Why yeast? Part IIOctober 24, 2008
In Part I, I talked about the convenience of using yeast in terms of cost, space, ease of care, and regulations. And all of those are really good reasons to use yeast. But those aren’t the only things that have made yeast a popular model organism for cell and molecular biology and genetics research.
First, because it’s easy to grow and maintain and because yeast is small, you can easily grow up a lot of yeast at once. This can be important if you want to do something like purify a protein for biochemical analysis. In order to do biochemistry you often need a lot of protein and, depending on the protein, you can get that from yeast.
The yeast genome can be easily manipulated. One thing you can do is knockout a gene, that is, essentially remove it from the genome. This is useful if you want to study the function of that gene. That sounds counterintuitive but, basically, the idea is that you get rid of the gene then see what effect that has on the cell. From that you can infer the function of the gene. Another thing you can do is add DNA to the genome. Let’s say you want to be able to see where your favorite protein is in the cell. You can insert DNA encoding a fluorescent protein (a protein that glows when exposed to a particular kind of light) just after the gene that codes for your favorite protein in such a way that now the fluorescent protein will be attached to your protein of interest allowing you to watch the movement of your protein in the cell. These kinds of tricks are available in other model systems, but it’s much much more complicated to do them and it takes longer. It takes about three days to get the yeast you want after knocking out or adding a gene in yeast whereas in mice it takes weeks to months.
You can do genetics with yeast. This is probably one of the biggest advantages to working with yeast. Yeast can exist either as haploids (having one copy of every chromosome) or diploids (having two copies of every chromosome) and it is fairly easy for a researcher to manipulate the yeast into each of those states. Two haploid yeast can be mated to create a diploid and a diploid can be forced to undergo meiosis and become haploid. Let us say you want to see if a particular gene is essential for life. If you try to knockout that gene in a haploid, you may kill the cell if the gene is essential because you’ve gotten rid of the only copy of the gene the cell has. If you don’t get any cells after the knockout procedure, you won’t be sure if the gene is essential or if there was a problem with the procedure. However, you can use a diploid yeast strain and knock out one copy of the gene. Now the cell has one good copy to keep it alive. Then, you force the yeast to undergo sporulation during which the yeast goes through meiosis. The result is four haploid spores. These spores can be manipulated such that you can observe the growth of each individual spores. If the gene is not essential, all four spores will grow. If the gene is essential then some of the spores won’t grow.
Yeast are eukaryotes and have the same basic internal structures as other eukaryotes (including mammalian cells). All eukaryotes (unlike prokaryotes) have internal membrane structures called organelles. These organelles include the nucleus, the endoplasmic reticulum, the Golgi, and mitochondria among others. The advantage to yeast is that these structures are often simpler in form than in a higher eukaryote like mammals. For instance, there are only 3-6 of the particular subcellular structure I study present in yeast while there are hundreds in the mammalian cell. The protein that I’m interested in is present both in yeast and in humans, but in humans there are two different forms while in yeast there is only one. By studying the simpler system in yeast, we gain insights into how things might work in more complicated systems like mammalian cells. We (or some other researchers) can then take the information gained in the simple system as a starting point for investigating the same process in the more complex system.