[Music] hi guys welcome back to the road to success MCAT test prep Series in this video I will be discussing the reproductive system and how it is important to biology now this is a very very important subject as it basically covers everything you need to know moving forward I always be starting off by talking about mitosis and meiosis and there are similarities and differences I will then move forward with an anatomy lesson about the male reproductive system and the female reproductive system and how sperm and OVA are formed I will end this video off by talking about the menstrual cycle which is a very important and can be a very complicated task to learn that being said I'll break it down and I'll make it very easy or at least digestible for you all to learn we have a lot to cover today lots of anatomy lots of various different Cycles so let's get started and jump into mitosis all right let's start off with the cell cycle and mitosis starting with the cell cycle we'll start with this concept of interface interphase is the G1 s and G2 phases of the cell cycle and it's the longest part of the cell cycle which is about 90 of the time the cell will be existing there's also a g not stage where the cell is simply living and carrying out basic functions no preparation of division is occurring during this stage during interphase individual chromosomes are not visible within light microscopy because they're in a less condensed form known as chromatin this is because the DNA must be available to RNA perlamarase polymerase so that genes can be transcribed during mitosis however the DNA will be condensed so starting off with a G or the G1 stage the cell will create organelles for energy and protein production while also increasing its size in addition passage into the S phase is governed by a restriction point certain criteria such as containing the proper complement of DNA must be met for the cell to pass the Restriction point and enter the synthesis stage during the S stage which is for a synthesis the cell will replicate its genetic material so that each daughter cell will have identical copies after replication each chromosome consists of two identical chromatids that are bound together at a specialized region known as centromere now I'll take a quick break and talk to you guys about the difference between chromatids and chromosomes so the initial like one part right here is known as a chromatid and two of them together like this this is a chromosome lastly what connects them those little circles right there we're going to call centromeres all right now that we got that out of the way I only wanted to stop here because we're going to be talking a lot about chromatids and chromosomes moving forward but with that out of the way let's talk about the ploidity of the cell in the S phase deployee will not be changing even though the number of chromatids has doubled but so in other words in humans we'll still have 46 chromosomes but we'll have 92 chromatids the G2 phase uh the cell will be passing through another quality control checkpoint where the cell ensures that there's enough organelles in cytoplasm for the two daughter cells that will be made also the cell will check to make sure that DNA replication occurred properly so that there's no errors or mutations that will be passed on to the progeny now lastly let's talk about mitosis there are four stages to mitosis referred to by students as pmat so make sure you memorize that uh that acronym right there starting with prophase this is the most complex one so bear with me prophase is the first phase in mitosis and it involves the condensation of Chromatin into chromosomes the centriole pairs will separate and move towards opposite ends of the pole once the centrioles move to opposite ends they'll form spindle fibers which are made of microtubules some microtubules will form Asters that anchor the centriole to the cell membrane While others will extend toward the middle of the cell lastly the nuclear membrane dissolves during prophase the nucleoli might be less distinct or may disappear entirely in metaphase the centriole pairs are now at opposite ends of the cell kinetochore fibers will interact with the fibers of the spindle apparatus to align the chromosomes at the metaphase plate note that they don't split here yet because that's what occurs during anaphase the sister chromatids will separate and they will be pulled towards the opposite ends of the cell tlfas and cytokinesis occur immediately after each other telophase is a reverse of prophase where the spindle apparatus will disappear while the nuclear membrane will reform the nucleoli reappear chromosomes on coil that will assume their interface form etc etc each of the two new daughter cells will have a complete copy of the genome identical to the original genome that was present cytokinesis is a separation of the cytoplasm and organelles this gives each daughter cell enough of material to survive on their own each cell undergoes a finite amount of Divisions before in program death so for human cells this will usually be about 20 to 50 uh Cycles after that they will there they can't divide one thing I do want to talk about is that mitosis occurs in somatic cells only and that's important because our next topic is meiosis meiosis is a bit different from mitosis in that it's two rounds of division as opposed to one round and it also creates haploid cells or end cells as opposed to the 2N diploid cells these occur as you can see right here it occurs in sex cells and homologous pairs or homologous chromosomes pair on the metaphase plate as opposed to homologous or sorry sister chromatids now let's talk about meiosis one meiosis II is very similar to mitosis so I'll just write here that's AKA mitosis meiosis one it's very very different so in prophase one the chromatin will condense into chromosomes and the spindle apparatus will form and the nuclei uh will behave similar to how it did in prophase of mitosis but the major difference here is a homologous chromosomes will come together and intertwine in a process that's known as synapsis each synaptic pair will contain four chromatids and they are referred to as a tetrad homologous chromosomes are held together by a protein or a group of proteins called the synaptom snap synaptomenal complex and sorry about that it's quite the tongue twister chromatids of homologous chromosomes break at the point of contact called the chiasma and they exchange equivalent pieces of DNA this process is known as crossing over and is characterized by crossover events that occur on one strand of DNA note that crossing over occurs between homologous chromosomes and not sister chromatids if it happened on sister chromatids it wouldn't cause no change because they are identical now I want to point out that crossing over occurs on prophase one and it's part of Mandel's Second Law of Independent Assortment so I'll write here second law crossing over in metaphase 1 the homologous pairs or tetrads will align on the metaphase play and each pair is attached to a separate spindle fiber on mitosis each chromosome is lined up on the metal metaphase plate by two spindle fibers while in meiosis homologous chromosomes are lined up from across each other and are held by one spindle fiber during anaphase one homologous pairs separate and are pulled to opposite poles by the cell or opposite poles of the cell this process is called disjunction and it accounts for Mendel's first law sorry about that sort of overlapped here I'll rewrite it again because I don't want to cover up the diagram and this is the first law during disjunction each chromosome of paternal origin separates from its homologue of maternal origin and each chromosome can end up in either daughter cell this separation of the two homologous chromosomes is also referred to as segregation lastly telophase 1 a nuclear membrane forms around each new nucleus it's essentially very similar to how telophase occurs in mitosis after uh that occurs the cell will divide into two dollar cells by cytokinesis and in between uh meiosis one miosis two there will be a short rest period called interkinesis that being said meiosis II I already discussed basically saying that it's like mitosis so we don't need to cover that again moving on let's talk about the male reproductive system one thing I want to start off with is by saying that the male uh chromosome the male sex chromosome is a y well the female sex chromosome is an X so that being said let's move straight into it you can see right here there's a lot of anatomy going on and you can study this on your own time but I will want to focus on a pathway called Seven Up which is essentially the sperm pathway from your testes with all the way exiting out the urethra so I'll say this well I'll say this slowly but it starts off with this in the seminiferous tubules I don't believe it's labeled on this one but that means you can study the Anatomy on your own time please focus on just the pathway itself the seminiferous tubules then go into the epididymis then after that it'll go through the vas deferens traveling all the way across here up to the ejaculatory duct here after the ejaculatory duct n stands for nothing and then it'll pass through the urethra right here and then exit out through the penis I know I went a little bit faster so I'll say it one more time slowly seminiferous tubules into epididymis to the vas deferens to the ejaculatory duct to the urethra to the penis that being said let's jump into the process of spermatogenesis the formation of haploid cells through meiosis in males diploid stem cells are known as spermatogonia and after replicating their genetic material in the S stage they will be developing into diploid primary spermatocytes the first meiotic division will result in haploid secondary spermatocytes which then undergo meiosis II to degenerate haploid spermatitis finally the spermatids undergo undergo maturation to become matures for matazoa experimatogenesis results in four functional sperm for each spermatogonium I want to point out that mature sperm is very compact they consist of a head which contains all the genetic material and a mid-piece which serves as a Connecting Point an ATP generator for the tail I also want to point out that each sperm head is also covered by a little cap known as the acresome the acrosome is derived from the Golgi apparatus and it's necessary to penetrate the ovum once a male reaches sexual maturity during puberty about 3 million sperm cells are produced every day and we'll be doing that we'll be doing that men will be doing the uh that over the course of their lifetime so from 13 on or 12 on whenever you hit puberty men will be creating sperm for the rest of their life and it'll be healthy as it was from day one or not day one for when you were 13. moving on to the female reproductive system there's a lot less Anatomy here but there's a lot more complicated stages so bear with me so I'll just talk really quick about the uh the pathway of an egg so they're created in the ovary and they're eventually ejected and go through the fallopian tube where they'll eventually be implanted in the uterus for fertilizer or for uh fertilization and growth I thought I had a little bit more for you guys but that's it apparently so moving on let's talk about the the process of oogenesis by birth all the oogonia have already undergone DNA replication and they're considered to be primary oocytes these are two n like their primaries spermatocytes and they're arrested in uh in prophase one so all of these primary oocytes are in prophase one after menarchy or the first metro menstrual cycle one primary oocyte will be released every month to complete meiosis one producing a secondary oocyte and a polar body the division is characterized by unequal cytokinesis which unfortunately the polar body receives none of the main organelles the polar body eventually will be just atrophied it will not uh serve any major purpose the secondary oocyte on the other hand will stay in metaphase 2 and it will not complete the remainder of meiosis II unless fertilization occurs so I'll just write it here metaphase two the last phase I want to talk about is the menstrual cycle this is going to be tested heavily on the MCAT so please pay attention to this section the menstrual cycle can be broken down into four different stages starting with the follicular phase this is where menstrual flow sheds the uterine lining of the previous cycle GnRH secretion from the hypothalamus increases in response to the decreased concentrations of estrogen and progesterone which fall off towards the end of the cycle the higher concentrations of GnRH cause increased secretions of both FSH and LH these two hormones will then work together to develop several or Varian follicles the follicles will then begin to produce estrogen which has negative feedback effects and causes the GnRH and LH as well as FSH to fall off estrogen stimulates regrowth of the endometrial lining stimulating vascularization and glandularization of the decidua so I know there was a lot that we I just said right there bear with me I'll write down basically shorthand what happens here so the increase in GnRH will cause an increase in FSH and LH which will then cause an increase in estrogen which then serves as a negative feedback loop decreasing GnRH which then leads to a decrease in FSH a decrease in LH follicle stimulating hormone and ludial hormone I'll be covering the specifics sorry the specifics of those in a later video about the endocrine system anyways the next stage is the ovulation uh stage estrogen here actually is pretty interesting in that it creates both a negative and a positive feedback loop the developing follicles will secrete higher and higher concentrations of estrogen eventually reaching a threshold where they will paradoxically create positive feedback and GnRH LH and FSH will start to spike the surge in LH is important as that is what is inducing ovulation so I'll shorthand this as well so the positive feedback loop here will cause an increase in estrogen which will then cause an increase in GnRH causing an increase in FSH and a massive surge in ludial hormone eventually leading to ovulation moving on to the ludial phase the lah will cause a ruptured follicle follicle to form the corpus luteum this is what's going to be secreting progesterone progesterone is what maintains implantation while estrogen regenerates the uterine lining progesterone levels begin to rise while estrogen levels remain high the high levels of progesterone cause negative feedback on GnRH FSH and luteal hormone preventing the ovulation of multiple eggs I'm so sorry I realized just here it says gluteal phase not luteal phase but anyways it's luteal phase and moving on towards the last step the menstruation occurs menstruation assuming that implantation does not occur the corpus luteum will lose its stimulation from luteal hormone and progesterone levels will Decline and the uterine lining will Slough off loss of high levels of estrogen and progesterone remove the block on GnRH so the next cycle can begin so I'll write just here I'll finish it off because I already wrote it for these two so increase in progesterone causes a negative feedback loop leading to a decrease in GnRH decrease gluteal hormone and FSH finally right here since there's a decrease in LH there's going to be a decrease in progesterone which will eventually cause sloughing off or menstruation I know there's a lot of stuff here but it's very important for the MCAT so please study this slide but that being said that's all I have for you guys today we covered the menstrual cycle as well as a male and female reproductive systems we talked about meiosis and mitosis and we also talked about the cell cycle this was a very dense passage but it's very important for the MCAT so I hope you guys were able to learn something I hope I was helpful to you guys that being said I hope you guys have a great rest of your day and happy studying