PCR inhibitors

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== Template inhibition ==
== Template inhibition ==
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Since inhibitors often enter the PCR reaction via the template preparation, decreasing template volume may improve PCR results indirectly by reducing the quantity of inhibitors.
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The amount of template and the ratio of template to primer is an important parameter to optimise for difficult PCRs. Adding more template can in some cases decrease the amount of PCR product obtained. However, since inhibitors often enter the PCR reaction via the template preparation, decreasing template volume may improve PCR results indirectly by reducing the quantity of inhibiting contaminations.
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Typical starting amounts for genomic DNA template are 25-1000 ng in a 25 µl PCR reaction. Starting amounts for plasmid templates are generally much lower at 5-50ng per 25µl PCR reaction.
== See also ==
== See also ==
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* [[Image:2stars.png]] [http://www.neb.com/nebecomm/products/faqproductM0273.asp NEB trouble shooting guide]
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* [[Image:2stars.png]] [http://www.med.yale.edu/genetics/ward/tavi/Trblesht.html PCR trouble shooting guide by O. Henegariu from Yale]
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* [[Image:2stars.png]] [http://www.neb.com/nebecomm/products/faqproductM0273.asp PCR trouble shooting guide by NEB]
* [[Image:1star.png]] [http://en.wikipedia.org/wiki/Polymerase_chain_reaction_inhibitors PCR inhibitor at the Wikipedia]
* [[Image:1star.png]] [http://en.wikipedia.org/wiki/Polymerase_chain_reaction_inhibitors PCR inhibitor at the Wikipedia]
* [[Image:1star.png]] [[Maheshri:PCR]] - has some tips on how to avoid inhibition
* [[Image:1star.png]] [[Maheshri:PCR]] - has some tips on how to avoid inhibition
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* [[Image:1star.png]] [http://www.embl-hamburg.de/~geerlof/webPP/genetoprotein/clo_pcr_strategy/optimizing_PCR.html PCR optimization guide by A Geerlof, EMBL Hamburg]
<small>(stars indicate relevance of the links)</small>
<small>(stars indicate relevance of the links)</small>

Current revision

Schematic of competitive inhibition of an enzyme. Note that active site inhibition is only one of many reasons why a PCR may not work.
Schematic of competitive inhibition of an enzyme. Note that active site inhibition is only one of many reasons why a PCR may not work.

Many compounds also contained in the PCR template beside the target DNA can interfere with the polymerisation of nucleotides by the Taq or a related polymerase. This may result in false negative results, i.e. no PCR product even though the template is present.

Contents

Mechanisms of inhibition

A multitude of mechanisms can cause a PCR to fail. Binding of an inhibitor to the active site of a DNA polymerase is schematically depicted on the right. Other reasons include sequestration of essentials co-factors like Mg2+, e.g. by chelators like EDTA used in TE buffer. Many PCR also fail due to difficult template sequences, for example GC-rich, that cause the polymerisation process to be inefficient or even fail. In many cases, several factor only decrease the efficiency of the PCR but together lead the PCR product falling below the detection threshold.

Inhibitors from the purification process

Inhibitors from the source tissue

  • components of blood: heme, hemoglobin, lactoferrin, immunoglobin G (IgG)
  • liver, digestive tract, feces: bile salts, polysaccharides
  • connective tissue, skin: collagen, melanin
  • urine: urea

See Rådström et al. 2004 (PMID 14764939) for a review on how to avoid inhibition from tissue components.

Template inhibition

The amount of template and the ratio of template to primer is an important parameter to optimise for difficult PCRs. Adding more template can in some cases decrease the amount of PCR product obtained. However, since inhibitors often enter the PCR reaction via the template preparation, decreasing template volume may improve PCR results indirectly by reducing the quantity of inhibiting contaminations.

Typical starting amounts for genomic DNA template are 25-1000 ng in a 25 µl PCR reaction. Starting amounts for plasmid templates are generally much lower at 5-50ng per 25µl PCR reaction.

See also

(stars indicate relevance of the links)

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