[Music] hi and welcome back to free science lessons by the end of this two-part video you should be able to describe the stages of cell division by meiosis you should then be able to describe how meiosis can lead to genetic variation now i should warn you that meiosis contains quite a lot of detail and that's why i've split this topic over two videos but you'll see that it's not too difficult when you get the basic ideas okay in the last two videos we looked at cell division by mitosis if you haven't seen those videos then you should watch them now remember that cell division by mitosis starts with a diploid cell in other words a cell with pairs of chromosomes and at the end of cell division by mitosis we've got two identical diploid cells now a key idea you need to understand is that cell division by mitosis happens in almost all cell types we see it when an organism grows or repairs itself we also see cell division by mitosis and asexual reproduction in this video we're looking at cell division by meiosis now unlike mitosis cell division by meiosis only takes place in sex organs and that's because cell division by meiosis is only ever used to produce gametes in other words sperm and eggs in mammals and pollen in eggs and plants in cell division by meiosis we start with a diploid cell and at the end we have four haploid gametes so these cells contain individual chromosomes not pairs now it's really important that gametes are haploid cells and that's because during fertilization gametes fuse together to produce a fertilized egg or zygote and because gametes are haploid the zygote has the diploid number of chromosomes another kid you need to understand is that all of the gametes made by meiosis are genetically different and we're going to look at why this happens later in this video okay now we're going to look at the stages of meiosis in the next video but before we do that we need to explore some essential ideas about chromosomes as we've seen diploid cells contain chromosomes in pairs and i'm showing you a pair of human chromosomes here these are the chromosome 9 pair now scientists refer to a chromosome pair like this as homologous chromosomes remember that one chromosome in a homologous pair comes from your mother and the other chromosome in a homologous pair comes from your father now there are a couple of key ideas about this that you need to understand firstly both chromosomes in a homologous pair have exactly the same genes so for example both copies of chromosome 9 have the gene which determines blood group however you can inherit different alleles from each parent looking at the blood group gene you could inherit the a allele from your mother and the b allele from your father or the b allele from your mother and the a allele from your father or you couldn't have an a allele from both parents or a b alley from both parents so even though homologous chromosomes have the same genes the alleles can be different okay now as we said before the gametes produced by meiosis are all genetically different so let's look at one reason why this is the case i'm sure you have a homologous pair of chromosomes in a cell that's about to undergo meiosis i'm showing the maternal chromosome in other words the one from your mother in orange and the paternal chromosome in other words the one from your father in yellow now before the nucleus divides by meiosis all the chromosomes are copied in interphase and i'm showing you that here remember that the sister chromatids remain joined at the centromere now at an early stage in meiosis the two chromosomes in a homologous pair come together like this and the chromatids of the two chromosomes wrap around each other scientists call this process crossing over and the attached pair of chromosomes are called a bivalent the points where the chromatids are joined are called kaya's mortar now the key idea is that parts of the chromatids can break off an exchange between the hemolytics chromosomes like this so as you can see the maternal and paternal chromosomes have now exchanged dna scientists say that these are recombinant chromosomes now because these chromosomes have exchanged dna that means that they can exchange alleles looking at the blood group gene imagine that the maternal chromosome contained the a allele and the paternal chromosome contained the b allele after crossing over both the maternal and paternal chromosomes now contain one chromatid with the a allele and one chromatid with the b allele and bear in mind that chiasmata can form at multiple points so a large number of alleles can be exchanged and this exchange of alleles is a major source of genetic variation in meiosis in the next video i'm going to take you through the stages of meiosis [Music]