Stages of Meiosis

<h1>Text Preview</h1> <p>In addition to reducing the chromosome number, the goal of meiosis is to rearrange the parental chromosomes to create genetically diverse gametes. We’ll observe meiosis in a diploid cell with three chromosomes. We’ll keep a running tally of the chromosome number. Our cell is 2n. “n” is the chromosome number, 3, while “2” indicates that the cell is diploid. There are 2 times 3, or 6, total chromosomes in this cell. </p> <p>When meiosis begins with Interphase I, DNA synthesis takes place so that we have TWO copies of each chromosome. So are there 12 chromosomes total? No, because sister chromatids are considered part of the same chromosome when they’re connected by a centromere. The total chromosome number is determined by counting the centromeres. In our cell, there are 6 centromeres, so there are 6 total chromosomes.</p> <p>The names of the stages in meiosis are similar to those in mitosis. Meiosis consists of two rounds of cell division, meiosis I and meiosis II, which result in FOUR daughter cells. Two unique and important things happen in prophase I which can take days or even longer to complete. Synapsis is the pairing of homologous chromosomes, each made up of two sister chromatids. Crossing over is the exchange of corresponding segments of nonsister chromatids at numerous places along the chromosome, forming what are called chiasmata. Crossing over creates genetic variation. The rest of prophase I is similar to mitosis prophase: the nucleolus and nuclear envelope disappear, and a spindle apparatus forms.</p> <p>In metaphase I, chromosome pairs randomly line up along the metaphase plate. Spindle fibers from one pole attach to the kinetochore of one member of each homologous pair, while fibers from the other pole attach to the other member.</p> <p>In anaphase I, the spindle fibers separate the two members of each chromosome pair. The random alignment of chromosome pairs along the metaphase plate ensures that independent assortment, the random distribution of the maternal and paternal members of each homologous chromosome pair to the daughter cells, occurs. Independent assortment also contributes to genetic variation.</p> <p>By telophase I, chromosomes have reached the poles. Cytokinesis immediately follows, involving a cleavage furrow in animal cells and a cell plate in plant cells. In some species, nuclear envelopes and nucleoli re-form. </p> <p>A rest period called interkinesis or interphase II follows in some organisms, but no DNA synthesis occurs. </p> <p>In other organisms, the daughter cells immediately proceed to the second division or meiosis II. Each daughter cell now has a haploid set of chromosomes, and each chromosome is composed of two sister chromatids. The purpose of meiosis ii is to separate the sister chromatids. It’s very similar to mitosis.</p> <p>In prophase II, a spindle apparatus forms. If the nucleolus and nuclear envelope reformed during interkinesis, they now disappear. In metaphase II, the chromosomes align along the metaphase plate, and each chromosome attaches to spindle fibers from both poles. In anaphase II, the spindle separates the sister chromatids, which move toward opposite poles. </p> <p>Finally, in telophase II, nuclei form at opposite poles, and cytokinesis follows. At the end of meiosis I and II, there are four cells total, each with a haploid number of unreplicated chromosomes.</p> <p>Let’s recap the differences between the phases of meiosis I and II and those of mitosis. Two events in meiosis prophase I don’t occur in mitosis prophase: the pairing up of homologous chromosomes and crossing over. The remaining events are comparable. </p> <p>In metaphase I, pairs of chromosomes line up along the metaphase plate, and each chromosome becomes connected to only one pole. In mitosis metaphase, single chromosomes line up, and each chromosome connects to both poles. Chromosome pairs are separated during anaphase I, while in mitosis anaphase, sister chromatids split up. Telophase I and mitosis telophase are very similar. Meiosis II and mitosis are almost the same process. However, meiosis results in four, non-identical, haploid cells, while mitosis yields two, identical, diploid cells.</p> <p id="TextCopyright">Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education</p>