How do you calculate molecular weight of DNA?

1 Answer
Aug 13, 2016


Here are the steps you might follow.


Step 1. DNA restriction

If you have a large DNA molecule, you will probably cut it into smaller fragments using a so-called DNA restriction enzyme.

Step 2. Gel electrophoresis

Then you will place properly prepared samples of the nucleic acid solution in the wells of an agarose gel electrophoresis system and apply a voltage for a specified amount of time.

In one of the wells, you also place a " DNA ladder": a sample that contains DNA fragments with known numbers of base pairs.

DNA Ladder

The finished gel might look like this:

The lanes on each end are the ladders. The distances travelled are proportional to the molar masses of the fragments.

Step 3. Calculate the numbers of base pairs in your fragments

Create a standard curve from your ladders by plotting log(base pairs) (bp) vs. distance travelled (d).

Assume your data were


You would get a graph like this:

Calibration Curve

If one of your fragments had migrated 1.80 cm, you would have calculated

#log("bp")= "-0.4637×1.80 + 4.3366" = 3.501#

#"bp" = 10^3.501 = "3176 bp"#

Step 4. Calculate the molar mass of the fragment.

The average mass of a base pair is 650 u.

Hence, the mass of the fragment is

#3176 color(red)(cancel(color(black)("bp"))) × "650 u"/(1 color(red)(cancel(color(black)("bp")))) = 2.06 ×10^6color(white)(l) "u"#

and the molar mass is #2.06 ×10^6color(white)(l) "g/mol"#.