After the first week of classes in graduate school, I’m a bit wrung out. I’ve taken graduate classes before, but not too many focused on paper-reading; the other classes had maybe two or three papers per week, but the current course I’m taking assigns two or three papers per class, which means around 10 hours of reading a week, assuming a little more than an hour per paper (we have to read them inside and out, understanding them in deep, experimental detail). With having around 6 or 7 seminar talks a week, two classes, and lab work to be done, and fellowships to write, I’m glad the weekend is here.
On the other hand, though, all the classwork means that I’ve been getting a lot of exposure to some very classic papers. The “fluctuation test” paper by Luria and Delbruck, for example, is from 1943, ten years before the structure of DNA was deciphered by Watson and Crick. Luria and Delbruck try to figure out whether bacteria become immune to viruses by mutations or because by chance some survive and acquire an immunity which is heritable, and they use some pretty clever math to do it.
The basic idea they use is that of the “jackpot”. Imagine that you’re playing a slot machine, and you win very rarely, but when you do, it’s a huge, huge payout: $100 million. Let’s say you pull the lever a million times, enough that you have some chance to actually win once or twice, but not enough that you’re sure to win. Now, let’s say that you have 20 people who all go to this machine and pull the lever a million times. Because one person might win three times, and another might not win at all, each person’s winnings will vary a huge amount from the others’. That’s the “jackpot” idea, that small differences get amplified a whole lot, because the payout is huge.
Now back to bacteria. There are two possible ideas, that bacteria all have a small chance of surviving the virus randomly, but that once they survive they’re immune (and pass that immunity on to their children), or that a small fraction of bacteria have a mutation that makes them immune, but most bacteria are susceptible.
The thing is, mutations are kind of like jackpots. If you start with one cell and it divides, acquiring mutations along the way, then in the early stages where there still aren’t that many cells, one cell might get a mutation. Since bacteria multiply quickly, that cell will then have lots of descendents that are also mutated, and so there’s a huge “payoff” for having an early mutation, because that change gets amplified. The number of mutations early on is random, so in two different experiments, there’s a good chance that the number of mutations you find in one is very different from the number in the other.
On the other hand, in the surviving and adapting viewpoint, there’s a low chance of surviving no matter how you go about it, and that characteristic doesn’t get amplified when bacteria have descendents. There’s no “jackpot” effect, and so the level of immune bacteria wouldn’t vary as much from experiment to experiment.
So, Luria and Delbruck did the experiment a lot of times. They took a small number of bacteria, grew them up, and tested them for immunity against a virus; and they saw the jackpot effect. Thus, mutations are the underlying cause for the resistance to virus. Bingo!