welcome back this is our third and final video for meiosis and we're going to be looking at life cycles so this was a picture the original authors of this slideshow put in here so I thought I'd leave it in here looks like a pretty nice camera that they use to take this picture but you can see the original Source down there sexual reproduction when we look at multicellular organisms there are very very few that divide only through a sexual reproduction so why do we need sexual reproduction what are the advantages that it provides for us this is really the variation that is created that we talked about in the previous video crossing over independent assortment random fertilization all of this generates variation in our Offspring those daughter cells and that's important to survival and reproduction and you'll examine this further in b4b in a section ual organisms that divide through asexual reproduction the only source of variation for them is really mutation so there are three main categories of life cycles for multicellular organisms some are diploid dominant like humans and other animals and things like this dragonfly some are haid dominant like these fungi over here so what does that mean diploid dominant means what you usually see like when I look at the human body and all of my somatic cells my body cells those are diploid so what you usually see are those cells diploid or haid so humans we're looking at diploid cells we only have gametes that are haid for the second one haid dominant when I'm looking at this mushroom over here the structure the main body these are haid cells and some of them go through something called alternation of generations we'll see this in some plants and algae or actually all plants and algae um this is when you have both you'll have diploid dominant stage and a haploid dominant stage as I mentioned most animals like humans have a diploid dominant stage or life cycle so you see that most of our body cells are diploid cells the only haid cells that we make are our gametes which are are sperm and egg germ cells and germ cells Are embryonic cells that have potential into uh the potential of developing into a gamt uh these are produced by the gonads the gonads are the sex organs like testes and ovaries these germ cells within the gonads use mitosis to propagate the cell line males will make sperm forever females actually make all of their eggs before they're born so they're making them while they're are still in their mom's uterus uh that will be more information for your next course though but gametes we know are going to be hpid and they're produced through meiosis when they come together so when the sperm fertilizes the egg we produce the zygote and the zygote will be diploid so there is no multicellular haploid stage these are going to be single cells the sperm and egg in most fungi such as uh this type of bread mold called black bread mold or it's called risus um and some algae the body of the organism it's hard to see here but if you zoom in you can see it more closely the body of the organism is haid so this is a haid dominant life cycle what happens is during sexual reproduction specialized haid cells from two different individuals that are designated the the positive or the negative mating type will join together to form a diploid zygote this will immediately go through meiosis to form four haid spores and the process continues although these hpid spores are like the parent cells that they came from they're actually genetically different because of the new genetic combination that was created when the zygote was formed so remember in my is we have that crossing over the independent assortment Etc these spores will eventually form a multi multicellular haploid structure and that happens through many rounds of mitosis and then finally we have our third type called alternation of generations this happens in all plants and some algae I think earlier in the previous slide I said all plants and algae it sounded like all plants and all algae I meant all plants and some algae so I'm going to talk about this in the next slide as we go through the picture all right so I'm going to go through the bullet points that you saw in the previous slide but for alternation of generations these are pretty cool because the plants have both a haploid body structure and a diploid body structure so the haid multicellular plants are called gyes and this is because they're going to produce gametes from certain specialized cells here when the sperm and egg come together through fertilization they're going to produce a zygote and that's going to go through mitosis to generate a diploid sporo plant sporo plant then specialized cells will undergo meiosis to generate those haid spores and those haid spores go through mitosis to again to regenerate that gight so this does not happen in humans but if I pretended it did if it was possible it would be like if our sperm and egg could go through mitosis and generate a human like a haid human I guess the egg would generate a genetically female human and the sperm would genetically generate a genetically male human uh but that does not happen so gametes cannot form whole structures like the plant can all right that takes us to the end of chapter 11 in chapter 12 we're going to be looking at mandelian genetics