Agarose and DNA
Location: Outside U.S.
Date: Fall 2013
How does agarose gel electrophoresis determine the molecular weight of DNA?
You can imagine agarose gel to be like a twisty, curvy network of
caves, and the DNA molecules as different sizes of travelers in the
If you're a person, it might take you a very long time to walk through
them because you might squeeze through one hole or have to find a way
around a hole that's too small. On the other hand, a mouse might move
through very fast, because the mouse can fit through more holes! And,
air (which is made up of VERY small particles) can move through
Big molecules of DNA move more slowly just like the people in the
example. Smaller molecules of DNA move faster just like the mouse. And
tiny pieces of DNA move fastest of all (like the air) because they are
Hope this helps,
Electrophoresis can be used to determine the molecular weight of a piece of DNA. DNA is negatively charged when it is in a neutral environment. When a current is run through a gel a piece of DNA will be drawn towards the anode (the positive end). Larger pieces travel more slowly than smaller pieces. A scientist can find the molecular weight of a piece of DNA by comparing it to other pieces of known molecular weight.
I hope this helps.
Andrea Kirk Ph.D.
Biologists often use a technique called agarose gel electrophoresis to separate molecules of DNA. Because DNA is negatively charged, DNA molecules will migrate through a gel towards the positively charged end of the gel when an electric field is applied. Shorter molecules travel more rapidly through the pores of the gel than longer molecules which tend to get trapped more often. You can get a good estimate of the size of a DNA molecule by comparing the distance it traveled to DNA of known molecular weight.
Separating DNA on an agarose gel is a very basic and commonly used technique for biologists - it can be used to help put new combinations of DNA together to understand what genes do, or in a forensics lab to analyze the content of a person's DNA to figure out who did the crime!
Hope that helps!
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Update: November 2011