Size of DNA Molecules Determined by Agarose Gel Electrophoresis
||Dr. Chuck Kim, Facilitator
Physical Science Department
Cherry Hill High School East
Cherry Hill, NJ 08003
Portrait by Abby Lichterman, Cherry Hill High
School East, 1999
|The distance DNA
molecules can travel through agarose gel depends on the size of DNA
molecules in electrophoresis. The next picture shows an example of gel
Sample agarose gel electrophoresis DNA fragments
order to determine the size of DNA molecules, we need to compile data of the
molecular size as a function of the migration distance and make a graph
using a semi-log scale for y-axis, the molecular size, of a standard DNA
such One Kilobase Ladder, as follow:
This graph was produced by Microsoft Excel taking the
Open Microsoft Excel.
Enter all the data, keeping the migration distance in the
first column and the size of molecules in the second column, as shown by
the above example.
Select the entire data and go to Chart Wizard.
Select Chart Wizard > Chart Type-X-Y Scatter,
and choose the graph with data points but not the ones with graph lines.
Click Next and select Column in Data Ranges
option. (Be patient and complete all five steps without skipping any
options. Look into every option tab.)
Click Next and enter all the information.
In the Gridline option, enable both Major
and Minor Gridlines.
Click Finish to produce your graph.
In order change the vertical scale to logarithmic scale,
right-click on the vertical scale numbers on your graph.
Choose Format Axis, open Scale tab, and
enable Logarithmic. You can adjust the Minor Unit to 10 to
produce enough grid lines.
Right-click again on the X-axis scale to change the
Minor Unit to 0.1 cm or 1 mm if you use mm as the minor unit in your
After making your graph, you need make an equation that
shows the relationship between the molecular size and the migration
Do not select the graph but right-click one of the data
point on your graph. This will open shortcuts for curve-fitting.
Select Add Trendlines.
Choose Power function as your Type. (Power
function is really "powerful" because it can predict the most probable
Click Options tab and enable both Display
Equation on Chart and Display R-Squared Value on Chart.
Finally click OK to see you Equation and
This type of curve-fitting is a powerful tool that our
students can use in any scientific investigation. For more examples, go to
Modeling in Biology.