lethal dose

The second experiment you will be setting up in your first 107 practical will involve exposing two different yeast strains to hydrogen peroxide - H2O2.

One of the yeast strains will be wild type. The other yeast strain will be mutant. The mutant strain has had three genes involved in DNA repair removed. This means that the mutant yeast will be less good at repairing damaged DNA.

Since H2O2 produces free radicals that love damaging DNA, you can imagine that your mutated yeast might not grow as well as the wild type yeast when exposed to H2O2.

So far so good. But what if you want to actually measure how much more susceptible the mutant yeast is to the H2O2 than the wild type yeast? 

This is where the LD50 comes in....

The LD50 (aka lethal dose) is how much of something it takes to kill half of a population, or, if you have a more positive mind, at what amount of something half of a population will still survive.

So, for our yeast and H2O2 experiment, the LD50 will be the concentration of H2O2 at which half of the yeast die. 

Calculating the LD50 is a really useful way to compare how well different strains of an organism cope under a certain stresses. When comparing LD50 values, a lower value shows that the organism was more susceptible to the toxin, as it means a smaller amount of toxin was required to cause death of half of the population.

Cheerful stuff!