Extension/Elongation Step The primers represent the starting point for the next step, called the extension step. During the extension, or elongation, step, Taq polymerase binds to each PCR primer and begins adding nucleotides. Note that Taq, like human DNA polymerase, can only add DNA nucleotides in one direction.

The extension time of PCR depends upon the synthesis rate of DNA polymerase and the length of target DNA. The typical extension time for Taq DNA Polymerase is 1 min/kb, whereas that of Pfu DNA polymerase is 2 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.

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.

Step : 3 Extension Generally, the reaction mixture is heated to a temperature intermediate between denaturation and annealing at this stage. 72°C is the ideal temperature for Taq polymerase. The polymerase extends the primers from 5' to 3'.

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.

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.

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 ).

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Extension Time In Pcr In Fairfax

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