Question

In lab, we are going to be tested about gel electrophoresis but I can't seem to understand how to predict how far fragments of DNA will move during electrophoresis. I know that the smaller the fragments are, the more they will travel but I'm not sure if that's the answer my lab instructor is looking for. I'd appreciate it if you had an example!

Answer 1

Say we have a mixture of DNA fragment of sizes 800bp, 500 bp, 200 bp, 100 bp and 50 bp.

The smaller the fragments are,the more they can pass through the pores of the agarose gel, and so they travel a longer distance in the gel. So here the 50 bp fragment will travel the furthest towards the anode. This band will be in the lowest region of the gel, the lowest band will be the 50bp band. (the exact distance migrated in milimeters depends on how long the gel is run and is proportional to the other bands as well. This is not something to be memorized)

Above the 50 bp band will be the 100 bp band. As we move up the gel the fragment size increases, and the topmost band will be the largest one, the 800 bp fragment. This is the concept that can be explained using some other examle of combination of fragments as well.