCR reaction and should be considered in combination with other additives and/ or forth with other modifications to cycling weather condition. Thus, in order to calculate the optimal annealing temperature the post-obit equation is employed: Ta OPT = 0.3 Tthousand Primer + 0.7 Tm Production -14.9 Tm Primer is calculated equally the Tone thousand of the less stable pair using the equation: Tm Primer = ((ΔH/(ΔS+R 10 ln(c/4)))-273.xv + 16.6 log[Chiliad+] Where ΔH is the sum of the nearest neighbour enthalpy changes for hybrids; ΔS is the sum of the nearest neighbour entropy changes; R is the Gas Constant (1.99 cal M-1 mol-1); C is the primer concentration; and [K+] is the potassium concentration. The latter equation tin exist computed using ane of the Tm calculators listed at the following website: http://protein.bio.puc.cl/cardex/servers/melting/sup_mat/servers_list.html Tm Product is calculated as follows: T g Product = 0.41(%G-C) + 16.6 log [K+] - 675/product length For most PCR reactions the concentration of potassium ([K+]) is going to be l mM.

  2. Hot starting time PCR is a versatile modification in which the initial denaturation time is increased dramatically (Tabular array four). This modification tin can be incorporated with or without other modifications to cycling atmospheric condition. Moreover, it is often used in conjunction with additives for temperamental amplicon formation. In fact, hot beginning PCR is increasingly included as a regular attribute of general cycling conditions. Hot start has been demonstrated to increase amplicon yield, while increasing the specificity and allegiance of the reaction. The rationale behind hot start PCR is to eliminate primer-dimer and not-specific priming that may result as a consequence of setting up the reaction below the Tm. Thus, a typical hot first reaction heats the sample to a temperature above the optimal Tm, at least to lx °C before any amplification is able to occur. In general, the Dna polymerase is withheld from the reaction during the initial, elongated, denaturing time. Although other components of the reaction are sometimes omitted instead of the DNA polymerase, here we will focus on the Dna polymerase. In that location are several methods which let the Dna polymerase to remain inactive or physically separated until the initial denaturation period has completed, including the use of a solid wax bulwark, anti-DNA polymerase antibodies, and accompaniment proteins. Alternatively, the Deoxyribonucleic acid polymerase may but be added to the reaction after the initial denaturation cycle is complete.

  3. Touchdown PCR (TD-PCR) is intended to take some of the estimate work out of the Tm calculation limitations by bracketing the calculated annealing temperatures. The concept is to design two phases of cycling atmospheric condition (Tabular array five). The showtime phase employs successively lower annealing temperatures every second cycle (traditionally 1.0 °C), starting at x °C above and finishing at the calculated Tm or slightly beneath. Stage 2 utilizes the standard 3-step atmospheric condition with the annealing temperature set up at five °C below the calculated Tm for another 20 to 25 cycles. The function of the first phase should alleviate mispriming, conferring a four-fold advantage to the right product. Thus, after 10 cycles, a 410-fold reward would yield 4096 copies of the correct product over any spurious priming.

    • Stepdown PCR is similar to TD-PCR with fewer increments in the beginning phase of priming. As an example, the first stage lowers annealing temperatures every 2nd cycle by 3 °C, starting at 10 °C above and finishing at 2 °C below the calculated Tm. Like TD-PCR, phase 2 utilizes the standard three-stride conditions with the annealing temperature prepare at 5 °C beneath the calculated Tm for another 20 to 25 cycles. This would let the correct production a 256-fold advantage over simulated priming products.

    • Slowdown PCR is merely a modification of TD-PCR and has been successful for amplifying extremely Chiliad-C rich (above 83%) sequences (Table 6). The concept takes into account a relatively new feature associated with modern thermal cyclers, which allows adjustment of the ramp speed besides every bit the cooling rate. The protocol also utilizes dc7GTP to reduce 2 °structure germination that could inhibit the reaction. The ramp speed is lowered to 2.5 °C s-one with a cooling rate of 1.5 °C southward-i for the annealing cycles. The first stage starts with an annealing temperature of 70 °C and reduces the annealing temperature by 1 °C every three rounds until information technology reaches 58 °C. The 2d phase and so continues with an annealing temperature of 58 °C for an additional 15 cycles.

  4. Nested PCR is a powerful tool used to eliminate spurious products. The employ of nested primers is particularly helpful when there are several paralogous genes in a single genome or when at that place is depression re-create number of a target sequence inside a heterogeneous population of orthologous sequences. The basic procedure involves two sets of primers that amplify a unmarried region of Dna. The outer primers straddle the segment of interest and are used to generate PCR products that are often non-specific in 20 to 30 cycles. A pocket-size aliquot, usually well-nigh 5 μl from the first l μl reaction, is then used as the template Dna for some other 20 to 30 rounds of amplification using the second set of primers that anneal to an internal location relative to the start ready.

Other PCR protocols are more specialized and go beyond the scope of this paper. Examples include RACE-PCR, Multiplex-PCR, Vectorette-PCR, Quantitative-PCR, and RT-PCR.

thirteen. Representative Results

Representative PCR results were generated by post-obit the basic PCR protocols described to a higher place. The results contain several troubleshooting strategies to demonstrate the result of various reagents and weather condition on the reaction. Genes from the budding yeast Saccharomyces cerevisiae and from an uncharacterized Mycobacteriophage were amplified in these experiments. The standard 3-step PCR protocol outlined in Tabular array ii was employed for all 3 experiments described beneath.

Before setting up the PCR experiment, the genomic DNA from both S. cerevisiae and the Mycobacteriophage were quantified and diluted to a concentration that would allow between 10iv and 107 molecules of DNA per reaction. The working stocks were prepared as follows. A genomic yeast DNA training yielded 10iv ng/μl. A dilution to 10 ng/μl was generated by adding 48 μl into 452 μl of TE pH eight.0 buffer. Since the S. cerevisiae genome is well-nigh 12.5 Mb, 10 ng incorporate 7.41 10 tenv molecules. The genomic Mycobacteriophage Dna preparation yielded 313 ng/μl. A dilution to 2 ng/μl was generated by adding six.4 μl into 993.6 μl of TE pH 8.0 buffer. This phage Dna is about 67 Kb. Thus, 1 ng contains 2.73 X 107 molecules, which is at the upper limit of Dna generally used for a PCR. The working stocks were so used to generate the Chief Mix solutions outlined in Table 7. Experiments varied cycling conditions equally described below.

In Figure 3a, genomic Dna from S. cerevisiae was used every bit a template to dilate the GAL3 factor, which encodes a protein involved in galactose metabolism. The goal for this experiment was to determine the optimal Mg2+ concentration for this set up of reagents. No MgCltwo was present in the original PCR buffer and had to be supplemented at the concentrations indicated with a range tested from 0.0 mM to 5.0 mM. As shown in the figure, a PCR product of the expected size (2098 bp) appears starting at a Mgii+ concentration of 2.5 mM (lane half-dozen) with an optimal concentration at 4.0 mM (lane ix). The recommended concentration provided by the manufacturer was 1.v mM, which is the amount provided in typical PCR buffers. Possibly surprisingly, the necessary concentration needed for product germination in this experiment exceeded this corporeality.

A different Dna template was used for the experiment presented in Effigy 3b. Genomic DNA from a Mycobacteriophage was used to dilate a conserved 566 bp DNA segment. Like the previous experiment, the optimal Mg2+ concentration had to exist determined. As shown in Effigy 3b, amplification of the desired PCR product requires at least 2.0 mM Mgii+ (lane 5). While in that location was more variability in the amount of product formed at increasing concentrations of MgCl2, the most PCR product was observed at iv mM Mgii+ (lane 9), the aforementioned concentration observed for the yeast GAL3 gene.

Discover that in the experiments presented in Figures 3A and 3B, a discrete band was obtained using the cycling conditions thought to be optimal based on primer annealing temperatures. Specifically, the denaturation temperature was 95 °C with an annealing temperature of 61 °C, and the extension was carried out for 1 minute at 72 °C for thirty cycles. The final 5 infinitesimal extension was then done at 72 °C. For the third experiment presented in Figure 3c, three changes were fabricated to the cycling conditions used to amplify the yeast GAL3 factor. First, the annealing temperature was reduced to a sub-optimal temperature of 58 °C. 2nd, the extension time was extended to 1 minute and 30 seconds. Third, the number of cycles was increased from 30 to 35 times. The purpose was to demonstrate the effects of sub-optimal amplification weather condition (i.e., reducing the stringency of the reaction) on a PCR experiment. As shown in Effigy 3c, what was a discrete ring in Figure 3a, becomes a smear of non-specific products nether these sub-optimal cycling conditions. Furthermore, with the overall stringency of the reaction reduced, a lower amount of Mg2+ is required to form an amplicon.

All iii experiments illustrate that when Mg2+ concentrations are likewise depression, there is no amplicon product. These results also demonstrate that when both the cycling conditions are correctly designed and the reagents are at optimal concentrations, the PCR experiment produces a unimposing amplicon corresponding to the expected size. The results show the importance of performing PCR experiments at a sufficiently high stringency (e.one thousand., discreet bands versus a smear). Moreover, the experiments indicate that changing one parameter can influence another parameter, thus affecting the reaction result.

Reagent Concentration of stock solutions Volume 13X ** Main Mix Final Concentration
Sterile H2O Q.Southward. to 50 μl Q.S. to 650 μl
PCR Buffer 10X 5 μl 65 μl 1X
dNTP's x mM one μl xiii μl 200 μM
MgCl2 25 mM iii μl 39 μl 1.5 mM
Forward Primer xx μM = xx pmol/μl 1 μl 13 μl 20 pmol
Contrary Primer 20 μM = 20 pmol/μl 1 μl thirteen μl xx pmol
Template Dna Variable Variable Variable ~10five Molecules
Taq Deoxyribonucleic acid Polymerase 5 Units/μl* 0.five μl 6.5 μl ii.5 Units
50 μl/Reaction

Table 1. PCR reagents in the social club they should be added.

*Units may vary between manufacturers

** Add all reagents to the Master Mix excluding any in need of titration or that may be variable to the reaction. The Master Mix depicted in the above tabular array is calculated for 11 reactions plus 2 extra reactions to arrange pipette transfer loss ensuring there is enough to aliquot to each reaction tube.

Standard 3-step PCR Cycling
Bike footstep Temperature Fourth dimension Number of Cycles
Initial Denaturation 94 °C to 98 °C 1 minute 1
Denaturation Annealing Extension 94 °C 5 °C below Tgrand 70 °C to 80 °C ten to threescore seconds 30 seconds Amplicon and DNA polymerase dependent 25-35
Concluding Extension 70 °C to 80 °C v minutes 1
Hold* 4 °C ∞ ane

Table 2. Standard 3-step PCR Cycling.

* Near thermal cyclers have the ability to pause at four°C indefinitely at the terminate of the cycles.

2-footstep PCR Cycling
Cycle step Temperature Time Number of Cycles
Initial Denaturation 94 °C to 98 °C 1 infinitesimal 1
Denaturation Annealing/Extension 94 °C 70 °C to 80 °C 10 to 60 seconds Amplicon and DNA polymerase dependent 25-35
Terminal Extension 70 °C to 80 °C 5 minutes 1

Table 3. 2-step PCR Cycling.

Hot Get-go PCR Cycling
Bike step Temperature Time Cycles
Initial Denaturation 60 °C to 95 °C 5 minute then add Deoxyribonucleic acid polymerase one
Denaturation Annealing Extension 94 °C 5 °C beneath Tk 70 °C to 80 °C x to 60 seconds 30 seconds Amplicon and DNA polymerase dependent 25-35
Final Extension 70 °C to lxxx °C 5 minutes i

Tabular array 4. Hot Kickoff PCR Cycling.

Touchdown PCR Cycling
Cycle footstep Temperature Time Cycles
Initial Denaturation 94 °C to 98 °C one infinitesimal 1
Denaturation Annealing Extension 94 °C X =10 °C in a higher place T1000 70 °C to 80 °C 10 to 60 seconds xxx seconds Amplicon and DNA polymerase dependent 2
Denaturation Annealing Extension 94 °C X-ane °C reduce ane °C every other bicycle lxx °C to 80 °C ten to 60 seconds 30 seconds Amplicon and polymerase dependent 28
Denaturation Annealing Extension 94 °C five °C below Tthousand lxx °C to 80 °C 10 to 60 seconds 30 seconds Amplicon and Dna polymerase dependent 20-25
Final Extension seventy °C to eighty °C five minutes 1

Tabular array v. Touchdown PCR Cycling.

Slowdown PCR Cycling
Wheel step Temperature Time Cycles
Initial Denaturation 94 °C to 98 °C one minute 1
Denaturation Annealing Extension 94 °C X °C =10 °C above T1000 70 °C to eighty °C ten to 60 seconds 30 seconds Amplicon and polymerase dependent ii
Denaturation Annealing Extension 94 °C X-one °C reduce 1 °C every other bicycle lxx °C to 80 °C* 10 to sixty seconds 30 seconds Amplicon and polymerase dependent 28
Denaturation Annealing Extension 94 °C 5 °C below Tm seventy °C to lxxx °C 10 to 60 seconds thirty seconds Amplicon and polymerase dependent 20-25
Last Extension lxx °C to lxxx °C 5 minutes ane

Tabular array 6. Slowdown PCR Cycling.

*For slowdown PCR, the ramp speed is lowered to 2.5 °C s-i with a cooling rate of 1.5 °C s-i for the annealing cycles.

Stock Solution Volume added to l μl reaction xiii Ten Yeast Master Mix 13 Ten Phage Master Mix Concluding Concentration
Sterile H2O q.s. to 50 μl = 31 μl or 30.5 q.south. to 650 μl = 396.5 q.southward. to 520 μl = 403 μl
PCR Buffer 10X five μl 65 μl 65 μl 1X
dNTP'due south x mM 1 μl xiii μl 13 μl 200 μM
MgCl2 Titration Added to each reaction Added to each reaction Added to each reaction Variable see titration
Forward Primer xx μM = 20 pmol/μl 1 μl 13 μl 13 μl twenty pmol
Contrary Primer 20 μM = twenty pmol/μl 1 μl 13 μl 13 μl twenty pmol
Template DNA 2 ng/μl phage or 10 ng/μl Yeast 0.five μl Phage or 1 μl Yeast half dozen.five μl 13 μl ~107 Molecules Phage or ~105 Molecules Yeast
Polymerase 0.5 Units/μl** 0.5 μl vi.v μl 6.five μl 0.5 Units/Reaction
40 μl + 10(Titration) μl/ Reaction
TITRATION
[MgCl2] 0.00 mM 0.5 mM one.0 mM ane.5mM 2.0 mM 2.5 mM 3.0 mM 3.5 mM four.0 mM 4.5 mM 5.0 mM
MgCl2 0.00 μl 1.00 μl 2.00 μl three.0 μl four.00 μl 5.00 μl half dozen.00 μl 7.00 μl eight.00 μl 9.00 μl 10.00 μl
HtwoO ten.00 μl 9.00 μl 8.00 μl 7.00 μl vi.00 μl v.00 μl four.00 μl three.00 μl 2.00 μl 1.00 μl 0.00 μl

Table 7. Titration of Mg2+ used in Figure 3.

An external file that holds a picture, illustration, etc. Object name is jove-63-3998-0.jpg

Figure one. Common issues that arise with primers and 3-footstep PCR amplification of target DNA. (a) Self-annealing of primers resulting in formation of secondary hairpin loop structure. Notation that primers do not e'er anneal at the extreme ends and may form smaller loop structures. (b) Primer annealing to each other, rather than the Dna template, creating primer dimers. Once the primers anneal to each other they will elongate to the primer ends. (c) PCR cycles generating a specific amplicon. Standard 3-stride PCR cycling include denaturation of the template DNA, annealing of primers, and extension of the target DNA (amplicon) by DNA polymerase.

An external file that holds a picture, illustration, etc. Object name is jove-63-3998-1.jpg

Figure 2. Ice bucket with reagents, pipettes, and racks required for a PCR. (i.) P-200 pipette, (two.) P-g pipette, (three.) P-twenty pipette, (4.) P-10 pipette, (5.) 96 well plate and 0.2 ml thin walled PCR tubes, (half dozen.) Reagents including Taq polymerase, 10X PCR buffer, MgCl2, sterile water, dNTPs, primers, and template Dna, (7.) one.eight ml tubes and rack.

An external file that holds a picture, illustration, etc. Object name is jove-63-3998-2.jpg

Figure three. Example of a Mgtwo+ titrations used to optimize a PCR experiment using a standard 3-footstep PCR protocol. (a) Due south. cerevisiae Yeast genomic Deoxyribonucleic acid was used every bit a template to dilate a 2098 bp GAL3 gene. In lanes 1 - 6, where the Mg2+ concentration is likewise low, in that location either is no product formed (lanes 1-five) or very little product formed (lane 6). Lanes vii - 11 represent optimal concentrations of Mgtwo+ for this PCR experiment every bit indicated past the presence of the 2098 bp amplicon production. (b) An uncharacterized mycobacteriophage genomic Deoxyribonucleic acid template was used to amplify a 566 bp amplicon. Lanes 1 - 4, the Mg2+ concentration is likewise low, every bit indicated by the absence of product. Lanes v - 11 represent optimal concentrations of Mg2+ for this PCR as indicated past the presence of the 566 kb amplicon product. (c) . S. cerevisiae Yeast genomic Deoxyribonucleic acid was used as a template to amplify a 2098 bp GAL3 cistron as indicated in panel a. All the same, the annealing temperature was reduced from 61 °C to 58 °C, resulting in a non-specific PCR bands with variable lengths producing a smearing effect on the agarose gel. Lanes 1 - four, where the Mg2+ concentration is also low, there is no production formed. Lanes 5 - 8 represent optimal concentrations of Mg2+ for this PCR as seen by the presence of a smear and band around the 2098 kb amplicon product size. Lanes 9 - 11 are indicative of excessively stringent conditions with no production formed. (a-c) Lanes 12 is a negative control that did not incorporate any template DNA. Lane M (marker) was loaded with NEB 1kb Ladder.

An external file that holds a picture, illustration, etc. Object name is jove-63-3998-3.jpg

Effigy 4. Sterile tubes used for PCR. (1.) 1.8 ml tube (2.) 0.two ml individual thin walled PCR tube, (3.) 0.two ml strip sparse walled PCR tubes and caps.

An external file that holds a picture, illustration, etc. Object name is jove-63-3998-4.jpg

Figure v. Thermal cycler. Closed thermal cycler left paradigm. Correct image contains 0.2 ml thin walled PCR tubes placed in the heating block of an open up thermal cycler.

Discussion

PCR has become an indispensible tool in the biological science arsenal. PCR has altered the course of scientific discipline allowing biologists to yield power over genomes, and brand hybrid genes with novel functions, assuasive specific and accurate clinical testing, gaining insights into genomes and diversity, too equally merely cloning genes for further biochemical assay. PCR awarding is limited only by the imagination of the scientist that wields its power. There are many books and papers that depict new specialized uses of PCR, and many more will be developed over the adjacent generation of biological science. However, regardless of the anticipated approaches, the fundamental framework has remained the aforementioned. PCR, in all its grandeur, is an in vitro application to generate large quantities of a specific segment of Deoxyribonucleic acid.

Designing a PCR experiment requires idea and patience. The results shown in Figure 3 exemplify i of the major challenges when designing an optimization strategy for PCR. That is, as one parameter of PCR is changed, it may bear on another. As an instance, if the initial PCR was carried out at the sub-optimal annealing temperature (58°C) with an optimal Mg2+ concentration of 2.0 mM, so the result would produce a smear as seen in Figure 3c. An attempt to resolve the smear might involve setting up PCR weather condition with reactions containing 2.0 mM MgCl2 and adjusting the annealing temperature to 61°C. Even so, as seen in Figure 3a, this would not yield any product. Consequently, information technology is appropriate to titrate reagents, rather than adding 1 concentration to a single reaction, when troubleshooting spurious results. Too, the near common adjustments that are required for optimizing a PCR experiment are to change the Mg2+ concentration and to correct the annealing temperatures. Notwithstanding, if these changes practise not minimize or countervail aberrant results, titration of additives and /or changing the cycling condition protocols as described in Tables ii-6 may convalesce the trouble. If all else fails, redesign the primers and try, attempt again.

Disclosures

I have nothing to disclose.

Acknowledgments

Special thank you to Kris Reddi at UCLA for setting up reagents and pouring gels and to Erin Sanders at UCLA for inspiration, guidance, and support and proofreading the manuscript. I would besides like to cheers Giancarlo Costaguta and Gregory South. Payne for supplying the yeast genomic DNA and primers to amplify the GAL3 gene. I would likewise like to give thanks Bhairav Shah for taking pictures of the lab equipment and reagents used to make figures 2 - four. Funding for this project was provided by HHMI (HHMI Grant No. 52006944).

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How The Concentration Of The Template Affect Pcr Product,

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