# Northern blot band size calculation - (May/21/2007 )

Hello all,
This is my first post here. How exciting to be able to discuss lab issues with all of you!!!

I would like to know if anyone could give me an insight to how to determine the size of an unknown band on a northern blot gel. I just developed my film. I know the size of the 28S and 18S bands (4718 [2.0cm] and 1874bp [2.9cm], respectively). A lab member told me that the calculation is based on proportions. Do anybody know how do I calculate the size of my unknown band (3.9cm)?

Thank you so much!!!

-peabiria-

QUOTE (peabiria @ May 21 2007, 11:22 AM)
Hello all,
This is my first post here. How exciting to be able to discuss lab issues with all of you!!!

I would like to know if anyone could give me an insight to how to determine the size of an unknown band on a northern blot gel. I just developed my film. I know the size of the 28S and 18S bands (4718 [2.0cm] and 1874bp [2.9cm], respectively). A lab member told me that the calculation is based on proportions. Do anybody know how do I calculate the size of my unknown band (3.9cm)?

Thank you so much!!!

One method is to imagine a graph with distance migrated on the x-axis and size on the y-axis. With your 28S and 18S bands you can plot two points on the graph and deduce the equation of the line between them (y=mx+ b ). Plug the distance traveled from your band of interest into this equation and solve for y to get its size.

I'm sure there's a more elegant way of doing it, but this works for me.

-Cassio-