Posted by There are two different ways that cells divide, mitosis and meiosis. Mitosis is the fundamental process of life while Meiosis is the process that essential for sexual reproduction in that it creates egg and sperm cells. Meiosis is a very interesting process that involves not only one but two rounds! Meiosis I starts with one diploid parent cell and ends with two haploid daughter cells which halves the number of chromosomes, while meiosis II starts with two haploid parent cells and ends with four haploid daughter cells which maintain the number of chromosomes within the cell.
In meiosis I, we start with prophase I and one diploid parent cell. The homologous chromosomes attach themselves at their tips to the nuclear envelope with proteins. Eventually, the nuclear envelope begins to break down and these proteins bring the pairs of chromosomes together. The homologous chromosomes line up next to each other in order to divide, this is so that each sister chromatid can be paired with another daughter cell. After this, the process of crossing-over begins. Crossing-over gets assistance from the synaptonemal complex, which is what actually holds the homologous chromosomes together. If you look under a microscope, you can actually see crossing-over occur in the form of chiasmata! This is just another thing that holds the chromosomes together after the synaptonemal complex breaks all the way down. The actual placement of crossing-over is random, you never really know where it’s going to happen.
Now we get into metaphase I which is actually not as jam packed with events like prophase I is. This is just where the chromosomes randomly line up in the middle of the cell in order to be grabbed by the newly developed spindles. In anaphase I, the spindles (microtubles) pull these chromosomes apart. While this is all happening, the chiasmata are broken. The chromosomes get pulled apart to the opposite sides of the cell where they can then begin telophase I and cytokinesis. This is basically just where the chromosomes meet at the opposite sides and the cytoplasm splits because of cytokinesis. We then get our two haploid daughter cells.
Now that we have these two cells, we can begin meiosis II! Everything that happens here is relatively the same as what happens in meiosis I, so that makes it a short and sweet process to follow. In prophase II, the exact same thing happens! Only, you start with two haploid daughter cells instead of one diploid parent cell. New spindles are also formed here as well. In metaphase II, the chromosomes meet in the middle. In anaphase II, they are pulled away from each other, yet again, but just like in meiosis I the sister chromatids will stay attached at their centromere. And finally in telophase I and cytokinesis, the chromosomes are pulled to the opposite sides of the cell and the cytoplasm is split in order to create our four haploid cells, thus ending the process of this meiosis. Now we have either sperm or eggs cells and the creation of life.
References
Lenormand, T., Engelstädter, J., Johnston, S. E., Wijnker, E., & Haag, C. R. (2016). Evolutionary mysteries in meiosis. Philosophical transactions of the Royal Society of London. Series B, Biological sciences, 371(1706), 20160001. https://doi.org/10.1098/rstb.2016.0001
“Meiosis | Cell Division | Biology (Article).” Khan Academy, 2017, www.khanacademy.org/science/ap-biology/heredity/meiosis-and-genetic-diversity/a/phases-of-meiosis#:%7E:text=In%20each%20round%20of%20division,metaphase%2C%20anaphase%2C%20and%20telophase
“11.1 The Process of Meiosis – Biology | OpenStax.” The Process of Meiosis, 29 Oct. 2015,
https://cnx.org/contents/GFy_h8cu@9.87:GYZS3DDP@8/The-Process-of-Meiosis
Ellie digitally drew the steps of meiosis. Meiosis is the process where sperm and egg cells are created; sperm and egg are haploid, meaning that they only have one set of DNA (either from the father or the mother). Meiosis has two parts, meiosis 1 and meiosis 2. The entire process begins with 1 diploid cell, where the nuclear envelope breaks in prophase 1. then comes Then the chromosomes line in the middle of the cell, crossing over occurs (where the homologous chromosomes exchange DNA at random so that each offspring will have a mixture of father and mother genes). In anaphase, the chromosomes are pulled to opposite ends of the cell, and the two new cells split off, and we have two haploid cells. these two cells then go through meiosis 2, where the process is relatively the same as meiosis 1, but we end up with four haploid daughters.
Ellie created slides of drawing that she created depicting with descriptions of each phase of Meiosis. Beginning with one diploid cell starting in prophase I where the chromosomes start to condense, causing the nuclear envelope to break. This is also where crossing over occurs at random between the chromosomes. Moving onto Metaphase I the chromosomes align at the meiotic spindle where in Anaphase I the pairs are pulled to opposite polls of the cell. where telophase and cytokinesis occur when the cells divide and become two haploid cells. Those two haploid cells then continue on to phases II of Meiosis which is a repeat of Meiosis I, however ultimately at the end of Cytokinesis II the cells have been divided and give us four haploid daughter cells.