hi everybody and welcome back to miss Angie's biology class I am Miss angler in today's video we are going to look at abnormal meiosis we're going to take a quick recap on how regular normal meiosis should take place and then we're going to look at how abnormal meiosis takes place what are the results and then how to explain yourself in exams and tests because I think that's all what we really want to know is is how do we get full marks if we have to explain this now if you are new here don't forget to give this video a thumbs up and make sure you're subscribed with your notifications turned on I post every Tuesday and Thursday for grade 8 to 12 biology if you're in metric and you're thinking about improving your mark this year and doing really well in your finals you should think about getting my study guide it's available on my website or you can join my membership which has loads of members only YouTube videos as well as free access to my study guide so I think it's really important at this point in the video that we clarify the point of meiosis so we understand better what happens when it goes wrong so in this diagram here we have normal meiotic division in the First Column we then have non-disjunction in meiosis one in the second and then non-destrunction happening in meiosis two now if you are not so sure about the differences between meiosis one and two you should go and click the videos I'm going to link above now so that you're better versed on what happens when now let's get back to normal meiotic division remember the point of my arctic division is to take one cell and half the chromosomes into two separate cells and then take those two cells and half them again so we end up with four and those four cells must be non-identical but when we look at the number of chromosomes they have we need to focus in on the fact that if we started off with eight chromosomes in the first cell the uh two cells produced would have four each and then those two cells produce another two cells each with four chromosomes in them now if that's really lost you then it's a really good idea to go and watch my meiosis videos to clarify that now that's normal meiosis we're taking chromosomes we are dividing them in half and then we pull apart the chromatids so that everyone gets one chromosome and we end up with four non-identical cells but each have the same amount of chromosomes however sometimes non-disjunction can take place which is what we see in the two other Columns of the diagrams here and if you actually notice both of these show non-disjunction however they show them at different points so I'm going to look at the middle column and what I want to show you is in this Central picture over here what happens in meiosis one particularly and this is occurring most often in anaphase and I'll go into more detail about that later you will notice that the homologous pairs fail to separate which is what you see here however in meiosis II you will notice a little bit lower down in the diagram over here it's not so much the homologous pair failing to separate it's actually the chromatids failing to separate in this one now if we look at the resultant cells you will notice that we annotate any of the cells that have too many chromosomes within plus one or if they have too few chromosomes in minus one and that is because obviously these cells ended up with an extra chromosome and they stole that extra chromosome from these cells which are now short one chromosome but what exactly is non-disjunction and so you need to have a really good grasp of what it is before we can actually apply to a situation so essentially non-disjunction is when there is a failure of chromosomes to separate and this most often occurs in anaphase which I am going to go into more detail but essentially non-disjunction occurs where you have homologous pairs of chromosomes like we see here and they separate generally into separate cells you can never have both homologous Partners in one cell they must be separate as we see here we've pulled them apart but at some point during meiosis II in this diagram what's happened is these two chromatids have not separated and they've ended up in the same cell leaving us with one cell that is now empty and has no chromosomes now non-disjunction is the process it's the word we use to describe it but when does it actually happen well let's have a look at that next now to answer that question we need to make sure that we're very familiar with the order which the phases occur so I think we're all very familiar with pmat which is prophase metaphase anaphase and telophase and this is the order in which the phases of meiosis or mitosis need to occur in but what's really important now is what happens between metaphase and anaphase because this is where non-dysunction actually takes place now if you're not sure about what I'm going to say next which is the fact that I already know this is metaphase II and this is anaphase two if you're like how did you know that well you need to go back again like I said and watch my meiosis 2 video and it'll show you how to actually tell the difference between the phases and whether you're in meiosis one or two but what I want to highlight here is the fact that non-disjunction takes place during anaphase one or in this instance anaphase two now starting off with where it begins metaphase which is this phase over here is when our chromosomes align the Equator the spindle fibers has grown out of the centrioles and they are now attaching themselves to either side of this chromosome and they intend to pull the chromatids apart which is what we see here in anaphase two we see the chromatids being pulled apart and it creates this space in between here however if you look a little bit lower down you will see that this chromosome still has two chromatids attached to each other which is not what we want because we want to separate them from each other at the end of meiosis too and so you'll see a spindle fatty here with nothing on it right which is not going to be great because if I drew the resultant cells of these two in this cell we would have one two three four chromosomes but in this cell we would have one two okay and I got that based off of whichever direction the chromatids were being pulled towards their pole okay and so that's why we end up with four in the one but only two in the other now let's get into the results of non-disjunction we know what non-disjunction is and how it or when it occurs but now we need to see like what are the products so this diagram here has three potential outcomes the first one is actually just regular meiosis and then fertilization where we have one haploid ovum joining with one haploid sperm cell to make a regular diploid cell but what happens when a cell that has no chromosomes or one less chromosome like we see here in this one joins with a regular sperm cell we get something called monosomi which means they are missing a chromosome the third one is what we're going to focus the most on because this is what is linked most often to the non-disjunction we do in grade 12 but when you have a cell that has an extra chromosome and it fuses with a regular gamete you now have three or extra chromosomes we call Trisomy and that means you have inherited an extra chromosome now let's do the most important bit right the bit that you are going to get marks for how are you going to explain this in exams or tests so first things first you must always state that the spindle fibers are shortening right they're pulling what are they doing what are they pulling well they are pulling both chromosomes to the same pole now what does that look like well if we have a cell here and we've got our centriole and our centriole and these are our spindle fibers growing out of them we end up getting something like this let's say we have two successful chromatids separating however we've got a chromosome that's still stuck together and so when they do finally go through cellular cytoplasm division will end up with one cell having three chromosomes in it and one cell having one which means it's short it should have two two but now one's got three and the other one's got one and so we call this one Trisomy and this one mono because it's missing one and please don't just get confused because I use the word three and try it's not always going to be three and try or one and mono um essentially the word mono means one less and try means one more now getting back to the explanation it's also really important in your description to state that non-disjunction occurs during anaphase one or two depending on the question you will have to decide did this happen during anaphase one or two and sometimes the diagram is going to help you decide that and finally you actually need to make a statement by saying this all results in Trisomy now lastly what we need to look at is the real world applications of looking into examples of Trisomy and a word that you may also be familiar with or may have heard in class is called anuploidy and essentially anuploidy means any organism that does not have the correct chromosome number we also call this a chromosomal mutation now what you're seeing here are karyotypes and karyotypes are when we lay out all your chromosomes your pairs of chromosomes and what we can see here is we have a karyotype of a healthy human that has the full correct number of chromosomes and then you have an embryo with extra chromosomes or what we call anuploidy specifically we are going to focus on number 21 if you get an extra number 21 chromosome you are going to get a syndrome called Down syndrome which we can see here in this picture of this young lady and you don't actually need to know all of the signs and symptoms of down syndrome you just need to Simply know that an example of non-disjunction and what it results in is anuploidy of chromosome number 21 which is Down syndrome but how do you actually explain how Down syndrome occurs well you can take the non-disjunction example we just spoke about now how to explain that now specifically you may be asked to explain Down syndrome and this is how we are going to answer it in any test or exam so first of all we must always state that due to non-disjunction you must acknowledge that that has happened we've now ended up with a sperm cell or an ovum that has 24 chromosomes instead of 23. now this ovum or sperm cell with 24 chromosomes is fertilized by a sperm or ovum with 23 which remember is the correct number this results in a zygote with 47 chromosomes remember we're supposed to be 46. this is the most important part in your answer specifically an extra number 21 chromosome is present in Down syndrome and this obviously results in having Trisomy now as always I like to wrap up my lessons with a little bit of terminology recap there wasn't too much terminology in this section because a lot of it falls under the meiosis but first things I want to speak about is the chromosomal mutation which remember is when you have too many or too few chromosomes and this is because of non-disjunction which is the failure of chromosomes to separate during anaphase 1 or anaphase two and those are really important phases to know and this results in a condition called anuproydi anuploidy simply means you have too many or too few chromosomes individual singular chromosomes now if you have too many we call this Trisomy but you also remember I spoke about monosomi which is when you are missing a chromosome or you have one less now please remember if you like this video don't forget to give it a thumbs up and make sure your notifications are turned on so you get notified when I post a new video every week on Tuesday and Thursday I'll see you all again soon bye