Yeasts – Fungi

by Vincent Racaniello, PhD

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    00:01 Okay, so that's filamentous fungi. There is one other general type and those are called yeasts and these are typically single cell organisms. There are about 1500 species of yeast, that's only 1% of all the fungi that we know of, so you can see the yeasts are in the minority. Yeasts are not only beneficial to us, we use them to make bread and wine and so forth, but they're also highly studied in the laboratory, because they're great models for eukaryotic cells but they're relatively easy to grow and easy to manipulate genetically.

    00:40 We have a lot of uses for yeasts in our world, saccharomyces cerevisiae is also called baker's yeast or brewer's yeast. So from that name you can guess we often use it for bread production, we use it to make wine and we use it to make beer. So once again the next time you see one of your favorite food or beverages, you should thank the yeast for making them for you. We also use yeast to produce ethanol fuel from sugars like sugarcane and field corn, so the next time you put gasoline into your car, it may be 10% ethanol, that ethanol came from a sugar and yeast helped to produce that ethanol. This lifestyle of a yeast is quite different from that of a filamentous fungus. The yeasts reproduce by budding, they divide asymmetrically, so if you think of a single spherical cell and we have some illustrations of that here on the slide, it can divide to make two cells of equal size or it can divide asymmetrically, which means one of the new cells is initially smaller than the other.

    01:56 You can see in this illustration, the process of budding. We have a spherical yeast and then a smaller yeast is coming out of that, that is asymmetrical budding. So yeasts divide by budding asymmetrically, but they also have a sexual stage, because there are haploid and diploid stages. There are stages where there is only one copy of each chromosome, that's the haploid stage, and then there are stages where there are two copies, that is the diploid stage. There also two mating types, A and alpha and they're shown here on this diagram, on the top you see alpha yeast, on the bottom you see A yeast. This is sort of like male and female in animals and these two come together to form diploid organisms.

    02:45 Each mating type, the alpha or the A, produces a pheromone, it's a chemical that attracts the other mating type. So an A and an alpha find each other, they get together and they fuse, they then form a diploid organism and that diploid cell can replicate or bud as a diploid or it can bud and replicate as a haploid organism. So this is a very unusual lifestyle compared to the filamentous fungi.

    03:15 Now many fungi produce spores in order to replicate. Here's an example of spores that are produced in what we call an ascus. The ascus is simply the sack that contains the spores, you can see here the spores are shown in green and there are many of them within the ascus, and the spores are called ascospores. So this is a way of reproducing, you produce a spore, the spore is released at some point, it goes into the environment and then can grow into a new fungus. We call fungi that reproduce in this way, by making spores in an ascus, we call them ascomyceytes. And ascomyceytes include yeasts, they include molds, they include many human pathogens and also some mushroom producing fungi that we like to eat, like truffles and morels, shown on the right of this slide. So they're all ascomyceytes, they make spores within a sac. Some examples of yeasts that are pathogenic for humans.

    About the Lecture

    The lecture Yeasts – Fungi by Vincent Racaniello, PhD is from the course Microbiology: Introduction.

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    Author of lecture Yeasts – Fungi

     Vincent Racaniello, PhD

    Vincent Racaniello, PhD

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