hi everyone and welcome to learn a level biology for free with mr. ik so here's one of the requested videos in the subscribers fortnight and it is on meiosis for a level biology if you are new here then click subscribe button to keep up to date with all the latest videos and I hope you find this helpful today and if you do give it a thumbs up so meiosis is a type of cell division you would have learned about meiosis and mitosis at GCSE but at a level you go into it in more detail learning how the variation is actually introduced so meiosis produces daughter cells which are genetically different so introducing variation which is the opposite of mitosis it involves two nuclear divisions which we can see in this diagram we start off here with our parent cell the original cell it divides once and then it divides twice so those are our two nuclear divisions and as a result we end up with four haploid daughter cells so haploid meaning you only have one set of the chromosomes and if we look back at our parent cell here before interphase you can see we have two copies of the longer chromosome one blue one red and two copies of this short chromosome one blue one red but at the end of meiosis we have our four daughter cells but each one only has one copy of each chromosome so one long one short so we described these as being haploid cells whereas the parent cell is diploid because it has two copies so that's what this diagram here is just showing us a basic overview of meiosis our parent cell interfaces acting within the cell cycle rather than meiosis but that's when the chromosomes would double and then we would have round 1 round 2 of our division so let's have a look at then how this variation is introduced there's two mechanisms that occur to introduce the variation firstly independent segregation of the homologous chromosomes and the second is crossing over between the homologous chromosomes now both of these happen in the first division of meiosis so in meiosis 1 between this stage here and creating these two cells here this is when independent segregation and crossing over will be occurring so independent segregation fest my ace is one homologous pairs will be lighting up opposite each other at the equator which just means the middle of the cell and when we say homologous pair that means two chromosomes which have exactly the same genes on them but they'll have different alleles so your maternal and paternal versions of the chromosome and that's what we can see here they're lining up up see each other at the equator but it's random which side of the equator they end up they'll always be in their pairs but you could have all of the paternal chromosomes on one side and all of their maternal chromosomes on the other or it could be random you could have some paternal some maternal on either side and every single time they line up at the equator it's completely random which side they end up lying on so when that first nuclear division occurs you'll have the pair's being separated to either side of the cell so then in the cells here we can see you'll only have one copy of each of those Maalik as pets now if we think about this in terms of possible combinations then that you could end up with there aren't many possible combinations in this example that I'm showing you here because I'm only showing you two homologous pairs of chromosomes but you can actually use a mathematical formula to work out how many possible combinations there are for which chromosomes you end up with in your gametes and that is - it's the power of n now we use two because we're talking about homologous pairs and that means you have two chromosomes each time n is the number of homologous pairs which you have so for a human we have pairs of chromosomes that's why it's a two but we have 23 pairs of chromosomes so if we want to work out how many possible different combinations we could make in the gametes it would be 2 to the power of 23 and that means we have over 8 million possible combinations of which chromosome of the homologous pairs will have in the gametes so already just considering independent segregation this accounts for creating over 8 million different possible gametes or different combinations of chromosomes in the gametes which is a huge amount of variation now that's not the only thing that introduces variation there is also crossing over crossing over happens at the same stage is in meiosis 1 when these homologous pairs line up opposite each other at the equator and here we can actually see demonstrated the homologous pairs they've got the same genes represented by these same letters but they have different versions they have different alleles the paternal one here is shown with the dominant allele and the maternal is shown with the recessive now what happens is sometimes when these homologous pairs line up opposite each other they actually get tangled and twisted and it's these chromatids that start to twist around with each other and get tangled when that happens that puts tension on the chromatids and actually causes part of the chromatid to snap and break off now where they break off they're able to recombine and often what happens is it breaks off but it recombines on the other chromosome of the pair and we can see here what that means is they have swapped sections of their chromatids and that results in a new combination of alleles on this chromatid and on the whole chromosome because we now have capital a4 dominant lowercase a for excessive and the officers over there as a result in the gametes we end up with new combinations of alleles so this is just showing us again we put a homologous pair of chromosomes you see where they start to cross over the chromatids and you'd have to be specific saying it's the chromatids which sort over and it's the chromatid which breaks and recombines and as a result we've now got new combinations of alleles in the gametes so those are two types of variation so just to have a summary of meiosis compared to mitosis meiosis is to nuclear divisions whereas mitosis is only one because we have two divisions we actually only end up with one set of chromosomes which we call haploid cells whereas we have one round of division in mitosis so you get two sets of chromosomes still which is a diploid cell so di die meaning - meiosis introduces genetic variation through independent segregation and crossing over whereas mitosis results in genetic identical cells or planes and we have a very brief summary here just showing that however in this diagram we don't see the impact of crossing a box they still all have exactly the same combination of allele sir now the next thing which is on the specification link to meiosis is they state that they could give you a completely unknown lifecycle of an organism and you have to draw on the diagram where meiosis is occurring and what they're checking is do you know that meiosis involves a diploid parent cell becoming a haploid cell so all you have to look for in the diagram is where cells go from being diploid which can be represented as two end meaning two copies of each chromosome so they go from being diploid to becoming haploid meaning only one copy of each chromosome so for example we've got this here for algae we've got lots of different stages in their lifecycle but at each point they're showing you whether it's haploid diploid eight haploid haploid haploid haploid and so on and sometimes it's a bit misleading because they say gametes are being made or we've got here haploid gametes and that can lead to confusion because genes might jump straight away just to see okay it's a gammy let's label it there but what you're looking for is where you have a 2n or diploid cell becoming a haploid cell and here that is because it's labeled as meiosis another example over here sorry that versions bit blurry so we've got a different example here which is much more crisp to see and this time it's in creating spores for a particular organism and we're told that all of this part of the diagram is 2n diploid and this part is haploid and where that change from diploid two haploid occurs is this section of the life cycle so you'd be labeling on that meiosis is this section so you don't need to read any of the other labels apart from haploid and diploid because there's 2n 2n so they'll always give you an example where it is misleading where you can see here they're creating gametes but for some organisms they actually create gametes by mitosis it's only certain animals like humans which will do it by meiosis so ignore the creating of gametes you're looking for 2n 2n so if a little bit is there's even more variation introduced from meiosis and that is the resulting gametes will then fuse in fertilization so sperm egg will fuse so we already said in terms of the gametes there were 2 to the power of n possible combinations of chromosomes in the gametes so for humans we said there was over 8 million to 2 power 23 but then we have a random egg a random sperm fusing and because it's round in which egg and sperm fuse we actually have 2 to the power of n squared possible combinations of chromosomes which then gives us a huge amount of variation so huge range of possible combinations of chromosomes you could get the resulting organism so for humans you see that's seven point zero four times 10 to power of 13 possible combinations and this is why you don't get humans which are genetically identical unless they are identical twins so this is before we even take into consideration the variation introduced by crossing over so in reality you actually have even more variation than that they're so meiosis the key takeaway from this is it introduces genetic variation and that is it for my OSIS a level so head over to miss Esther ich komm for practice questions and you can go to mrs. 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