Meiosis I

in the next two lectures we're going to begin our discussion on a type of sexual reproduction process that takes place in eukaryotic cells known as meiosis now when we discussed mitosis we said that mitosis is a cell division process that's somatic cells undergo so somatic cells are the eukaryotic cells that divide via mitosis now those eukaryotic cells that divide via meiosis are known as gamete assigns shogunate Desai's divide via meiosis and somatic cells divide via mitosis now both of these processes are a type of cell division the question is what exactly is the major difference between mitosis and meiosis so in mitosis our somatic cell divides into two genetically identical cells that are diploid and that means the chromosome number does not actually change if we begin with 46 chromosomes we're going to end up with cells that contain 46 chromosomes however in the process of meiosis our gametes aside divides into four genetically different haploid cells and that means if we begin with 46 chromosomes the chromosome number will be divided by two so we're going to end up with cells with haploid cells that contain only 23 chromosomes now in humans our male gamete aside is known as a sperm at asad while our female gamete aside is known as the oocyte now before meiosis actually takes place and before the cell divides via meiosis the cell undergoes a process known as interphase which is similar to the interphase that takes place in somatic cells before they divide via mitosis now during interphase we have a phase known as the s-phase and during the S phase of interphase we have the DNA that is replicated and in humans where we have 46 chromosomes all 46 chromosomes or 23 pairs of homologous chromosomes are basically replicated during the process of S phase in interphase so our DNA is actually replicated before meiosis actually takes place in the same way that the DNA is replicated before mitosis takes place now what exactly do we mean by this replication process let's take a look at the following diagram so let's take one of these 23 pairs as shown in the following diagram so this is our pair of homologous chromosome so we have homologous chromosome 1 and homologous chromosome 2 now these two chromosomes are genetically different from one another one of these chromosomes let's say the purple one came from the male and the other one let's say the brown one came from the female parent so these are genetically different but they are homologous and what that means is both of these chromosomes carry genes that code for the same exact trait so what that means is let's suppose that the purple chromosome contains the gene that codes for the hair color that means the brown one will also contain the gene that codes for the hair color that's exactly why these are homologous so during S phase of interphase before meiosis actually takes place we replicate each one of these individual chromosomes so we replicate the purple one and we produce this and we replicate the brown one and we produce this now this is identical with respect to this and that's exactly why we call an sister crome kids so these two sister chromatids are identical with respect to one another now on this chromosome we also have two identical sister chromatids and these two chromosomes just like these two individual chromosomes are set to be homologous with respect to one another so we basically double the number of chromatids but the number of pairs remains the same we have one pair and one pair here but we have four chromatids here and only two chromatids here so that means during S phase we double the number of chromatids so now once our gamete decides actually undergo the process of s phase those gametes asides are known as primary gametes assigned so our male gamete aside are known as primary spermatocytes while our a female give me the sides are known as primary oocytes now before the female organism before the female human is actually born although asides basically become primary gametes asides or primary asides and we and we'll talk more about that when we'll discuss the process of sexual reproduction now let's actually get into the process of meiosis now as we said meiosis is the process by which a single cell a single gamete sucide divides into four genetically different haploid cells now we can divide the process of meiosis into two stages we have meiosis one and meiosis two in this lecture we're going to focus only on meiosis one in the next lecture we're going to discuss the process of meiosis two now just like mitosis can be divided into four stages meiosis one can be divided into four individual stages we have prophase 1 metaphase 1 we have an affair one and telophase one so let's take a look at each one of these individual phases and describe what takes place in each one of the phases and let's begin with prophase one of meiosis now prophase 1 of meiosis is somewhat similar to prophase of mitosis in that in prophase 1 of meiosis we have the two centrioles that begin to move to opposite ends and as they begin to move to opposite ends they begin to synthesize our mitotic spindle apparatus and they begin to synthesize the spindle fibers which begin to grow and move into the nucleus of our cell now at the same exact time the chromatin condenses into the chromosomes and the nuclear membrane as well as our nucleolus begins to disappear and that's exactly what allows our spindle fibers to make its way into the nucleus area of our cell now the major difference between prophase 1 of meiosis and prophase of mitosis is the following so the major process that takes place within within prophase 1 is a time of genetic recombination process known as crossing over and we'll see what crossing over is in just a moment now before crossing over actually takes place our two homologous pairs of chromosomes actually have to find one another and basically orient themselves side-by-side and then that allows crossing over to actually take place so before crossing over takes place homologous pairs must find each other and move side by side with respect to each other and this pairing this movement of the homologous chromosomes next to one another is known as synapses and this is shown in the following diagram so the the staff of synapses is the following these two homologous pairs of chromosomes that we spoke of earlier actually have to find each other and move side by side and then when they move side by side these chromatids of these two chromatids are chromosomes have to overlap they have to intertwine as shown in the following diagram so during the process of synapses the point at which our two chromatids intertwine or overlap is known as our chiasma and in this diagram the key ozma is basically this point here it's the point at which our two chromatids this chromatid and this two are in this chromatid basically overlap now because we have four individual chromatids we have one chromatid to three chromatid for this entire structure is known as a tetran so basically the Tetra refers to the orientation of these individual four chromatids that are very close with respect to one another now one synapse this actually takes place and we form our key Ozma then crossing over actually takes place so crossing over is a type of genetic recombination process in which we have the exchange of genetic information from this chromatid and this chromatid so basically crossing over produces two new recombinant chromosomes as shown in a following diagram so this section ends up on this chromatid and this section ends up on or this section ends up on this chromatid to form the following two recombinant chromosomes so crossing over produces our two recombinant chromosomes and notice that each one of the chromatids on the chromosomes are genetically different from one another so this chromis or this chromatid is there from this crow than this chromatid and this chromatid is different than this one and it's different than this one so each one of these chromatids have their own unique genetic information and that's exactly what our recombinant genetic recombination actually does so this is the major process that takes place in prophase one that differentiates prophase one from far from prophase of mitosis now let's move on to metaphase one of meiosis so during metaphase one the spindle fibers actually attach themselves to the kinetochore region of each one of these tetrax and what these spindle fibers basically do is they align the tetras along the center along the equatorial line of our cell as shown in the following diagram so let's suppose our cell contained only six pairs of homologous chromosomes so that means we're going to have three tetras as shown in the following diagram that are aligned along the center notice that made a metaphase one is not exactly the same as metaphase of mitosis in metaphase of mitosis we actually align these individual chromosomes along our equator so in metaphase of mitosis we would have six of these aligned along our equator but in metaphase one of meiosis we only have three because we form these tetras and these tetras are basically aligned along our equator so we have 23 tetras that are basically aligned along the equator and together we have 23 x 2 or 46 chromosomes now let's move on to anaphase one of meiosis in anaphase we have the process of disjunction and disjunction basic means our spindle fibers begin to pull on our chromosomes and we separate our chromosomes so these three chromosomes begin to go onto the left side and these other three begin to move to the right side so we separate our tetras so during anaphase 1 this junction occurs that is the chromosome pair in the tetran are separated to opposite poles now notice one important point so let's take a look at this tetra tier so this chromatid and this chromatid are basically the same chromatid that we began with in this diagram before the S phase actually took place so these are the two original homologous chromosomes where one came from the mother and one came from the father and notice what happens in the process of anaphase so our chromosome that came from the Father is separated from the chromosome that came from the mother and this means that our genes that are homologous that code for the same type of traits are separated during the process of anaphase and this is known as the law of segregation so notice that the original maternal and paternal homologous pair are separated so this is separated from this this is separated from this and this green one is separated from this orange one this process is absolutely random and that basically means this can end up on this side or it can end up on this side now this process is random so either one can end up on either side and this process of segregating our alleles our genes that code for similar traits that came from the female and the male parent this segregation process is known as the law of segregation and we'll talk more about the law of segregation when we'll get into genetics now let's move on to the process known as telophase one so following anaphase one of meiosis we have telophase one of meiosis so during telophase one the nuclear membrane begins to reform around each set of chromosomes so we form two nuclei on the left side of the cell and the right side of the cell at the same time the nucleolus basically reforms our spindles begin to basically deteriorate and what also happens is our nuclear membrane begins to basically separate into two and the cytoplasm also begins to divide so cytokinesis begins to take place now once the cell actually divides one of the cells will have this information and the other cell will have this information and notice our initial cell had 1 2 3 4 5 6 chromosomes and after this division each cell will have 1 2 3 chromosomes and 1 2 3 chromosomes so during telophase 1 we have our nuclear division that basically transforms our diploid cell into a haploid cell so in humans we begin with 46 chromosomes and when telophase 1 takes place we're going to end up with a haploid number of chromosomes and that's exactly why telophase 1 of meiosis is also known as the reduction division because it reduces the chromosome number from 46 to 23 chromosomes so this is the process known as meiosis 1 next is meiosis 2 and we're going to focus on meiosis 2 in the next lecture

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Keywords: meiosis, meiosis I, steps in meiosis I, stages in Meiosis I, prophase I, metaphase I, anaphase I, disjunction, disjunction in meiosis I, tetrads, crossing over, genetic recombination, synapsis, chiasma, recombinant chromosomes, telophase I, cytokinesis, reduction division, haploid cells

Id: ZBqEtIoQmQY

Length: 17min 37sec (1057 seconds)

Published: Sun Sep 14 2014