Hi. In this video, we're going to look at a few ways that eukaryotic life cycles can play out. We'll see these ideas come up again when we take a look at the various lineages of protists, and also when we look at plants and animals, as well as fungi, later on. Now, we've already talked about alternation of generations briefly when we were talking about sexual life cycles, and we mentioned alternation of generations in connection to plants, but it actually plays out in protists as well. And basically, the way that alternation of generations works is that, in the course of the life cycle of these organisms, there will be a haploid and diploid stage and both of these will be multicellular or will have a multicellular component, I should say. So, looking at our image here, you can see here we have our haploid phase, and down here we have our diploid phase. Now, the haploid multicellular stage is known as the gametophyte. And you can see we have our gametophytes right here, and these are going to produce our gametes. Our gametes will get together and fertilization occurs, and we form a zygote. Now, the zygote will undergo mitosis and form the sporophyte. Right? The sporophyte is the diploid multicellular stage, and this is what is going to undergo meiosis and produce spores. Spores are a unit of asexual reproduction. They're generally haploid and unicellular. And we'll see that sporophytes are going to produce spores. These spores will undergo mitosis to form those gametophytes that we started off talking about. And of course, those gametophytes will then produce gametes and the cycle continues and we continually alternate between generations, the gametophyte and the sporophyte generation. So, we'll see this again when we talk about various protists and also when we talk about plants. Now what's kind of cool about alternation of generations is in some species, the sporophyte and gametophyte are structurally different. They look different, you know, they have distinctly different structures and we call these heteromorphic, basically meaning different shape. Right? Different form. Now in some species, the sporophyte and gametophyte will actually look the same. Even though, of course, in the sporophyte, the cells are all diploid, and in the gametophyte, the cells are all haploid. Pretty crazy, I know. These organisms are called isomorphic or same form. Pretty cool and you can see here we have two images. Both of these are actually brown algae. So these are both protists, both types of brown algae. This one over here is isomorphic though. So both the sporophyte and gametophyte are going to look like that, and this one over here is heteromorphic. And, this is a different type of brown algae. You don't really need to worry about the names or anything about these types of brown algae. Just an example of how two organisms that are from sort of the same lineage. Right? These are both types of brown algae. How they can use these two different types of strategies. With that, let's flip the page.
Protist Life Cycles - Video Tutorials & Practice Problems
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Alteration of Generations
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Haploid and Diploid Dominant Life Cycles
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Not all eukaryotes undergo an alternation of generations. Some have life cycles that are either dominated by a diploid or haploid phase. In the haploid dominant life cycle, what we'll see is that the zygote is the only part of the life cycle that is diploid. The zygote is going to form from the fusion of gametes like normal, and then it's going to undergo meiosis and reform haploid cells. These life cycles can be experienced by unicellular organisms or multicellular organisms. That's what's meant by "individuals or more cells". Now, these haploid cells will reproduce asexually through mitosis and cell division, and they can also form gametes through mitosis. Again, those gametes will fuse to form the zygote, and then the zygote will undergo meiosis. So, basically, the dominant part of the life cycle of these organisms is haploid.
In a diploid dominant life cycle, which is what humans have, the only haploid cells are the gametes. These are going to be formed by some type of mature cell that's going to undergo meiosis, like sperm and egg formation. Gametes are the only haploid part of this organism's life cycle, and they will fuse together to form a zygote, and then the individual, which is all going to be diploid. The difference between these two life cycles is that in the haploid dominant life cycle, it's the zygote itself that's going to undergo meiosis to produce the haploid cells. They are not actually the gametes, but the point is the zygote undergoes meiosis directly. Whereas in a diploid dominant life cycle, the zygote will undergo mitosis to form the organism, and then certain mature cells in that organism will be the ones that undergo meiosis to form gametes. So the zygote does not undergo meiosis. With that, let's conclude this video.