Author: Karen Kyker
Woodrow Wilson Biology Institute
Imagine a huge swimming pool full of water and many entangled nets (this is the agarose gel). A dump truck comes along and dumps its load into one end of the pool. That load contains an extensive collection of long, skinny things of varying lengths and sizes (analogous to DNA of varying sizes), e.g. from boa constrictors to smaller snakes to worms to microscopic bacteria.
There is a vacuum at the other end of the pool (analogous to the electric current), which can be turned on and off. When on, the vacuum pulls all of the creatures across the pool. But because of the netting, movement of the skinny things is impeded.
After describing this, I ask the students which creatures will reach the other end of the pool first. They always answer correctly and explain that the longer creatures have to wind in and out and around the holes in the netting to move; therefore, their progress is slower. When the vacuum stops, the smaller things will have moved further than the larger ones.