First, 50 ng/ul is not a very low concentration; depending on your DNA composition and target, that should still work. It makes most sense to keep the annealing temperature the same as in a regular PCR, but to increase the number of cycles.
What source is your DNA template derived from? Ideally, you should use 1-10 ng of template if your DNA is from a plasmid, 10-100 ng of template if cDNA, and 100-500 ng if template is genomic DNA. If your template is below these suggested concentrations, you could adjust the volume of template and water in your reaction, increasing the amount of DNA while decreasing the volume of water. When using low concentrations of template, there is a risk of generating base substitutions. This is discussed in the following paper. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1867510/
As Dr. Bik stated, increasing the number of cycles could be helpful if you're unable to increase the concentration of your template. If this doesn't work for you, you could indeed try lowering the annealing temperature, but you run the risk of non-specific annealing, and this should probably be used as a last resort.
Best of luck with your research!.
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