RNA concentration weight and molarity - (Sep/12/2019 )

Hi everyone,

I have simple question that bothers me for a while so I would like to sort it out.

If I have 2 different RNAs with very different length than I think 10ug of one RNA size sample is not the same as 10ug of the RNA of different size.

If I want to comapre them in experiments I think I would need to calculate their molarity in order to have same number of molecules in a sample. 

Am I right?

If so, (or if not), my next question would be in what condition would I need their weight concentration actually?

Thank you! 

You are correct - RNAs (or DNAs) of different lengths have different concentrations for the same mass. This is because the concentration is dependent on the number of molecules per volume, not the mass per volume. If you have 1 molecule of 10 nucleotide length, its molecular mass will be half that of one of 20 nucleotides. For the same mass dissolved in the same volume, the shorter one will have a higher molarity because it takes more molecules to make up the same mass.

You can't determine the concentration exactly because any population of RNA or DNA is likely to contain some longer and some shorter molecules, and you would need to take these into account in your calculations... which you can't do because you don't know the exact amount of them. However, what you can do is determine the average length and then use that to calculate a concentration from average molecular mass based on the length.

Thank you so much for your answer.

So the correct way to compare two experiments using different RNAs would be for example:

2 kb RNA = 10 ug is 7.7 pmol

and that would mean:

0.05 kb RNA = 250 ng is 7.7 pmol

So in this case these two samples can be compared? Because obviously 10 ug of 2 kb and 10 ug of 0.05 kb would be actually a very different sample.

Thank you!! 

That would be about correct. I haven't done the concentration calculations, but the concept is correct.

As to the comparison, it depends on what you want to do, but if you are looking at expression of a gene, that would probably work OK.

Thank you very much!