This is an example of a gel casting apparatus as described earlier. The casting apparatus consists of 3 parts--the tray, the support, and the comb. The tray is the actual mold which provides a shape for the gel as it polymerizes. After polymerization, we will want to move the gel out of the mold and submerse it in a tank of buffer to run.
Do you see a problem here? If you are thinking that the gel is probably too flimsy to pull out of the casting tray without breaking, give yourself a pat on the shoulder. That's what the support is for. The support, which is just a small piece of glass or plastic that rests snugly in the bottom of the tray, can be seen between Margaret's fingers in the photo above. When the gel is finished polymerizing, the support is gently pushed upwards out of the tray by pushing through the hole (the circle you see in the bottom of the tray).
In order to make our DNA migrate through the gel, we need to make sure the agarose polymerizes with small "wells" in it in which to put the DNA. This is the job of the comb, marked by the arrow in the photo. The comb is placed into slots in the tray, with the "teeth" down, when the agarose is still hot. Then when the agarose cools and hardens, the comb is gently lifted up out of the gel, leaving the spaces previously occupied by the teeth. This gives us several places to load different samples.
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