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Meiosis vs Mitosis
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In previous lecture we discussed what is mitosis today we will be discussing the role of meiosis. What is meiosis? In this lecture I will assume that you have watched that previous lecture of mitosis if you haven't please watch that first because it will be very easy for you to understand meiosis if you know what is Mitosis. So let's start, start the today's Lecture. Hello I am Dr. Aizaz from medicovisual.com welcome to this visual Lecture.
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So you know from the previous lecture that this is a sperm and this is an ovum and they will fuse together through the process of fertilization to form a single cell called zygote. Right, and you know sperm has 23 chromosomes, right, and ovum also has 23 chromosome when they will combine 23 plus 23 they will form 46 chromosomes. So zygote has how many Chromosomes? Zygote has 46 chromosomes. Right, now from zygote onwards what happens
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that zygote undergoes a type of cell division that is called mitosis and with the help of mitosis zygote forms first two daughter cells then from 2 to 4, 4 to 8, 8 to 16 and so on ultimately complete multi-trillion celled organism is formed, complete human, adult human is formed and interestingly through the, through the process of mitosis the daughter cells have same number of chromosomes that is zygote has 46 chromosomes and all the subsequent cells that are formed from mitosis up to this
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complete organism all of them has how many chromosomes? 46 chromosome so the same number of chromosomes is there. Now this organism if he is a male he will form sperms in his testes if she is a female she will form ovum in her ovaries. Right, so here now all of his or her cells they have 46 chromosomes including those cell that are precursor of these gametes but these gametes will have half number of chromosomes than that of those precursor cell cells. Right, so from 46 chromosomes
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in the precursor cell of these gametes 23 chromosomes cell sperm or ovum is formed and that is due to presence of meiosis. Right, so we all say this what kind of cell division ? It is a reduction division as per chromosomes is concerned. It is a reduction division and that is chromosome number is halved with the help of meiosis. Why so? Why don't he or she just forms the gametes with the help of mitosis? So let's assume that gametes are formed with the help
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of mitosis then let's see what happens in that imaginary scenario so now what will happen that let's suppose mitosis is undergoing so 46 chromosomes gametes will be formed that is sperm and ovum will have how many Chromosomes? 46 chromosome. Right, when they will fuse they will have 46 plus 46 how many chromosomes in zygote? 92 Chromosomes. Then in the complete human each cell will have 92 chromosomes and when
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again this will divide, when again this will he or she will form gametes it will have again 92, 92 and 92 plus 92 is I don't know what is the figure I am not good at math but the point is that up till now if there, if meiosis never existed up till now we might have trillions of chromosomes in our cells. It should not happen like that that is why meiosis exist. That is the beauty of meiosis that it keeps the number of chromosomes constant among Species.
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species will have a particular number of chromosomes and that remain constant generation after generation. Right, this is the, this is the beauty of Meiosis. Another benefit, another significance of meiosis that we will discuss later that it imparts the genetic variance among the different members of a species. Right, so let's talk about meiosis. Here I will like to tell you that this 46, 46 chromosome is called diploid and when it's number is
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half that is in sex cells in gametes sperm and ovum it's number is half and that is called haploid. So all of our body cells they have diploid number of chromosomes, they have diploid chromosomes, while gametes are haploid. So meiosis, it consists of meiosis one and meiosis two. So here is a cell which has to undergo Meiosis. Which cell is this? Is it any cell of body somatic cell? Yes what do you think? No. No it is not. Yes you are
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right. It is a cell that is the precursor of gamete. This is a cell that has to form either sperm or ovum from meiosis. It occur in only, in sex cells while mitosis occur in all the body cells, all the somatic cells of our body. So from previous knowledge you know what is Prophase. In prophase of mitosis you might have heard that chromosomes are formed firstly the DNA is in the form of Chromatin, thread, thin thread like form of thin thread
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like structure the Chromatin. Chromatin further condenses to form chromosomes in prophase. So here is the start of prophase and all the chromosomes they are now condensed. Right, now each of our cells including this precursor cells, gamete precursor cell it will have how many chromosomes? Yes 46 chromosomes and out of this 46 chromosomes 23 chromosomes are those chromosomes that came from sperm and 23 are those that came from ovum. 23 that
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came from sperm are those cells* that chromosome that came from father and 23 from ovum are those that came from Mother. Right, now you can see from this diagram actually I should have shown 46 but of course that is not possible so I am only showing six chromosome that represents total 46 chromosomes. Right, so here you can see some of the chromosome they are quite similar to each other in shape for example here you can see these two they look similar
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to each other. Right, and they also look similar to each Other. Letís suppose this is chromosome number one let's assume this is chromosome number one, right, and in this chromosome number one they are in the form of pair this one, pink one it came from mother and this came from father, blue one. Right, now they are not exactly alike but their genes, genes are same. I mean for example here at this place it may code for here there may be a
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gene for eye color then on maternal chromosome here there will also be the gene for eye color.
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If here let's suppose there is a gene for hair colour here also there must be a gene for hair colour. So all genes are present at the same location or loci. Right, and all genes are same but their phenotypic expression may be different. Right, for example this maternal chromosome here this eye color gene may code for let's say black eye color but this paternal gene it may code for blue eye colour. So gene is same but their phenotypic
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expression may or may not be Same. Right, so these are actually, what they are called? These are called homologous pair. So we have how many homologous pair? Yes 1,2,3 yes we have 23 homologous pairs. 23 from father 23 from Mother. In mitosis there is no clear cut there is no concept or demarcation of homologous chromosome that is only important in meiosis we will discuss details later. So these are homologous chromosome that are
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homologous to each other right one exception to this rule they look like each other but one exception to this role is sex chromosome, that is x and y chromosomes. 22, we have 22 somatic chromosomes and 23rd Paris sex chromosome their X and Y chromosome they are homologous but they are not exactly alike. Their shape is quite different from each Other, their genes are also different from each other but some part of their genes they are same so
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they are actually homologous chromosome they undergo crossing over as well but not the whole Chromosome, a part of that chromosome undergo crossing over. What is crossing Over? We are now just going to discuss that. So you are clear about homologous chromosomes concept? These two chromatids are their sister chromatids. Letís suppose this chromatid and this chromatid, they are sister chromatids, connected to each other with and centromere.
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Right, now they are exactly alike for example here the gene will be letís say eye colour and here is also for eye colour, and here also eye colour and here as well. Now here if eye colour gene is coding for blue, eye colour, here there must also be the gene that is coding for blue eye colour but here that may be different in this maternal chromosome. It may code for let's say black eye colour and here also let's say black eye colour.
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So, these are sister chromatids of the same chromosome and these are sister chromatids for another chromosome. These two chromosomes are homologous to each other but chromatids, this chromatid and this chromatid they are sister chromatids or non-sister chromatid? Yes, they are non-sister Chromatids. These two are sister Chromatids, these two are sister chromatids but they this and this and this these are non-sister chromatids. So are you
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really clear about the concept of homologous chromosomes and sister and non-sister chromatid? yes this is very important concept in meiosis so let's move forward
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so I have shown only a single pair of homologous chromosomes rest I have removed for the sake of simplicity now these sister chromatids, not Chromatids! these homologous chromosomes they come close to each other and then they come to lie on the top of each other, right, and then there is a special bond that is formed between them this is not just an emotional bond that is being formed between them but Actually, there is a, there is a thread like
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structure that is formed in between Them. There are certain proteins that form this thread like structure between these homologous chromosomes what is this this pairing? This pairing of homologous chromosomes is called synapsis and this thread like structure that is formed is called synaptonemal complex what it is called? Synaptonemal complex Nemal means thread, synapses mean pairing Synaptonemal complex. So now what they are doing they're
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undergoing Synapsis. They have formed synapsis and that is demarcated by presence of Synaptonemal Complex. So after they have came very close to each other lying on the top of each other then they will do something that a part of their body I mean not Body, a part of their structure that is Exchanged, right. What happens here is that you can see this part of maternal chromosome it is transferred to this paternal chromosome here and this part of paternal
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chromosome it is shifted, it is exchanged to this part of maternal chromosome so they are doing something to exchange their body parts right clear now something interesting happened here now Synaptonemal complex is lost and now they try to separate from each other but when even when they are separated from each other they are still attached at this part at which they exchange the material so this exchange of material is called crossing
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over. What it is called? crossing over! and when Synaptonemal complex disappears when Synaptonemal complex disappears and they try to separate from each other till that part is attached with each other and it formed a cross like structure you can see here it is forming a cross like structure this cross like structure, that is formed it is called, what it is called it is called Chiasma. So, what is this? Chiasma. So these terms you
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must be very clear about what is synapsis. when they come close to each other they pair with each other this is synapsis. When they exchange a part of their structure a part of their genome. This is called crossing over and after crossing over has occurred and Synaptonemal complex has lost they try to separate from each other but their part with which they exchange they are still Attached. This is called. This part is called Chiasama. So,
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you should not mix these terms. yes this is chiasma, then what is chiasmata? Letís bring other chromosomes here, by now they have also done something to exchange their parts, right, so they are back after doing something at molecular level and here you can see what is this thing yes chiasmata and here is also chiasmata and here is also chiasmata usually in a single pair single homologous chromosome spear there are more than one Kozma for example
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you can see here is a Kozma and here is a Kozma that is called as a whole it is called chaos metaphor mission he has meta is plural term right it is simple English word that is plural and Kozma if we talk about a single single this thing single cross like this structure we will call it Kozma so Kozma is singular chaos metals plural and this process is called chaos meta formation right so up till now you you should be very clear about this term
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should we move forward yes so this was ro phase right all these things happened in prophase another thing that happened in prophase is that nuclear membrane it also disappears so that subsequent phases becomes easier it becomes easier to align the screw Muzammil player on metaphase plate we will discuss that later so let's review what happens in prophase of meiosis one or prophase one of meiosis what happens first chromatin condenses further
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to form chromosome then homologous chromosomes pair up to form 2 what this is called synapses then they exchange their genetic material what is it it is called cross crossing over then they try to separate with each other but they are still connected at the parts where crossing over has occurred this is called Kozma and during synapses that thread like structure that is formed this synaptic Oh nimble complex right and finally nuclear membrane
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disappears so all these things they are here they happen in prophase interestingly you can see still they are attacked with each other specially at these Cosmo cosmetics that's more forward not all these chromosome chromosomal pair homologous chromosomes pair they are arranged in the midline and this is called arrangement on mitotic player they rained on midline and centers on from centrosome spindle fibers arises that attaches to kinetochore
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right and the other non kinetochore spindle fibers also arise right they are called non kinetochore spindle fibers are polar spindle fibers now here's the interesting thing is that these two will act as a single unit these two chromatids and these two will also act as a single unit and here kinetochore fibers when they will when these kinetochore fibers will shorten as it happens in mitosis when they will shorten they will not pull a single
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sister chromatid but they will pull these two chromatids hold this chromosome and hold this chromosome they will pull them right so what happens in next ways the in this phase it starts their kinetic or fiber they start to shorten and these non kinetic or fibers they will elongate they will then turn up so what will happen these these fiber these chromosomes homologous chromosome this beautiful couples they will separate from each other
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unfortunately or fortunately they will they will be separated from each other and due to lengthening of polar fibres hell with and on gate so what will happen at sea so here what happens they are separated from each other no in this at this point you can see in metaphase they were still connected with each other right at Kozma but in anaphase the Kozma is lost he has mice lost now homologous chromosomes are separate from each other right
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no what is so interesting here is that now the chromosomes they are not they are not the same this chromosome it has some part from this Roma's own paternal chromosome and this chromosome also has some part from this maternal chromosome similarly here you can see now what you can see here is that we have recombinant chromosomes that are formed chromosomes with a newer combination chromosomes are not the same as that they were before now these
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chromosomes are recombinant chromosome and because of this you can see Jenner attic variance is there in different species in the same member in different members of same species for example he did not look exactly like your father or mother this is because of this process lead let's move forward finally in Halo phase due to hard work of kinetochore and non kinetic or spindle fiber they finally separate from each other they are far from each other this
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chromosome they're far from each other and finally nuclear membrane reappear so up till now this was meiosis one final thing is cytokinesis that it will be separated into two daughter cells I took I Ness is the same as in mitosis that contract try a contractile ring appears which separates it into two cells this is meiosis one now what happens in meiosis two we also saw is exactly like mitosis the only difference is that before mitosis what happens
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their DNA replicates before mitosis DNA replicates but before meiosis two DNA does not replicate why what is the reason as it already has sister chromatid with it yeah you are right in mitosis the problem is that we only have single chromatid like this right to divide them we need to make a copy of the sister chromatids like this and then we will separate those sister chromatids we will separate those sister chromatids with the help of mitosis but here you can
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see we already have two sister chromatids here also two sister chromatids we only need to separate them right we will only need to separate them so first I had to explain meiosis two I will just enlarge this cell Liat and let's see what happens it's very easy to understand chromatin condenses to form chromosome the first stage prophase then arrangement on metaphase plate this is metaphase findell fibers arise mitotic spindle arise this is also happening
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in metaphase sister chromatids are separated from each other this is NFS and finally he love phase we're and then there operated with the help of side to Kinesis right this was that complete meiosis now you involves the complete video of complete animation of meiosis I will not speak you will just try to understand I I hope you will be able to understand this
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from your says one is there
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our single word yes malformation deformation
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metaphase face your face I token aces you Oh myiasis - that is like mitosis
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you you
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liyan oh one thing more I would like to explain about meiosis is that here you can see we have homologous chromosomes and in meiosis one versus one what will happen we will form two daughter cells and these two daughter cells they will have the chromosomes homologous chromosome separated from each other and finally their chromatids will be separated right and ultimately after meiosis two we will have how many daughter cells in total one two three
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four so here another thing that I would I would like to discuss is there what are the differences between meiosis and mitosis yes what do you think in meiosis the early somatic variation but that doesn't happen in mitosis yeah most important thing is that first let's start with the reduction degree and that chromosome number is half right for example here you can see how many chromosomes we have 1 2 3 4 5 6 and if we consider sister chromatid
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as a chromosome we will have for many needs daughter cell 1 2 3 so from 6 we have now 3 actually here is 46 and here is 23 so for the sake of simplicity I am only showing six chromosomes here so first thing is that it is reduction division chromosome number is shaft second as you have told that there is crossing over which imparts genetic variability another difference can you highlight very easy it forms four chromosomes are not chromosome
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four dot essence at the end of my ourselves we have four daughter cell at the end of mitosis we have how many daughter cell to two daughter cells easy any other difference the cells that undergo mitosis and meiosis they are different mitosis occur in all that somatic cells meiosis is occurring only recessive precursor of precursor of gametes the for differences any other difference I think it's complete finally here again I would like to
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mention that there are six chromosome let's say here and here is three so this is this is diploid number of chromosomes this is applied number of chromosome Liat thank you so much for watching this video if you like this video please press the thumbs up button please port me on patreon please subscribe to my channel and please give your precious comments regarding this video thank you so much
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