Upper limit of MgCl2 in PCR

[Ng Hian Cheong]

Ng Hian Cheong (medp3019 at leonis.nus.sg) wrote:

What is the reason for the existence of an upper limit of MgCl2 
concentration during PCRs ?? What is this value, 10 mm or 15 mM ?

I have experience this "upper limit of MgCl2 conc. in PCRs b4:

	I was studying a family of genes by PCR amplifying their genes 
segments from genomic DNA. At 1.5 mM, I identified the first gene. At 3.0 
mM, the second gene together with the first gene were amplified. At 4.5, 
6.0 & 7.5 mM, only the second gene segmemt was amplified. It was also 
noticed that PCR product at 7.5 mM was lesser in amount than those at 6 
mm as judged from intensity of their bands in agarose gels. At 9.0 mM 
MgCl2, NO PRODUCTS was observed. What is the reason for this absence of 
PCR products at high MgCl2 conc. ? Any suggestions. You are welcome to 
reply directly to medp3019 at leonis.nus.sg.

Thousand of thanks