1. Hi, I get a lot of questions like this in my genetics practicals and I keep getting them wrong so I know I'm doing the wrong method but I have no idea how to do it and we've never been taught.

The concentration of the nucleotides in a given PCR master mix is 200 uM. How many microlitres (ul) of a 10 mM stock solution do you need to add to prepare 1 ml of mastermix?

I convert it all to SI and do:
(200x10-6) x (1x10-3) = 2x10-7moles of nucleotides in the mastermix

Therefore:
(2x10-7) / (10x10-3) = (2x10-5)L therefore you need 20uL but this is wrong and the correct answer is 50 and I don't see how.

2. (Original post by LeaX)
Hi, I get a lot of questions like this in my genetics practicals and I keep getting them wrong so I know I'm doing the wrong method but I have no idea how to do it and we've never been taught.

The concentration of the nucleotides in a given PCR master mix is 200 uM. How many microlitres (ul) of a 10 mM stock solution do you need to add to prepare 1 ml of mastermix?

I convert it all to SI and do:
(200x10-6) x (1x10-3) = 2x10-7moles of nucleotides in the mastermix

Therefore:
(2x10-7) / (10x10-3) = (2x10-5)L therefore you need 20uL but this is wrong and the correct answer is 50 and I don't see how.

Hi,

I used the C1V1 = C2V2 method and came to the same answer as you (20 uL). I too can't seem to figure out how the supposed answer is 50 uL, and would be interested in finding out, as dilution calculations such as these may well come up in my calculations exam soon. Would you be able to email one of your lecturers and ask them whether 50 uL is indeed the correct answer?

Thanks

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