Extension Time In Pcr In Pima
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Using too few PCR cycles can lead to insufficient amplification. Use 20–35 cycles. Use fewer cycles when template concentration is high, and use more cycles when template concentration is low. If the extension time is too short, there will be insufficient time for complete replication of the target.
Increasing the final extension time improves full-length replication and yield of a 0.7-kb, GC-rich PCR fragment from human gDNA in these experiments. The smear under the desired band in 0 minute final extension suggests incomplete extension of the PCR amplicon by the DNA polymerase.
Extension is achieved by using the loosened nucleotides of each base to grow the complementary DNA strand. The end result is two double-stranded products of DNA. The temperature that is used during the extension phase is dependent on the DNA polymerase that is used.
The final stage is the extension step (20 sec to 1 min at 72 °C), which is performed so that the DNA polymerase extends the primer sequences from the 3' of each primer to the end of the amplicon. A 1 min extension is typically sufficient to synthesize PCR fragments up to 2 kilobases (kb).
Extension Time Extensions are normally performed at 68°C. As a general rule, use extension times of one minute per 1000 base pairs (e.g. 3 minutes for a 3 kb product) For products less than 1 kb, use 45-60 seconds. Products greater than 3 kb, or reactions using more than 30 cycles, may require longer extensions.
A typical PCR cycle includes an extension step at 72°C after denaturation of double-stranded DNA and annealing of oligonucleotide primers. At this temperature the thermostable poly-merase replicates the DNA at an optimal rate that depends on the buffer and nature of the DNA template ( 1 ).
Many of the common problems with PCR and RT-PCR are identified during agarose gel electrophoresis of the reaction products. These include the absence of the expected amplification product, the presence of nonspecific products, excessive smearing, and the presence of a “primer dimer” band.
Use 20–35 cycles. Use fewer cycles when template concentration is high, and use more cycles when template concentration is low. If the extension time is too short, there will be insufficient time for complete replication of the target. Generally, use an extension time of 1 min/kb.
If the extension time is too short, there will be insufficient time for complete replication of the target. Generally, use an extension time of 1 min/kb. If the annealing time is too short, primers do not have enough time to bind to the template. Use an annealing time of at least 30 sec.
Increase the annealing temperature to improve specificity. The optimal annealing temperature is usually no less than 3–5°C below the lowest primer Tm. Optimize the annealing temperature stepwise in 1–2°C increments, using a gradient cycler when available. Consider touchdown PCR to enhance specificity.
