hello everyone and welcome to learn a level biology for free with mr. stroke in this video I'll be going through chromosome mutations for a level biology if you are new here make sure you click Subscribe to keep up to date and if you find this helpful please give a thumbs up so first of all just what we mean by the chromosome mutations now this is where there is a change in the number of chromosomes and just like with gene mutations this change happens spontaneously and it's by a process called nondisjunction in meiosis so we'll go through what nondisjunction is so this is when either the chromosomes in meiosis 1 do not separate equally or the chromatids in meiosis 2 do not separate equally in anaphase and as a result the resulting gametes which is made in meiosis will not have the correct number of chromosomes and there's two types of effects that this nondisjunction can have you can either have polyploidy or aneuploidy and polyploidy poly meaning many is when every single chromosome is affected so you'll get a whole additional set of chromosomes so instead of creating a diploid gametes you would create a triploid gametes aneuploidy is when just one single chromosome is affected so you will still create a gametes which is haploid but it might have one additional or one too few chromosomes so we'll go through both of these with diagrams just to fully explain that so polyploid you just again I said it's a change in the whole sets of chromosomes so if meiosis occurs without chromosome mutations the result should be haploid gametes now I've labeled this is human gamo gametes as Lloyd however I'm only actually showing three chromosomes there instead of 23 just because it wouldn't be able to fit 23 in I went to human haploid gametes fuse you end up with a diploid cell meaning two copies of every chromosome and that's what this 2n is referring to and that's what a non mutated human body cell or somatic cell should have however if polyploidy occurs which is when there's mutation resulting in an extra set of chromosomes you could get triploid or tetraploid zygotes now in humans that would be fatal so if you did have that occurring the zygote wouldn't successfully develop into a fetus but you do see this quite commonly in plants so we'll go through how the nondisjunction occurs to create a tetraploid tour trip point so first of all we'll just stick to three homologous chromosomes because i won't be able to fit 23 in a diagram but you would have 23 homologous pairs in humans and at this stage we can see each pair is doubled and that's because i'm starting from the point of DNA replication already occurring in interphase so in this case i'm going to show nondisjunction occurring in the first round of division which we call meiosis 1 and all of the chromosomes failed to separate equally if it's polyploidy nondisjunction so after meiosis 1 we now have all of the chromosomes in one cell and this cell is empty of chromosomes now meiosis 2 I'm going to say happens normally meaning that all of the chromatids do you separate equally now for this one there were no chromosomes here so we don't have any chromatids separating and there's no chromosomes in either of those gametes however in this one this cell had all of the chromosomes moving into it now my ice is too there is normal division so now all of these chromatids will separate equally to the opposite poles in anaphase and that means the resulting gametes we do see the chromatids are separated to these single chromosomes but we have two copies of every chromosome because there were both of the homologous pairs in this cell so instead of creating the N or haploid gametes we now have diploid gametes so if we have a look then at what would happen if one of these diploid gametes fused or fertilized with a normal haploid gametes so here's our mutated gamete which is diploid here is a normal hat ployed gametes where you only have one copy of each monikers pair when they fuse we join all of those chromosomes together and fertilization and that is how we now have three copies of every single chromosome and that's what we call a triploid and that is how we get this change in the whole set of chromosome and we have three sets now if that happened again you could end up with four sets as I said this is polyploidy when you get an additional set of chromosomes but you mainly see implants it wouldn't occur in humans if it did this zygote that was created wouldn't survive now just to show you again this can happen with the nondisjunction in meiosis two as well so we have this time normal division and all of the chromosomes separate equally in meiosis one however in meiosis two for this cell i've said there is normal division so that all the chromatids separate equally and we get two haploid gametes however this cell the chromatids do not separate equally they're all pulled to one pole and as a result we get a 2n or diploid gamete again and this creams this one has no chromosomes so the second type of chromosome mutation is the aneuploidy this is where there's just the change in the number of one individual chromosome so this time just one of the individual homologous pairs and the chromosomes felt separate during meiosis and as a result the zygote that would be made from these gametes would have either one extra or one too few chromosomes and this is an example on example is Down syndrome now again I'm going to demonstrate all of that with the diagrams so again we're gonna have three homologous chromosomes they're already doubled because of DNA replication nondisjunction meiosis one but because this is aneuploidy it's just one of the chromosomes does not separate equally and I've shown that it's this red one and both of those red homologous pairs are pulled to the left pull the purple and the yellow do separate equally so there's one of each of those and monica's pairs in the new cells meiosis 2 happens normally so all of the chromatids separate equally and this is the resulting gametes are made so this time they're all haploid cells because we still only have one set however these ones are missing the red chromosome so you described this as n minus 1 meaning it is a haploid gametes but it's missing one chromosome these two are haploid gametes except they have one additional chromosome so they have one additional red chromosome so if we have a look what happens when either of these types of gametes fuse we can see here the n plus one gamete if it is fusing fertilizing with a normal gamete we end up with a diploid cell plus one extra chromosome and this links back to what I was saying with the introduction on aneuploidy so we now have on fertilization the zygote is haploid but it has an A shanell chromosome and this is one form of Down syndrome so we have three copies and in Down syndrome that would be three copies of chromosome 21 if we have a look at if it was the n minus one gametes again we have a diploid zygote made but it's 2 n minus 1 to n meaning it's diploid two copies of each chromosome however it's minus 1 because we're missing the second copy of that red chromosome now as I did with the polyploidy just the point outs you can get the nondisjunction happening in meiosis 2 so if we have a look we have normal division and the chromosomes separate equally this time in meiosis 1 but in meiosis 2 I'm showing that in this one all of the chromatids do separate equally and we get two haploid gametes however this is the cell where one of the chromatids fails to separate equally and we end up with n minus 1 and n plus 1 so it does it for chromosome mutations I hope you understand [Music]