| PCR | | 点击： 作者： 来源： 时间： 2006-11-06 本站论坛 |
|  |
PCR
1) Add the following to a microfuge tube:
10 ul reaction buffer
1 ul 15 uM forward primer
1 ul 15 uM reverse primer
1 ul template DNA
5 ul 2 mM dNTP
8 ul 25 mM MgCl2 or MgSO4 (volume variable)
water (to make up to 100 ul)
2) Place tube in a thermocycler. Heat sample to 95 °C, then add 0.5 -1 ul of enzyme (Taq, Tli, Pfu etc.). Add a few drops of mineral oil.
3) Start the PCR cycles according the following schemes:
a) denaturation - 94 ° C, 30-90 sec.
b) annealing - 55 °C (or -5° Tm), 0.5-2 min.
c) extension - 72 °C, 1 min. (time depends on length of PCR product and enzyme used)
repeat cycles 29 times
4) Add a final extension step of 5 min. to fill in any uncompleted polymerisation. Then cooled down to 4- 25 °C.
Note:
Most of the parameters can be varied to optimise the PCR (more at ):
a) Mg++ - one of the main variables - change the amount added if the PCR result is poor. Mg++ affects the annealing of the oligo to the template DNA by stabilising the oligo-template interaction, it also stabilises the replication complex of polymerase with template-primer. It can therefore also increases non-specific annealing and produced undesirable PCR products (gives multiple bands in gel). EDTA which chelate Mg++ can change the Mg++ concentration.
b) DNA concentration - PCR is very powerful tool for DNA amplification therefore very little DNA is needed. But to reduce the likelihood of error by Taq DNA polymerase, a higher DNA concentration can be used, though too much template may increase the amount of contaminants and reduce efficiency.
c) Enzymes used - Taq DNA polymerase has a higher error rate (no proof-reading 3' to 5' exonuclease activity) than Tli or Pfu. Use Tli, Pfu or other polymerases with good proof-reading capability if high is needed. Taq, however, is less fussy than other polymerases and less likely to fail. It can be used in combination with other enzymes to increase its fidelity. Taq also tends to add extra A's at the 3'end (extra A's are useful for but needs to be removed if blunt end ligation is to be done). More enzymes can also be added to improve efficiency (since Taq may be damaged in repeated cycling) but may increase non-specific PCR products. Vent polymerase may degrade primer and therefore not ideal for mutagenesis-by-PCR work.
d) dNTP - can use up to 1.5 mM dNTP. dNTP chelate Mg++, therefore amount of Mg++ used may need to be changed. However excessive dNTP can increase the error rate and possibly inhibits Taq. Lowering the dNTP (10-50 uM) may therefore also reduce error rate. Larger size PCR fragment need more dNTP.
e) primers - up to 3 uM of primers may be used, but high primer to template ratio can results in non-specific amplification and primer-dimer formation (note: store primers in ).
f) Primer design - check primer sequences to avoid primer-dimer formation. Add a GC-clamp at the 5' end if a restriction site is introduced there. One or two G or C at the 3' end is fine but try to avoid having too many (it can result in non-specific PCR products). Perfect complementarity of 18 bases or more is ideal. See .
g) Thermal cycling - denaturation time can be increased if template GC content is high. Higher temperature may be needed for primers with high GC content or longer primers (calculate Tm). Using a (if your PCR machine permits it) is a useful way of determining the annealing temperature. time should be extended for larger PCR products; but reduced it whenever possible to limit damage to enzyme. Extension time is also affected by the enzymes used e.g for Taq - assume 1000 base/min (also check suppliers' recommendations, actual rate is much higher). The number of cycle can be increased if the number of template DNA is very low, and decreased if high amount of template DNA is used (higher template DNA is preferable for PCR cloning - lower error rate in the PCR).
h) Additives - - Glycerol (5-10%), formamide (1-5%) or (2-10%) can be added in PCR for template DNA with high GC content (they change the Tm of primer-template hybridisation reaction and the thermostability of polymerase enzyme). Glycerol can protects Taq against heat damage, while formamide may lower enzyme resistence.
- 0.5 -2M (stock solution - 5M) is also useful for PCR over and (/ ). Perform a titration to determine to optimum concentration (1.3 M recommended). Reduce melting temperature (92 -93 °C) and annealing temperature (1-2°C lower). It may be useful to use betaine in combination with other reagents like . Betaine is often the secret (and unnecessarily expensive) ingredient of many commercial kits.
- >50mM TMAC (tetramethylammonium chloride), TEAC (tetraethylammonium chloride), and TMANO (trimethlamine N-oxide) can also be used.
- BSA (up to 0.8 µg/µl) can also improve efficiency of PCR reaction.
- See also Dan Cruickshank's and Alkami for more.
i) PCR buffer
- of PCR buffer may be used to improve efficiency.
- This buffer may work better than the buffer supplied from commercial sources.
16.6 mM ammonium sulfate
67.7 mM TRIS-HCl, pH 8.89
10 mM beta-mercaptoethanol
170 micrograms/ml BSA
1.5-3 mM MgCl2
j) The PCR product may be purified using a number of commercially available products or by if the template needed to be removed. It can also be .
k) Trouble shooting see , , , and
l) PCR methods - - to reduce non-specific amplification. Can also be done by separating the DNA mixtures from enzyme by a layer of wax which melts when heated in cycling reaction. A number of companies also produce hot start PCR products, See .
- - start at high annealing temperature, then decrease annealing temperature in steps to reduce non-specific PCR product. Can also be used to determine DNA sequence of known protein sequence.
- Nested PCR - use to synthesize more reliable product - PCR using a outer set of primers and the product of this PCR is used for further PCR reaction using an inner set of primers.
- - for amplification of regions flanking a known sequence. DNA is digested, the desired fragment is circularise by ligation, then PCR using primer complementary to the known sequence extending outwards.
- - methods for creating genomic fingerprints from species with little-known target sequences by amplifying using arbitrary oligonucleotides. It is normally done at low and then high stringency to determine the relatedness of species or for analysis of Restriction Fragment Length Polymorphisms (RFLP).
- - using RNA-directed DNA polymerase to synthesize cDNAs which is then used for PCR and is extremely sensitive for detecting the expression of a specific sequence in a tissue or cells. It may also be use to . See also , , , .
- (rapid amplificaton of cDNA ends) - used where information about DNA/protein sequence is limited. Amplify 3' or 5' ends of cDNAs generating fragments of cDNA with only one specific primer each (+ one adaptor primer). Overlapping RACE products can then be combined to produce full cDNA. See also .
- (differential display) - used to identify differentially expressed genes in different tissues. First step involves RT-PCR, then amplification using short, intentionally nonspecific primers. Get series of band in a high-resolution gel and compare to that from other tissues, any bands unique to single samples are considered to be differentially expressed.
- - 2 or more unique targets of DNA sequences in the same specimen are amplified simultaneously. One can be use as control to verify the integrity of PCR. Can be used for mutational analysis and identification of pathogens.
- Q/C-PCR (Quantitative comparative) - uses an internal control DNA sequence (but of different size) which compete with the target DNA (competitive PCR) for the same set of primers. Used to determint the amount of target template in the reaction.
- Recusive PCR - Used to synthesise genes. Oligos used are complementary to stretches of a gene (>80 bases), alternately to the sense and to the antisense strands with ends overlapping (~20 bases). Design of the oligo avoiding homologous sequence (>8) is crucial to the success of this method.
-
- In Situ PCR
-
- Far too many to list properly.
For more information, protocols and links, go to , , , , and , See also , and PCR manual at and .
Other PCR links - , ,
上一篇：Extrachromosomal elements, plasmids, selectable markers 下一篇：Polymerase Chain Reaction (PCR) cont. | | |
|
|
|
