A step-by-step breakdown of meiosis, the cell division that makes sperm and eggs. The lecture covers both meiosis I and II, with focus on crossing over, where chromosomes swap DNA segments to create genetic variation in the four haploid cells produced.
In this lesson, we'll explore the details of what happens during the phases of meiosis. Meiosis, sometimes called reduction division, is the type of cell division that produces gametes. By gametes we mean sex cells such as sperm cells in males and egg cells in females. Meiosis is broken down into two stages of cell division called meiosis 1 and meiosis 2. Meiosis I has four prophase 1, metaphase I, anaphase I and telophase I and meiosis II also has four prophase ii, metaphase ii, anaphase II and telophase ii. Lets look at what happens during meiosis I. Prophase I starts with a diploid cell. Its chromatin contains two uncoiled spread out sets of chromosomes, one from each parent. After the DNA in the chromatin replicates, it condenses into the more familiar X shaped chromosomes. The replicated DNA is the same in the identical sister chromatids of each chromosome. In a process called synapsis, each chromosome pairs up with and binds to its corresponding homologous chromosome, forming a tetrad. A tetrad is the group of four sister chromatids in paired homologous chromosomes. The chromosomes contain genetic information called genes. These genes were inherited from each parent and different versions of the same gene on each chromosome are called alleles. In a process called crossing over, chromatids from each homologous chromosome exchange segments of alleles, also called recombination. Crossing over randomly happens on every chromosome, resulting in different gene combinations. This explains why every gamete is genetically different from every other gamete. Crossing over results in genetic variety in offspring. This is why children are different from their biological parents as well as from their biological siblings. Continuing on with prophase one, the nuclear membrane disappears, the centrioles move to opposite ends of the cell and spindle fibers fan out from them. Next, in metaphase I, the homologous chromosomes line up at the equator and attach to spindle fibers from opposite poles. During anaphase I, spindle fibers separate the homologous chromosomes in each tetrad and pull them to opposite poles of the cell. The cell enters telophase I with one chromosome from each homologous pair at separate poles. However, each chromosome still consists of sister chromatids. Keep in mind that each chromosome's sister chromatids are no longer identical because of the allele exchange that happened during crossing over. Then spindle fibers disappear and the nuclear membrane reforms around the chromosomes. Finally, cytokinesis occurs. Meiosis 1 ends with two genetically different haploid daughter cells. Each haploid cell contains only one set of chromosomes consisting of paired sister chromatids. Both cells now enter the next stage, meiosis ii. However, unlike meiosis I, DNA does not replicate before meiosis II begins once again. In prophase ii, the nuclear membrane disappears and spindle fibers fan out from the two sets of paired centrioles. During metaphase ii, the chromosomes in each cell line up at the equator and attach to spindle fibers from both poles. During anaphase ii, the sister chromatids of each chromosome separate and move to opposite poles. Once the sister chromatids separate, they are called chromosomes. Finally, during telophase ii, the spindle fibers disappear and nuclear membranes reform and cytokinesis occurs in both cells. Meiosis 2 ends with four genetically different haploid daughter cells, each containing only one set of chromosomes. Some key points to remember about it begins with a diploid cell. Meiosis only produces gametes. Gametes are genetically different haploid cells, sperm cells in males and eggs in females. Meiosis has two stages of cell division called meiosis 1 and meiosis 2. During meiosis 1, homologous chromosomes separate to produce two haploid cells, each containing chromosomes in the form of paired sister chromatids. In meiosis 2, the sister chromatids separate in both cells, becoming individual chromosomes. Cytokinesis of these cells produces four genetically different haploid gametes and Here are some key points to remember about prophase I. The pairing of homologous chromosomes, called synapsis, occurs. Each pair of homologous chromosomes consisting of four chromatids is called a tetrad. During the process of crossing over chromosomes in homologous pairs exchange segments of alleles. Crossing over results in genetic differences in gametes. All gametes produced by meiosis are haploid.
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