Centrifuge
A centrifuge is a machine designed to separate heavy material from
light material. When the substance is a very fine solid in a solution,
separating parts of the mixture can be more difficult than a solid. In our
case, we will use a centrifuge to separate DNA from a liquid so we can focus
just on the DNA.
A centrifuge works by spinning the substance at high speed. You
have probably seen the trick where you fill a bucket of water about halfway
full, spin it around, and watch as the water stays in the bucket. The water is
“pushed” against the bottom of the bucket by what is colloquially known as
“centrifugal force.” Though not a real force, but a reaction to the
centripetal force, centrifugal force pushes against the bottom of the bucket.
This causes the particles in the solution to clump at the bottom of the tube.
That solid clump is called a pellet, and the solution above it is called the
“supernatant”.
http://commons.wikimedia.org/wiki/File:Tabletop_centrifuge.jpg
Here is a picture of a modern centrifuge. Notice the
different parts of the machine: there are holes for centrifuge tubes to be inserted, the numbers of which vary
machine to machine; the cap, to secure the tubes inside, preventing them from flying out at high RPMs
(Revolutions Per Minute); the lid, which further protects both the samples and the experimenter should something
go wrong; a way to control the RPM; and finally an on/off switch. Although RPM is commonly used for setting
centrifuge speeds, a more accurate measurement is actually called RCF or relative centrifugal force. 1 RCF is
independent of the radius between the center of the centrifuge and the holes for the samples.
To use a centrifuge, put a test tube in one of the slots. However, it is very important that the centrifuge be
balanced! If the centrifuge is not balanced the centrifuge will be unstable, and it will not spin at the right speed! To
balance a centrifuge, put samples in the centrifuge so they are directly across from each other. If you only have one
tube to centrifuge, you should balance it with a “blank” tube (a tube filled with water). If you have three tubes, you
should put them in 1-7-13, so on and so forth. Once the centrifuge is balanced, make sure the cap is screwed on
tightly, close the lid, set the desired RPM and time, and click start.
Questions
1. Why does the DNA settle towards the bottom of the tube after our sample tube has been centrifuged?
2. Would centrifugation work for liquid mixtures? Liquid solutions? Why or why not?
3. Can you name another case in which a centrifuge is used in science?
References
1. Source: http://www.ehow.com/how_5018706_convert-centrifuge-rpm-rcf-_g_force_.html
2. Source: https://secure.wikimedia.org/wikipedia/en/wiki/Centrifuge