[Music] hi and welcome back to free science lessons this is the second part of a two-part video on meiosis you should be able to describe the stages of meiosis and how meiosis can lead to genetic variation in the last video we looked at crossing over and how that produces genetic variation in meiosis if you haven't watched that video then you need to watch it now remember that in meiosis we start with a diploid cell in other words with chromosomes and pairs and we produce four haploid gametes these gametes have individual chromosomes not pairs of chromosomes now a key idea you need to understand is that meiosis actually involves two rounds of nuclear division in meiosis 1 homologous chromosomes are separated from each other and in meiosis 2 sister chromatids are separated from each other let's look at the stages of meiosis before meiosis starts the cell will have been through interphase during interphase the cell copies the chromosomes in the organelles remember though that the chromosomes are not visible as distinct structures during interphase okay first the cell enters meiosis one and the first stage of this is prophase one during prophase one the chromosomes condense and become visible homologous chromosomes link together forming kaya's mortar remember that when the homologous chromosomes are paired like this we call this a bivalent at this point crossing over can take place exchanging alleles between the homologous chromosomes during prophase one the nuclear membrane also breaks down and the centrioles move to opposite poles of the cell spindle fibers also start to assemble into the spindle apparatus in metaphase one the pairs of homologous chromosomes are now lined up on the equator of the spindle apparatus okay now we have anaphase one during anaphase one the spindle fibers shorten and the homologous chromosomes move towards opposite poles for this to happen the chiras mata between homologous chromosomes break okay finally in meiosis one we've got telophase one in telophase one the chromosomes have now reached the poles of the cell at this point the nuclear membranes reform and the chromosomes uncoil back to their chromatin state at this point the cell undergoes cytokinesis dividing into two cells now these cells are haploid because they no longer contain pairs of homologous chromosomes okay now the cells enter meiosis ii in prophase ii the chromosomes condense and become visible again again the nuclear membrane breaks down and spindle fibers begin to develop in metaphase ii the chromosomes are lined upon the equator of the spindle apparatus in anaphase ii the centromere of each chromosome divides and the spindle fibers shorten the chromatids are now pulled towards opposite poles of the cell okay now in telophase two the chromatids have reached the poles of the cell and we now call them chromosomes just like before the nuclear membranes reform and the chromosomes uncoil back to their chromatin state and finally each cell undergoes cytokinesis to produce two haploid cells so as you can see meiosis starts with one diploid cell and produces four haploid cells because the chromosome number halves scientists say that meiosis is reduction division now each gamete made by meiosis is genetically different to the others and we've already seen that crossing over is a major source of genetic variation in meiosis but meiosis increases genetic variation in another way as well i'm showing you here the homologous chromosome pairs lined up on the spindle during metaphase one and to keep things simple i'm not showing crossing over the key idea you need to understand is that when homologous chromosome pairs line up on the spindle we cannot predict whether the paternal or maternal chromosome will end up in which gamete scientists call this independent assortment looking at cell a the homologous chromosome pairs have lined up so that the two paternal chromosomes are on the left and the two maternal chromosomes are on the right however looking at cell b in this case the homologous chromosome pairs have lined up so that one paternal and one maternal chromosome are now on both the left and right so when the homologous chromosomes separate we can produce four genetically different cells like this and each cell may have different alleles depending on whether it contains a paternal or maternal chromosome now i'm showing this for a cell which only has two chromosome pairs a lone pair and a short pair but human cells actually have 23 chromosome pairs now the number of genetically different gametes produced by independent assortment is 2 to the power of n where n is the number of homologous chromosome pairs 2 to the power of 23 gives us over 8 million genetically different gametes and remember that's only considering independent assortment of chromosomes when we factor in genetic variation due to crossing over the number of possible gametes becomes really enormous so meiosis has two ways to produce genetic variation in the gametes these are crossing over and independent assortment of chromosomes there is one final factor to consider most organisms produce a vast number of genetically different gametes during fertilization male and female gametes fuse randomly with each other in other words we cannot predict which male gamete will fuse with which female gamete and this random fusion of gametes introduces a whole extra level of genetic variation in the offspring okay so hopefully now you can describe the stages of meiosis and how meiosis can lead to genetic variation