and then we can contrast meiosis with that now remember mitosis and meiosis are just referring to the division of the nucleus not the division of the cell the division of the cell is cytokinesis all right division of the nucleus is either mitosis or meiosis so mitosis is used when we need cells that are identical to the parent cell or growth repair and replacement we don't want any changes we want them to be identical to each other and identical to the cell that gave rise to them and these are strictly for growth repair and replacement you cut yourself you need to um get the wound to heal and it's because you uh your cells go through mitosis and they produce new cells you take place the ones that died okay so um or you're growing from a child to an adult and you need new cells for that so with this we have the parent cell and the parent cell has 23 pairs of chromosomes all right I like using 23 pairs rather than 46 because that reminds me that one set comes from my mom and one set comes from my dad so all of your cells and your body except for the sperm and egg have 23 pairs of chromosomes half for Mom and half from your dad all right um and because of that we say that we are diploid and that is two copies of each chromosome the cells in our body that are haploid only have one copy and that would be the sperm and egg okay so the sperm and egg are haploid so for an organism n equals the number of unique chromosomes our n is 23. but different organisms can have different numbers of chromosomes um I think those the big strawberries that you get in the supermarket I think they are have eight chromosomes and I think they have three copies of each chromosome so they are triploid and they are um uh they have eight chromosomes rather than 23. so different organisms have different numbers of chromosomes and you can have two pairs three pairs you know one other there's different numbers too so the sperm and egg are n all the other cells in our body are two n two and that means diploid m is haploid head so we go through doubling of our DNA and this happens in the cell cycle and now we have 46 pairs of chromosomes and then we go through mitosis and cytokinesis meaning we split both the nucleus as well as the cytoplasm and we end up with two daughter cells that are identical to the parent cell and identical to each other okay we still have the parent cell you double the DNA you get 46 pairs of chromosomes you go through mitosis followed by cytokinesis and act as a 23 pairs of chromosomes in two daughter cells right meiosis on the other hand is used to make sperm and eggs and we want to end up with haploid gametes we also want um to create genetic diversity okay so we have two goals here we want to make haploid gametes that have half the information so if you have two cells that each have 23 chromosomes so here's an egg with 23 there's a little sperm with 23 and you put these two together now you have 23 pairs and that is the new individual okay um so we want to make haploid gametes and we also want to mix things up a little bit we want to introduce or create some genetic diversity genetic diversity makes us healthier okay we don't want to concentrate bad genes you can concentrate good genes but you would also be concentrating badging and a perfect example of that is um dogs when you have Hybrid dogs that are of a certain breed like a poodle or a boxer or a Rottweiler they are interbred and interbred and into bread to get those characteristics that we associate with debris like a dachshund has that long they're short legs and real long bodies um different different breeds of dogs have different shapes and sizes and that's because we've we've made them that way by interbreeding them and not only has that made them have those characteristics it also brings along a whole set of diseases and disorders that are specific to that group some of them is more of a consequence of their body shape than it is a genetic thing like dachshunds always tend to have back problems um German Shepherds tend to get hip dysplasia things like that but there are certain types of cancers that run in breeds like boxers I think get a certain kind of leukemia um so we don't want that we want to mix things up and that's why if you want a really healthy dog you go get one from the pound that's a Heinz 57 that you don't know what's in it that's a mixed breed it's going to be the healthiest generally speaking it's going to be healthier than a pedigree a dog that has a pedigree okay so we want to introduce some diversity we want to mix up our chromosomes I want to mix up my mom and dad's chromosomes and also take parts of my mom's chromosomes stick them on my dad's and vice versa and then mix up which ones go into each cell so that my offspring and um the the father also his body is doing the same thing so there hopefully our offspring are a little bit different than both of us you may see similarities but um the genetics are going to hopefully be a bit mixed up so that we can have healthy children all right so you've got a little line here so let's look at the sperm we start with the spermatogonium all right and the spermatogonium is the stem cell and the first thing it does is it undergoes mitosis so that you never run out of stem cells otherwise you'd end up using them up but if it undergoes mitosis so here's our spermatogonium there's 23 pairs of chromosomes and it undergoes mitosis okay one of these guys is going to continue in the process and this other guy is going to um just stick around until he's needed then he'll undergo mitosis so that you never run out okay so this guy is now our um primary put it underneath so that is our first cell is the spermatidonium once it goes through mitosis now we have our primary dermatocyte okay and the primary spermatocyte is going to double its DNA 36 pairs of chromosomes and then undergo meiosis one and cytokinesis I'm going to put CK for cytokinesis because it's okay to write it and then you produce two cells with 23 pairs of chromosomes and these are the secondary Grenada sites and each of these guys is going to undergo meiosis II followed by cytokinesis and you'll end up with four cells that are attached to these little cytoplasmic Bridges they have a little bit of a tail starting there and these are spermatids okay so here's our secondary spermatic diet it's from the kids and then the spermatids are going to mature into sperm cells notice how little the cytoplasm is now and that's because of Phagocytosis by the sutoli cells so these are called sperm or spermatocyte and the entire process is called from here here it's called um spermatogenesis