meiosis one the first part is prophase so remember our cell cycle we have geo where cell just goes out and does its job whatever its job happens to be and then we have interphase when it gets a signal a chemical signal it goes into interphase goes back into that cell cycle an interphase consists of g1 in which the organelles double s in which the dna doubles g2 in which all the proteins are made for the physical division of the cell following that it goes through mitosis or in this case meiosis and we have prophase metaphase anaphase telophase and then cytokinesis which is the physical pinching a part of the cells into two so in meiosis one we have prophase so here we've doubled our dna so instead of one strand there's two that are connected to the center through a little button called the center centromere so we have two copies of moms and here's two different chromosomes and two copies of dads and those are two different chromosomes so remember we only see chromosomes during cell division normally dna exists as chromatin which is that tangled up mass of dna wrapped around some proteins and when cells are getting ready to divide that tangled mass condenses it doubles and condenses into what we know as chromosomes so in prophase one chromosomes first become visible the mitotic spindle appears okay that's going to be here we have the centrosome are the centrioles and the centrioles are going to have microtubules coming out of them that are going to grab onto that centromere and rip those guys apart so this is called mitotic spindle and that appears first in prophase the things that happen in meiosis prophase one of meiosis one are two significant events synapsis and crossing over so synapsis um is when the copies homologous copies so this is chromosome one from mom it consists of two sister chromatids this is chromosome one we'll say from dad and two sister chromatids and we'll say this is chromosome 2. synapsis is when these guys pair off together and they sit next to each other in the metaphase plate when they sit next to each other they form what's called a tetrad okay tetra is four so one two three four copies of the gene two from mom two from dad and crossing over when they cross over they kind of cross their little legs and arms and switch parts and that increases genetic diversity so during synapsis all copies of the homologous chromosomes pair off they form a tetrad and then portions of the chromatids are exchanged between any members of the tetrads so you have maternal chromosomes exchanged with paternal ones and so we can end up with gametes that are unlike either parent this is really important for for genetic diversity and here you can see synapsis and what's going to happen with the crossing over so let's say that this is from mom and this is for eye color and hair color so she has brown hair and brown eyes dad has blonde hair and blue eyes now they crossed over and now we have brown hair and brown eyes but now we also have brown hair and blue eyes and blonde hair and brown eyes so you could have a child that has blonde hair and brown eyes or one that has any of the different combinations here because of the genetic recombination in metaphase one these guys are going to be pulled to opposite poles and they're going to be we're going to have anaphase telophase metaphase they're sitting next to each other anaphase they're pulled opposite poles telophase the nucleus starts to reform and then we have cytokinesis and the cells pinch apart so you have two cells with the haploid number of chromosomes in meiosis two then there's prophase two metaphase two anaphase two telophase two sort of just like we had but there's not gonna be any recombination going on here so here the centromeres now these are going to grab onto the center mirrors rather than just grab onto a chromosome and pull it to the side it's going to split the chromosomes into the sister chromatids and pull them to opposite sides so that after cytokinesis now we have four genetically unique haploid cells or gametes so as the sperm mature they move towards the lumen and you can see they get smaller and smaller until they are going to be released into the lumen of the cell and travel through that straight tubule to the rete testis to the efferent duct to the epididymis so within a seminiferous tubule you can see cells at all stages of sperm development and the supporting cells again those are the sertoli cells and then in the inner in the regions in between these tubules is where we find the lytic cells so the sertoli cells go from the basement membrane to the lumen they sort of look like a puzzle piece they form the blood testis barrier through these tight junctions they support the developing sperm cells produce fluid and control release of sperm into the lumen they also secrete inhibin and that slows down sperm production by inhibiting follicle stimulating hormone the spermatogonium give rise to two daughter cells by mitosis one is going to be kept in reserve so the first thing they do is go through mitosis and put one cell on the side so you always have a stem cell this one's going to differentiate become a primary spermatocyte primary spermatocyte enters meiosis one and we have two secondary spermatocyte and then these are made into goes through meiosis 2 and we have the the spermatids and the spermatids get start getting the flagellum on them and then the sertoli cell is going to get rid of this excess cytoplasm remember these guys are connected by a cytoplasmic bridge and when it eats up that excess cytoplasm and the sperm matures it becomes a spermatozoa or a sperm cell remember in meiosis one is when we have the dna replication tetrad formation and crossing over now half of the sperm then are going to be x chromosomes and half would be y when the dna replicates remember a guy is x y you're going to have two x's and two y's so two of the sperm that you ultimately form are going to be x have the x chromosome to have the y so there's a 50 50 chance of having a boy or a girl based upon the composition of the sperm the female only can give an x so there is female has no part whatsoever in whether the child's a boy or a girl it all is dependent upon the sperm so spermiogenesis is a maturation of spermatids into sperm developing the tails um and losing that excess cytoplasm getting a little cap on top of them and then spermiations when they're released and here you can see just another picture of the different stages and the sertoli or suspentacular cells are often called nurse cells because that's pretty much what they do the sperm has a head that has dna in it also has acrosome on the top and an acrosome has enzymes and it's like a little cap that has enzymes in the midpiece there's mitochondria because it needs a lot of energy to swim the tales of flagellum that's used for swimming and sperm are produced the rate of about 300 million per day once they're ejaculated they have a life expectancy of about 48 hours within the female reproductive tract once ovulation happens there's a short window during which the egg is capable of being fertilized so given these two time frames it really is amazing that pregnancy seems to happen as easily and frequently as it does considering how many events have to be correct in order for it to happen so here you can see the acrosomal cap nucleus mitochondria in the midpiece middle piece and then the flagellum and this is how it's going from this is the spermyogenesis where it's going from being a spermatid developing an acrosomal vesicle in the flagellum the acrosomal cap and shedding the excess cytoplasm and then maturing into a full-fledged spermatozoa or spermicide sperm cell it takes about five weeks for this to occur and this is what sperm looks like under a scanning electron micrograph here's another picture of it this is the same slide that was up before but it shows the acrosome filled with enzymes that help the sperm burrow into the egg the nucleus and the head then we have the mitochondria and then the tail or flagellum the goals for lecture two were to define spomatogenesis bermatagonium spermiogenesis spermiation spermatid primary spermatocytes secondary spermatocyte diploid haploid crossing over tetrad synapsis autosomes sex chromosomes and gametes draw a flow chart for meiosis and indicate the number of chromosomes found during each stage and where crossing over occurs draw a sperm cell label the following parts the acrosome head tail or flagellum and midpiece and answer the question how long can a sperm survive in the female reproductive tract following ejaculation