Friday, November 20, 2009


On Wednesday, I put up some tables of data with bad calculations of molecules per cell. Rosie's concerns about the data were misplaced; they should've been concerns about my brain!

I'd been saying that 8 ng of 200 bp DNA / 200 ul of competent cells was ~ 1 molecule per cell. But I was off. Not just because I calculated it for pg instead of ng, but because I'd screwed up other parts of the math...

Here's a more correct way to approximate the number of molecules per cell starting with one of Rosie's Universal Constants:

(1e-18 g / one 1 kb DNA) * (one 200bp DNA / five 1kb DNA) =
(2e-19 g / one 200 bp DNA) * (1e9 ng / 1 g) =
2e-10 ng / one 200 bp DNA.
So that's about how much a single 200 bp DNA molecule weighs.

Then the inverse gives molecules per ng:
1 / (2e-10 ng / one 200 bp DNA) =
(5e+9 molecules DNA / ng DNA) * (8 ng) =
4e+10 molecules DNA / 8 ng DNA.

Adding 8 ng of DNA to 2e+8 cells (in 200 ul of competent cells), then gives:
200 molecules per cell...

That's better. And more in-line with the lab's previous work...
I have changed the Tables in Wednesday's post to reflect this correction.

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