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Posted By Caulimovirus on July 25, 2006

.: One of the best ways for me to learn scientific concepts is to read about the actual experiments performed that brought to light the relevant facts in the first place (I suppose the best way to learn it is to actually do the experiments myself — but who has the time?). It’s much easier for me to picture the experiments in my head than to simply memorize their conclusions by rote. One of the most frustrating things about my science classes is that they tend to emphasize the knowledge gleaned from experiments while all but ignoring how we arrived at that information in the first place.

.: I remember learning in 9th grade that the speed of light was 299,792,458 m/s and wondering, “How do they know that?” I never asked, of course, and it took me a long time to realize that that was a question worth asking.

.: I hardly remember anything from 12th grade physics, but there’s one class I recall clearly: my teacher explained to us how, early in the 20th century, a scientist was able to measure the speed of light quite accurately. I can’t remember the exact mathematics off the top of my head, but if I had the right equipment I’m sure I could figure out the stuff by myself, because I remember how the experiment was setup:

.: Basically, a large rotating wheel with eight reflective sides was set up on one side of a valley, and two mirrors were set up on the other side. A concentrated beam of light was configured above the spinning wheel in such a manner that for a fleeting moment the beam would reflect off the surface at a 45 degree angle. It would then travel a great distance across the valley, reflect off the two mirrors set up there, and travel once again across the valley and strike the lower surface of the spinning wheel. The idea was to get the wheel spinning fast enough so that by the time the light had traveled there and back the wheel would make exactly 1/8th of a rotation and the beam would reflect directly into the receptor. If they spinned the wheel slightly too fast or slightly too slow they wouldn’t get a precise 1/8 of a rotation, and the beam of light would reflect at too wide or too shallow an angle to be sensed by the receptor (see: right).

.: Once they did that correctly, all they had to do was calculate the angular velocity of the rotating wheel, divide it by 8, measure the distance the light traveled (approximately 52 km in our example), and multiply it all together correctly and voila! The speed of light!

.: But then again, I’m a student of biology and chemistry — what do I care about the speed of light? Let’s instead focus on a rather brilliant experiment pertinent to biology: how did scientists find out which molecules held genetic information?

.: They knew genes were on chromosomes, and they also knew that chromosomes were made up of two types of molecules: proteins and nucleic acid. For a while it was thought that proteins held the genetic information, since they did just about everything in a cell. They also knew at this point that viruses were nothing but a protein and DNA. They realized that certain viruses will infect bacteria and use the bacterium’s cellular machinery to make copies of the virus. So which viral molecule was it that gave the instructions to the bacterium to make more viruses — the protein or the DNA?

.: First, some background on radioactivity. Radioactivity in elements can be easily detected, and a radioactive isotope doesn’t necessarily disrupt cellular activity like you might have thought after watching all those Troma movies. It was known at this time that DNA contains phosphorus, and proteins contain sulfur. Equally important, they also knew that DNA does not contain sulfur, and proteins do not contain phosphorous. So, if one were to, say, grow a batch of viruses in a solution that contains radioactive sulfur and another batch in a solution of radioactive phosphorus, one would have two batches of differently radioactive viruses ready for experimentation.

.: Everything seems simple from here: briefly expose the bacteria for a long enough time so that the viruses can inject their genetic essence, but then quickly separate the bacteria from the radioactive viruses before the bacteria burst with all new viruses. The separation is accomplished by a centrifuge, in which the unholy solution is spun round and round at several thousand RPM, eventually causing the heavier bacteria to aggregate at the bottom of the test tube while the fluid and viral leftovers float on top.

.: There are two possibilities at this point:

1.) If proteins carry the genetic information, then the pellet of bacteria at the bottom of the test tube containing radioactive sulfur would also be radioactive.

2.) If DNA carries the genetic information, then the pellet of bacteria at the bottom of the test tube containing radioactive phosphorus would also be radioactive.

.: The results agreed with the second prediction and the scientists now had evidence that showed DNA was the molecule that held genetic information.

.: Now how simple was that to follow? I didn’t even have to draw a picture for that one; it just makes sense. It’s one of the few experiments in biology my professors talked about in their lectures — everything else they mention is fact this, fact that. But so rarely are the processes used to glean these facts ever mentioned! It’s like learning that 2 + 2 = 4 without anyone ever bothering to explain what “+” means — they just expect you to know 2 + 2 = 4 as a single, isolated fact.

.: So I guess my question is this: do any of you out there reading this know of any good sites or books that explain actual experiments, preferably in biology and chemistry?

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