{"id":210,"date":"2014-11-12T20:25:58","date_gmt":"2014-11-12T20:25:58","guid":{"rendered":"http:\/\/courses.candelalearning.com\/novabiology\/?post_type=chapter&#038;p=210"},"modified":"2019-05-13T18:13:01","modified_gmt":"2019-05-13T18:13:01","slug":"the-process-of-meiosis","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/nemcc-biology1v2\/chapter\/the-process-of-meiosis\/","title":{"raw":"The Process of Meiosis","rendered":"The Process of Meiosis"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Objectives<\/h3>\r\nBy the end of this section, you will be able to:\r\n<ul>\r\n \t<li>Describe the behavior of chromosomes during meiosis<\/li>\r\n \t<li>Describe cellular events during meiosis<\/li>\r\n \t<li>Compare the differences between meiosis and mitosis<\/li>\r\n \t<li>Distinguish between the two instances of genetic variation<\/li>\r\n<\/ul>\r\n<\/div>\r\n<div>\r\n<div class=\"media-body \">\r\n<div id=\"os-content\">\r\n<p id=\"fs-id1457129\" class=\"para\">Sexual reproduction requires <span class=\"term\">fertilization<\/span>, the union of two cells from two individual organisms.\u00a0 As mentioned earlier, haploid cells contain one set of chromosomes, while diploid cells contain two sets.. For reproduction to continue, the diploid cell must reduce its number of chromosome sets before fertilization can occur.\u00a0 In addition to fertilization, sexual reproduction includes a nuclear division that reduces the number of chromosome sets.<\/p>\r\n<p id=\"fs-id1433868\" class=\"para\">\u00a0In each <strong><span class=\"term\">somatic cell<\/span><\/strong> of the organism, the nucleus contains two copies of each chromosome, called homologous chromosomes. Somatic cells are sometimes referred to as \u201cbody\u201d cells. Homologous chromosomes are matched pairs containing the same genes in identical locations along their length. Diploid organisms inherit one copy of each homologous chromosome from each parent . Haploid cells, containing a single copy of each homologous chromosome, are found only within structures that give rise to either gametes or spores. Gametes, sperm and egg, are the sex cells of animals and some plants.\u00a0 <strong><span class=\"term\">Spores<\/span><\/strong> are haploid cells that can produce a haploid organism or\u00a0 fuse with another spore to form a diploid cell.\u00a0 Some plants and all fungi produce spores for reproduction.<\/p>\r\n<p id=\"fs-id1801644\" class=\"para\">Meiosis is the nuclear division forming haploid cells and is similar to mitosis. As mentioned earlier, mitosis is the part of a cell reproduction cycle that results in identical daughter nuclei that are genetically identical to the original parent nucleus. In mitosis, both the parent and the daughter nuclei are diploid for most plants and animals. Meiosis employs many of the same mechanisms as mitosis. The starting nucleus is always diploid with the resulting nuclei being haploid. To achieve this reduction in chromosome number, meiosis consists of one round of chromosome duplication and two rounds of nuclear division.\u00a0 Because the events of meiosis are analogous to those of mitosis, the same\u00a0 names are assigned. However, there are two rounds of division in meiosis.<\/p>\r\n\r\n<section id=\"fs-id903510\">\r\n<h1 class=\"title\">Meiosis I<\/h1>\r\n<p id=\"fs-id1419274\" class=\"para\">Meiosis I is preceded by an interphase consisting of the G<sub class=\"sub\">1<\/sub>, S, and G<sub class=\"sub\">2<\/sub> phases, which are very similar to the phases preceding mitosis. The G<sub class=\"sub\">1<\/sub> phase is focused on cell growth. The S phase is when the DNA of the chromosomes is replicated. Finally, the G<sub class=\"sub\">2<\/sub> phase is the third and final phase of interphase where the cell undergoes its final preparations for meiosis.<\/p>\r\n\r\n<section id=\"fs-id1755317\">\r\n<h2 class=\"title\">Prophase I<\/h2>\r\n<figure id=\"fig-ch11-01-01\" class=\"ui-has-child-figcaption\"><\/figure>\r\n[caption id=\"attachment_1386\" align=\"alignright\" width=\"450\"]<img class=\" wp-image-1386\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/198\/2016\/11\/28162224\/Figure_11_01.jpg\" alt=\"This illustration depicts two pairs of sister chromatids joined together to form homologous chromosomes. The chromatids are pinched together at the centromere and held together by the kinetochore. A protein lattice called a synaptonemal complex fuses the homologous chromosomes together along their entire length.\" width=\"450\" height=\"343\" \/> Figure 1. Early in prophase I, homologous chromosomes come together to form a synapse. The chromosomes are bound tightly together and in perfect alignment at the centromere.[\/caption]\r\n<p id=\"fs-id1340707\" class=\"para\">Early in prophase I, the homologous chromosomes are attached to the nuclear envelope.\u00a0 As the nuclear envelope breaks down, proteins, associated with homologous chromosomes, bring the pair closer together. This tight pairing of the homologous chromosomes is called <strong><span class=\"term\">synapsis<\/span><\/strong>. In synapsis, the genes on the chromatids of the homologous chromosomes are aligned precisely with each other(Figure 1). In humans, even though the X and Y sex chromosomes are not homologous, they have a small region that allows them to pair up during prophase I. \u00a0 An exchange of chromosomal segments between <span style=\"text-decoration: underline\">non-sister<\/span> homologous chromatids, a process called crossing over, may occur during synapsis(Figure 2).<\/p>\r\n\r\n<figure id=\"fig-ch11-01-02\" class=\"ui-has-child-figcaption\"><\/figure>\r\n<\/section><section id=\"fs-id1662346\">\r\n\r\n[caption id=\"attachment_1387\" align=\"alignright\" width=\"449\"]<img class=\" wp-image-1387\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/198\/2016\/11\/28162312\/Figure_11_02-731x1024.jpg\" alt=\"This illustration shows a pair of homologous chromosomes that are aligned. The ends of two non-sister chromatids of the homologous chromosomes cross over, and genetic material is exchanged. The non-sister chromatids between which genetic material was exchanged are called recombinant chromosomes. The other pair of non-sister chromatids that did not exchange genetic material are called non-recombinant chromosomes.\" width=\"449\" height=\"629\" \/> <br \/>Figure 2. Cross over occurs between non-sister chromatids of homologous chromosomes. The result is an exchange of genetic material between homologous chromosomes.[\/caption]\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n<p id=\"fs-id1220575\" class=\"para\" style=\"text-align: left\">As prophase I progresses, the chromosomes begin to condense. The homologous chromosomes remain attached to each other at the centromere. Following crossing over, the connection between homologous pairs is removed. At the end of prophase I, the pairs are called <strong><span class=\"term\">tetrads<\/span><\/strong> because the four sister chromatids of each pair of homologous chromosomes are now visible(Figure 2).\u00a0 Crossing over is the first source of genetic variation in the nuclei produced by meiosis.\u00a0 Now, when that sister chromatid is moved into a gamete, some DNA from each parent moves forward.\u00a0 Recombinant DNA is a molecule made from a combination of maternal and paternal genes that did not exist before the crossover.<\/p>\r\n\r\n<h2 class=\"title\">Prometaphase I<\/h2>\r\n<p id=\"fs-id1461481\" class=\"para\">In prometaphase I, the main event is the attachment of the spindle fiber microtubules to the centromere with the kinetochore proteins.\u00a0 Microtubules grow from centrosomes placed at opposite poles of the cell. The microtubules move toward the middle of the cell and attach to one of the two fused homologous chromosomes. The microtubules attach at each chromosomes' kinetochores.\u00a0 Now, the microtubules can pull the homologous pair apart.\u00a0 At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome facing each pole.<\/p>\r\n\r\n<\/section><section id=\"fs-id1195147\">\r\n<h2 class=\"title\">Metaphase I<\/h2>\r\n<p id=\"fs-id1417670\" class=\"para\">During metaphase I, the homologous chromosomes are arranged in the center of the cell facing opposite poles. Random orientation of the homologous pairs occurs at the equator.\u00a0 This is important in determining the genes carried by a gamete.\u00a0 Each gamete will only receive one of the two homologous chromosomes.\u00a0 Homologous chromosomes are not identical. They contain slight differences in their genetic information, allowing each gamete to have a unique genetic makeup.<\/p>\r\n<p class=\"para\">\u00a0Consider that the homologous chromosomes of a sexually reproducing organism are originally inherited as two separate sets, one from each parent.\u00a0 In humans, one set of 23 chromosomes is present in the egg from the mother. The father provides the other set of 23 chromosomes in the sperm that fertilizes the egg. Every cell of the multicellular offspring has copies of the original two sets of homologous chromosomes. In prophase I of meiosis I, the homologous chromosomes form the tetrads. In metaphase I, these pairs line up at the midway point between the two poles of the cell to form the metaphase plate. There is an equal chance the microtubule fiber encounters a chromosome from mom or dad.\u00a0 The arrangement of the tetrads at the metaphase plate is random. Any maternally inherited chromosome may face either pole. Any paternally inherited chromosome may also face either pole. The orientation of each tetrad is independent of the orientation of the other 22 tetrads.<\/p>\r\n<p id=\"fs-id1311003\" class=\"para\">This random event, or independent assortment of homologous chromosomes during metaphase I, is the second mechanism that introduces variation.\u00a0 In each cell that undergoes meiosis, the arrangement of the tetrads is different.\u00a0 There are two possibilities for orientation at the metaphase plate.\u00a0 The number of variations is dependent on the number of chromosomes making up a set.\u00a0 The possible number of alignments equals 2<em class=\"emphasis\">n<\/em>, where <em class=\"emphasis\">n<\/em> is the number of chromosomes per set. Humans have 23 chromosome pairs, resulting in over eight million (2<sup class=\"sup\"><em class=\"emphasis\">23<\/em><\/sup>) possible genetically distinct gametes. This number does not include the variations created during crossing over.\u00a0 Given these two mechanisms, it is highly unlikely that any two haploid cells in meiosis will have the same genetic composition (Figure 3).<\/p>\r\n<p id=\"fs-id1021412\" class=\"para\">To summarize the genetic consequences of meiosis I, the maternal and paternal genes are recombined by crossover events that occur between each homologous pair during prophase I. In addition, the random assortment of tetrads on the metaphase plate produces a unique combination of maternal and paternal chromosomes that will make their way into the gametes.<\/p>\r\n\r\n\r\n[caption id=\"attachment_1388\" align=\"aligncenter\" width=\"1024\"]<img class=\"size-large wp-image-1388\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/198\/2016\/11\/28162405\/Figure_11_03-1024x904.jpg\" alt=\"This illustration shows that, in a cell with a set of two chromosomes, four possible arrangements of chromosomes can give rise to eight different kinds of gamete. These are the eight possible arrangements of chromosomes that can occur during meiosis of two chromosomes.\" width=\"1024\" height=\"904\" \/> Figure 3. Random, independent assortment during metaphase I can be demonstrated by considering a cell with a set of two chromosomes (<em>n<\/em> = 2). In this case, there are two possible arrangements at the metaphase plate in metaphase I. The total possible number of different gametes is 2<em>n<\/em>, where n equals the number of chromosomes in a set. In this example, there are four possible genetic combinations for the gametes. With <em>n<\/em> = 23 in human cells, there are over 8 million possible combinations of paternal and maternal chromosomes.[\/caption]\r\n<figure id=\"fig-ch11-01-03\" class=\"ui-has-child-figcaption\"><\/figure>\r\n<\/section><section id=\"fs-id1463196\">\r\n<h2 class=\"title\">Anaphase I<\/h2>\r\n<p id=\"fs-id1969953\" class=\"para\">In anaphase I, the microtubules pull the linked chromosomes apart. The sister chromatids remain tightly bound together at the centromere(Figure 4).<\/p>\r\n\r\n<\/section><section id=\"fs-id1458977\">\r\n<h2 class=\"title\">Telophase I and Cytokinesis<\/h2>\r\n<p id=\"fs-id1664445\" class=\"para\">In telophase I, the separated chromosomes arrive at opposite poles.\u00a0 Depending on the species, the other typical telophase events may or may not occur.\u00a0 The chromosomes may decondense and nuclear envelopes may form around the chromatids, \u00a0 Cytokinesis, the separation of the cytoplasmic components, may occur without reformation of the nuclei. As mentioned previously, in nearly all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow. While in plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate. This cell plate leads to the formation of cell walls that separate the two daughter cells.<\/p>\r\n<p id=\"fs-id1321580\" class=\"para\">The end of Meiosis I results in two haploid cells\u00a0 There is only one full set of chromosomes present, because at each pole, there is just one of each pair of the homologous chromosomes.\u00a0 But, each homologous chromosomes consists of two sister chromatids. Except for changes during crossing over, sister chromatids are merely duplicates of one of the two homologous chromosomes. In meiosis II, these two sister chromatids will separate.<\/p>\r\n\r\n<div id=\"fs-id1340075\" class=\"note ui-has-child-title textbox shaded\"><header>\r\n<h3 class=\"title\">Link to Learning<\/h3>\r\n<p class=\"title\">Review the process of meiosis, observing how chromosomes align and migrate, at <a href=\"http:\/\/www.cellsalive.com\/meiosis.htm\" target=\"_window\" rel=\"nofollow\">Meiosis: An Interactive Animation<\/a>.<\/p>\r\n\r\n<\/header><\/div>\r\n<\/section><\/section><\/div>\r\n<section id=\"fs-id1459778\">\r\n<h1 class=\"title\">Meiosis II<\/h1>\r\n<p id=\"fs-id1447227\" class=\"para\">In some species, cells enter a brief interphase-like state before entering meiosis II. <u>Interkinesis<\/u> lacks an S phase, so chromosomes are not duplicated.\u00a0 The two cells produced in meiosis I go through the events of meiosis II together. During meiosis II, the sister chromatids of the two daughter cells separate.\u00a0 Four new haploid gametes are formed.\u00a0 The mechanics of meiosis II is very similar to mitosis, except that each dividing cell has only one set of homologous chromosomes.<\/p>\r\n\r\n<section id=\"fs-id1357838\">\r\n<h2 class=\"title\">Prophase II<\/h2>\r\n<p id=\"fs-id1371327\" class=\"para\">What occurs in prophase II is highly dependent on the events of telophase I. If the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment. The centrosomes move away from each other toward opposite poles. New spindles are begin formation.<\/p>\r\n\r\n<\/section><section id=\"fs-id1300636\">\r\n<h2 class=\"title\">Prometaphase II<\/h2>\r\n<p id=\"fs-id1461181\" class=\"para\">The nuclear envelopes are completely broken down.\u00a0 The spindle is fully formed. Each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles.<\/p>\r\n\r\n<\/section><section id=\"fs-id806351\">\r\n<h2 class=\"title\">Metaphase II<\/h2>\r\n<p id=\"fs-id1416911\" class=\"para\">The sister chromatids are maximally condensed and aligned at the equator of the cell.<\/p>\r\n\r\n<\/section><section id=\"fs-id1220173\">\r\n<h2 class=\"title\">Anaphase II<\/h2>\r\n<p id=\"fs-id1488854\" class=\"para\">The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles. There is a physical elongation of the cell.<\/p>\r\n\r\n\r\n[caption id=\"attachment_1389\" align=\"aligncenter\" width=\"1024\"]<img class=\"size-large wp-image-1389\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/198\/2016\/11\/28162456\/Figure_11_04-1024x822.jpg\" alt=\"This illustration compares chromosome alignment in meiosis I and meiosis II. In prometaphase I, homologous pairs of chromosomes are held together by chiasmata. In anaphase I, the homologous pair separates and the connections at the chiasmata are broken, but the sister chromatids remain attached at the centromere. In prometaphase II, the sister chromatids are held together at the centromere. In anaphase II, the centromere connections are broken and the sister chromatids separate.\" width=\"1024\" height=\"822\" \/> Figure 4. The process of chromosome alignment differs between meiosis I and meiosis II. In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes.\u00a0 The homologous chromosomes are arranged at the metaphase plate of the cell in metaphase I. In anaphase I, the homologous chromosomes are separated. In prometaphase II, microtubules attach to the kinetochores of sister chromatids, and the sister chromatids are arranged at the metaphase plate of the cells in metaphase II. In anaphase II, the sister chromatids are separated.[\/caption]\r\n<figure id=\"fig-ch11-01-04\" class=\"ui-has-child-figcaption\"><\/figure>\r\n<\/section><section id=\"fs-id1470624\">\r\n<h2 class=\"title\">Telophase II and Cytokinesis<\/h2>\r\n<p id=\"fs-id1282360\" class=\"para\">The chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form around the chromosomes. Cytokinesis separates the two cells into four unique haploid cells. \u00a0 The cells produced are genetically unique due to crossing over in prophase I and the random assortment of the tetrads during metaphase I.\u00a0 The entire process of meiosis is outlined in Figure 5.<\/p>\r\n\r\n\r\n[caption id=\"attachment_1390\" align=\"aligncenter\" width=\"730\"]<img class=\"size-large wp-image-1390\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/198\/2016\/11\/28162535\/Figure_11_05-730x1024.jpg\" alt=\"This illustration outlines the stages of meiosis. In interphase, before meiosis begins, the chromosomes are duplicated. Meiosis I then proceeds through several stages. In prophase I, the chromosomes begin to condense and the nuclear envelope fragments. Homologous pairs of chromosomes line up, and chiasmata form between them. Crossing over occurs at the chiasmata. Spindle fibers emerge from the centrosomes. In prometaphase I, homologous chromosomes attach to the spindle microtubules. In metaphase I, homologous chromosomes line up at the metaphase plate. In anaphase I, the spindle microtubules pull the homologous pairs of chromosomes apart. In telophase I and cytokinesis, the sister chromatids arrive at the poles of the cell and begin to decondense. The nuclear envelope begins to form again, and cell division occurs. Meiosis II then proceeds through several stages. In prophase II, the sister chromatids condense and the nuclear envelope fragments. A new spindle begins to form. In prometaphase II, the sister chromatids become attached to the kinetochore. In metaphase II, the sister chromatids line up at the metaphase plate. In anaphase II, the sister chromatids are pulled apart by the shortening spindles. In telophase II and cytokinesis, the nuclear envelope forms again and cell division occurs, resulting in four haploid daughter cells.\" width=\"730\" height=\"1024\" \/> Figure 5. An animal cell with a diploid number of four (2<em>n<\/em>\u00a0=\u00a04) proceeds through the stages of meiosis to form four haploid daughter cells.[\/caption]\r\n<figure id=\"fig-ch11-01-05\" class=\"ui-has-child-figcaption\"><\/figure>\r\n<\/section><\/section><section id=\"fs-id1267128\">\r\n<h1 class=\"title\">Comparing Meiosis and Mitosis<\/h1>\r\n<p id=\"fs-id850564\" class=\"para\">Mitosis and meiosis are both forms of division of the nucleus in eukaryotic cells. They share some similarities, but exhibit distinct differences that lead to very different outcomes (Figure 6). Mitosis is a single nuclear division that results in two nuclei that are divided into two new cells. The nuclei are genetically identical to the original nucleus.\u00a0 In most plants and all animal species, it is typically diploid cells that undergo mitosis to form new diploid cells. In contrast, meiosis consists of two nuclear divisions resulting in four nuclei that are divided into four new cells. The nuclei resulting from meiosis are not genetically identical and they contain one chromosome set only.<\/p>\r\n<p id=\"fs-id1322104\" class=\"para\">The main differences between mitosis and meiosis occur in meiosis I, which is a very different nuclear division than mitosis. In meiosis I, the homologous chromosome pairs associate with each other, are bound together, and undergo crossing over between nonsister chromatids.\u00a0 They line up along the metaphase plate as tetrads.\u00a0 With pulling apart of the tetrad during anaphase I, the number of chromosomal sets has been reduced.\u00a0 Mitosis has not chromosomal reduction.<\/p>\r\n<p id=\"fs-id1418144\" class=\"para\">Meiosis II is much more analogous to a mitotic division. In this case, the duplicated chromosomes line up on the metaphase plate. During anaphase II, as in mitotic anaphase, the centromeres divide and one sister chromatid is pulled to one pole while the other sister chromatid is pulled to the other pole. If not for crossing over, the two products of each individual meiosis II division would be identical (like in mitosis). But there will always be some crossing over.\u00a0 Meiosis II is not a reduction division because although there are fewer copies of the genome. There is still one set of chromosomes, as at the end of meiosis I.<\/p>\r\n\r\n\r\n[caption id=\"attachment_1391\" align=\"aligncenter\" width=\"1024\"]<img class=\"size-large wp-image-1391\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/198\/2016\/11\/28162632\/Figure_11_06-1024x945.jpg\" alt=\"This illustration compares meiosis and mitosis. In meiosis, there are two rounds of cell division, whereas there is only one round of cell division in mitosis. In both mitosis and meiosis, DNA synthesis occurs during S phase. Synapsis of homologous chromosomes occurs in prophase I of meiosis, but does not occur in mitosis. Crossover of chromosomes occurs in prophase I of meiosis, but does not occur in mitosis. Homologous pairs of chromosomes line up at the metaphase plate during metaphase I of meiosis, but not during mitosis. Sister chromatids line up at the metaphase plate during metaphase II of meiosis and metaphase of mitosis. The result of meiosis is four haploid daughter cells, and the result of mitosis is two diploid daughter cells.\" width=\"1024\" height=\"945\" \/> Figure 6. Meiosis and mitosis are both preceded by one round of DNA replication; however, meiosis includes two nuclear divisions. The four daughter cells resulting from meiosis are haploid and genetically distinct. The daughter cells resulting from mitosis are diploid and identical to the parent cell.[\/caption]\r\n<figure id=\"fig-ch11-01-06\" class=\"ui-has-child-figcaption\"><\/figure>\r\n<div class=\"textbox key-takeaways\"><section>\u00a0<\/section><\/div>\r\n<div id=\"fs-id1244433\" class=\"note ui-has-child-title textbox shaded\">\r\n<h3><span class=\"title tight\">Link to Learning<\/span><\/h3>\r\nClick through the steps of this interactive animation to compare the meiotic process of cell division to that of mitosis: <a href=\"https:\/\/www.pbs.org\/wgbh\/nova\/body\/how-cells-divide.html\" target=\"_window\" rel=\"nofollow\">How Cells Divide<\/a>.\r\n\r\n<\/div>\r\n<\/section><section id=\"fs-id1361505\">\r\n<h2 class=\"title\">Section Summary<\/h2>\r\n<p id=\"fs-id1135317\" class=\"para\">Sexual reproduction requires that diploid organisms produce haploid cells.\u00a0 These then fuse during fertilization to form diploid offspring.\u00a0 Meiosis is a series of events that arrange and separate chromosomes and chromatids into daughter cells. During the interphases of meiosis, each chromosome is duplicated. In meiosis, there are two rounds of nuclear division resulting in four nuclei and usually four daughter cells, each with half the number of chromosomes as the parent cell. The first separates homologous chromosomes, and the second separates chromatids into individual chromosomes. During meiosis, variation in the daughter nuclei can occur due to crossing over(prophase I) and random alignment of tetrads(metaphase I). The cells produced by meiosis are genetically unique.<\/p>\r\n<p id=\"fs-id1664568\" class=\"para\">Meiosis and mitosis share similarities, but have distinct outcomes. Mitotic divisions are single. nuclear divisions producing daughter nuclei that are genetically identical with the same number of chromosome sets as the original cell. Meiotic divisions include two nuclear divisions producing four daughter nuclei that are genetically different, having one chromosome set instead of the two sets like the parent cell. The main differences between the processes occur in the first division of meiosis.\u00a0 The second division of meiosis is more similar to a mitotic division.<\/p>\r\nhttps:\/\/www.openassessments.org\/assessments\/478\r\n\r\n<section id=\"fs-id1207123\">\r\n<div class=\"textbox exercises\">\r\n<h3>Additional Self Check Questions<\/h3>\r\n<div id=\"fs-id1181228\" class=\"exercise\"><section>\r\n<div id=\"fs-id872298\" class=\"problem\">\r\n<p id=\"fs-id1606170\" class=\"para\">1.\u00a0 Define tetrad.<\/p>\r\n\r\n<\/div>\r\n<\/section><\/div>\r\n<div id=\"fs-id1321696\" class=\"exercise\"><section>\r\n<div id=\"fs-id1892091\" class=\"problem\">\r\n\r\n2.\u00a0 Name two methods of variation in cell division.\r\n\r\n3. \u00a0 In a comparison of the stages of meiosis to the stages of mitosis, which stages are unique to meiosis and which stages have the same events in both meiosis and mitosisAnswers\r\n\r\n<\/div>\r\n<\/section><\/div>\r\n<\/div>\r\n<\/section>\r\n<div>\r\n<div class=\"textbox exercises\">\r\n<div>1. A tetrad forms when homologous chromosomes pair up during synapsis.<\/div>\r\n<div><\/div>\r\n<div>2.\u00a0 Crossing over and random alignment of tetrads provides variation during cell division.<\/div>\r\n<div><\/div>\r\n<div>\r\n\r\n3.\u00a0 All of the stages of meiosis I are unique because homologous chromosomes are separated, not sister chromatids.\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<\/section><\/div>\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Objectives<\/h3>\n<p>By the end of this section, you will be able to:<\/p>\n<ul>\n<li>Describe the behavior of chromosomes during meiosis<\/li>\n<li>Describe cellular events during meiosis<\/li>\n<li>Compare the differences between meiosis and mitosis<\/li>\n<li>Distinguish between the two instances of genetic variation<\/li>\n<\/ul>\n<\/div>\n<div>\n<div class=\"media-body\">\n<div id=\"os-content\">\n<p id=\"fs-id1457129\" class=\"para\">Sexual reproduction requires <span class=\"term\">fertilization<\/span>, the union of two cells from two individual organisms.\u00a0 As mentioned earlier, haploid cells contain one set of chromosomes, while diploid cells contain two sets.. For reproduction to continue, the diploid cell must reduce its number of chromosome sets before fertilization can occur.\u00a0 In addition to fertilization, sexual reproduction includes a nuclear division that reduces the number of chromosome sets.<\/p>\n<p id=\"fs-id1433868\" class=\"para\">\u00a0In each <strong><span class=\"term\">somatic cell<\/span><\/strong> of the organism, the nucleus contains two copies of each chromosome, called homologous chromosomes. Somatic cells are sometimes referred to as \u201cbody\u201d cells. Homologous chromosomes are matched pairs containing the same genes in identical locations along their length. Diploid organisms inherit one copy of each homologous chromosome from each parent . Haploid cells, containing a single copy of each homologous chromosome, are found only within structures that give rise to either gametes or spores. Gametes, sperm and egg, are the sex cells of animals and some plants.\u00a0 <strong><span class=\"term\">Spores<\/span><\/strong> are haploid cells that can produce a haploid organism or\u00a0 fuse with another spore to form a diploid cell.\u00a0 Some plants and all fungi produce spores for reproduction.<\/p>\n<p id=\"fs-id1801644\" class=\"para\">Meiosis is the nuclear division forming haploid cells and is similar to mitosis. As mentioned earlier, mitosis is the part of a cell reproduction cycle that results in identical daughter nuclei that are genetically identical to the original parent nucleus. In mitosis, both the parent and the daughter nuclei are diploid for most plants and animals. Meiosis employs many of the same mechanisms as mitosis. The starting nucleus is always diploid with the resulting nuclei being haploid. To achieve this reduction in chromosome number, meiosis consists of one round of chromosome duplication and two rounds of nuclear division.\u00a0 Because the events of meiosis are analogous to those of mitosis, the same\u00a0 names are assigned. However, there are two rounds of division in meiosis.<\/p>\n<section id=\"fs-id903510\">\n<h1 class=\"title\">Meiosis I<\/h1>\n<p id=\"fs-id1419274\" class=\"para\">Meiosis I is preceded by an interphase consisting of the G<sub class=\"sub\">1<\/sub>, S, and G<sub class=\"sub\">2<\/sub> phases, which are very similar to the phases preceding mitosis. The G<sub class=\"sub\">1<\/sub> phase is focused on cell growth. The S phase is when the DNA of the chromosomes is replicated. Finally, the G<sub class=\"sub\">2<\/sub> phase is the third and final phase of interphase where the cell undergoes its final preparations for meiosis.<\/p>\n<section id=\"fs-id1755317\">\n<h2 class=\"title\">Prophase I<\/h2>\n<figure id=\"fig-ch11-01-01\" class=\"ui-has-child-figcaption\"><\/figure>\n<div id=\"attachment_1386\" style=\"width: 460px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1386\" class=\"wp-image-1386\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/198\/2016\/11\/28162224\/Figure_11_01.jpg\" alt=\"This illustration depicts two pairs of sister chromatids joined together to form homologous chromosomes. The chromatids are pinched together at the centromere and held together by the kinetochore. A protein lattice called a synaptonemal complex fuses the homologous chromosomes together along their entire length.\" width=\"450\" height=\"343\" \/><\/p>\n<p id=\"caption-attachment-1386\" class=\"wp-caption-text\">Figure 1. Early in prophase I, homologous chromosomes come together to form a synapse. The chromosomes are bound tightly together and in perfect alignment at the centromere.<\/p>\n<\/div>\n<p id=\"fs-id1340707\" class=\"para\">Early in prophase I, the homologous chromosomes are attached to the nuclear envelope.\u00a0 As the nuclear envelope breaks down, proteins, associated with homologous chromosomes, bring the pair closer together. This tight pairing of the homologous chromosomes is called <strong><span class=\"term\">synapsis<\/span><\/strong>. In synapsis, the genes on the chromatids of the homologous chromosomes are aligned precisely with each other(Figure 1). In humans, even though the X and Y sex chromosomes are not homologous, they have a small region that allows them to pair up during prophase I. \u00a0 An exchange of chromosomal segments between <span style=\"text-decoration: underline\">non-sister<\/span> homologous chromatids, a process called crossing over, may occur during synapsis(Figure 2).<\/p>\n<figure id=\"fig-ch11-01-02\" class=\"ui-has-child-figcaption\"><\/figure>\n<\/section>\n<section id=\"fs-id1662346\">\n<div id=\"attachment_1387\" style=\"width: 459px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1387\" class=\"wp-image-1387\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/198\/2016\/11\/28162312\/Figure_11_02-731x1024.jpg\" alt=\"This illustration shows a pair of homologous chromosomes that are aligned. The ends of two non-sister chromatids of the homologous chromosomes cross over, and genetic material is exchanged. The non-sister chromatids between which genetic material was exchanged are called recombinant chromosomes. The other pair of non-sister chromatids that did not exchange genetic material are called non-recombinant chromosomes.\" width=\"449\" height=\"629\" \/><\/p>\n<p id=\"caption-attachment-1387\" class=\"wp-caption-text\">Figure 2. Cross over occurs between non-sister chromatids of homologous chromosomes. The result is an exchange of genetic material between homologous chromosomes.<\/p>\n<\/div>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p id=\"fs-id1220575\" class=\"para\" style=\"text-align: left\">As prophase I progresses, the chromosomes begin to condense. The homologous chromosomes remain attached to each other at the centromere. Following crossing over, the connection between homologous pairs is removed. At the end of prophase I, the pairs are called <strong><span class=\"term\">tetrads<\/span><\/strong> because the four sister chromatids of each pair of homologous chromosomes are now visible(Figure 2).\u00a0 Crossing over is the first source of genetic variation in the nuclei produced by meiosis.\u00a0 Now, when that sister chromatid is moved into a gamete, some DNA from each parent moves forward.\u00a0 Recombinant DNA is a molecule made from a combination of maternal and paternal genes that did not exist before the crossover.<\/p>\n<h2 class=\"title\">Prometaphase I<\/h2>\n<p id=\"fs-id1461481\" class=\"para\">In prometaphase I, the main event is the attachment of the spindle fiber microtubules to the centromere with the kinetochore proteins.\u00a0 Microtubules grow from centrosomes placed at opposite poles of the cell. The microtubules move toward the middle of the cell and attach to one of the two fused homologous chromosomes. The microtubules attach at each chromosomes&#8217; kinetochores.\u00a0 Now, the microtubules can pull the homologous pair apart.\u00a0 At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome facing each pole.<\/p>\n<\/section>\n<section id=\"fs-id1195147\">\n<h2 class=\"title\">Metaphase I<\/h2>\n<p id=\"fs-id1417670\" class=\"para\">During metaphase I, the homologous chromosomes are arranged in the center of the cell facing opposite poles. Random orientation of the homologous pairs occurs at the equator.\u00a0 This is important in determining the genes carried by a gamete.\u00a0 Each gamete will only receive one of the two homologous chromosomes.\u00a0 Homologous chromosomes are not identical. They contain slight differences in their genetic information, allowing each gamete to have a unique genetic makeup.<\/p>\n<p class=\"para\">\u00a0Consider that the homologous chromosomes of a sexually reproducing organism are originally inherited as two separate sets, one from each parent.\u00a0 In humans, one set of 23 chromosomes is present in the egg from the mother. The father provides the other set of 23 chromosomes in the sperm that fertilizes the egg. Every cell of the multicellular offspring has copies of the original two sets of homologous chromosomes. In prophase I of meiosis I, the homologous chromosomes form the tetrads. In metaphase I, these pairs line up at the midway point between the two poles of the cell to form the metaphase plate. There is an equal chance the microtubule fiber encounters a chromosome from mom or dad.\u00a0 The arrangement of the tetrads at the metaphase plate is random. Any maternally inherited chromosome may face either pole. Any paternally inherited chromosome may also face either pole. The orientation of each tetrad is independent of the orientation of the other 22 tetrads.<\/p>\n<p id=\"fs-id1311003\" class=\"para\">This random event, or independent assortment of homologous chromosomes during metaphase I, is the second mechanism that introduces variation.\u00a0 In each cell that undergoes meiosis, the arrangement of the tetrads is different.\u00a0 There are two possibilities for orientation at the metaphase plate.\u00a0 The number of variations is dependent on the number of chromosomes making up a set.\u00a0 The possible number of alignments equals 2<em class=\"emphasis\">n<\/em>, where <em class=\"emphasis\">n<\/em> is the number of chromosomes per set. Humans have 23 chromosome pairs, resulting in over eight million (2<sup class=\"sup\"><em class=\"emphasis\">23<\/em><\/sup>) possible genetically distinct gametes. This number does not include the variations created during crossing over.\u00a0 Given these two mechanisms, it is highly unlikely that any two haploid cells in meiosis will have the same genetic composition (Figure 3).<\/p>\n<p id=\"fs-id1021412\" class=\"para\">To summarize the genetic consequences of meiosis I, the maternal and paternal genes are recombined by crossover events that occur between each homologous pair during prophase I. In addition, the random assortment of tetrads on the metaphase plate produces a unique combination of maternal and paternal chromosomes that will make their way into the gametes.<\/p>\n<div id=\"attachment_1388\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1388\" class=\"size-large wp-image-1388\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/198\/2016\/11\/28162405\/Figure_11_03-1024x904.jpg\" alt=\"This illustration shows that, in a cell with a set of two chromosomes, four possible arrangements of chromosomes can give rise to eight different kinds of gamete. These are the eight possible arrangements of chromosomes that can occur during meiosis of two chromosomes.\" width=\"1024\" height=\"904\" \/><\/p>\n<p id=\"caption-attachment-1388\" class=\"wp-caption-text\">Figure 3. Random, independent assortment during metaphase I can be demonstrated by considering a cell with a set of two chromosomes (<em>n<\/em> = 2). In this case, there are two possible arrangements at the metaphase plate in metaphase I. The total possible number of different gametes is 2<em>n<\/em>, where n equals the number of chromosomes in a set. In this example, there are four possible genetic combinations for the gametes. With <em>n<\/em> = 23 in human cells, there are over 8 million possible combinations of paternal and maternal chromosomes.<\/p>\n<\/div>\n<figure id=\"fig-ch11-01-03\" class=\"ui-has-child-figcaption\"><\/figure>\n<\/section>\n<section id=\"fs-id1463196\">\n<h2 class=\"title\">Anaphase I<\/h2>\n<p id=\"fs-id1969953\" class=\"para\">In anaphase I, the microtubules pull the linked chromosomes apart. The sister chromatids remain tightly bound together at the centromere(Figure 4).<\/p>\n<\/section>\n<section id=\"fs-id1458977\">\n<h2 class=\"title\">Telophase I and Cytokinesis<\/h2>\n<p id=\"fs-id1664445\" class=\"para\">In telophase I, the separated chromosomes arrive at opposite poles.\u00a0 Depending on the species, the other typical telophase events may or may not occur.\u00a0 The chromosomes may decondense and nuclear envelopes may form around the chromatids, \u00a0 Cytokinesis, the separation of the cytoplasmic components, may occur without reformation of the nuclei. As mentioned previously, in nearly all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow. While in plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate. This cell plate leads to the formation of cell walls that separate the two daughter cells.<\/p>\n<p id=\"fs-id1321580\" class=\"para\">The end of Meiosis I results in two haploid cells\u00a0 There is only one full set of chromosomes present, because at each pole, there is just one of each pair of the homologous chromosomes.\u00a0 But, each homologous chromosomes consists of two sister chromatids. Except for changes during crossing over, sister chromatids are merely duplicates of one of the two homologous chromosomes. In meiosis II, these two sister chromatids will separate.<\/p>\n<div id=\"fs-id1340075\" class=\"note ui-has-child-title textbox shaded\">\n<header>\n<h3 class=\"title\">Link to Learning<\/h3>\n<p class=\"title\">Review the process of meiosis, observing how chromosomes align and migrate, at <a href=\"http:\/\/www.cellsalive.com\/meiosis.htm\" target=\"_window\" rel=\"nofollow\">Meiosis: An Interactive Animation<\/a>.<\/p>\n<\/header>\n<\/div>\n<\/section>\n<\/section>\n<\/div>\n<section id=\"fs-id1459778\">\n<h1 class=\"title\">Meiosis II<\/h1>\n<p id=\"fs-id1447227\" class=\"para\">In some species, cells enter a brief interphase-like state before entering meiosis II. <u>Interkinesis<\/u> lacks an S phase, so chromosomes are not duplicated.\u00a0 The two cells produced in meiosis I go through the events of meiosis II together. During meiosis II, the sister chromatids of the two daughter cells separate.\u00a0 Four new haploid gametes are formed.\u00a0 The mechanics of meiosis II is very similar to mitosis, except that each dividing cell has only one set of homologous chromosomes.<\/p>\n<section id=\"fs-id1357838\">\n<h2 class=\"title\">Prophase II<\/h2>\n<p id=\"fs-id1371327\" class=\"para\">What occurs in prophase II is highly dependent on the events of telophase I. If the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment. The centrosomes move away from each other toward opposite poles. New spindles are begin formation.<\/p>\n<\/section>\n<section id=\"fs-id1300636\">\n<h2 class=\"title\">Prometaphase II<\/h2>\n<p id=\"fs-id1461181\" class=\"para\">The nuclear envelopes are completely broken down.\u00a0 The spindle is fully formed. Each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles.<\/p>\n<\/section>\n<section id=\"fs-id806351\">\n<h2 class=\"title\">Metaphase II<\/h2>\n<p id=\"fs-id1416911\" class=\"para\">The sister chromatids are maximally condensed and aligned at the equator of the cell.<\/p>\n<\/section>\n<section id=\"fs-id1220173\">\n<h2 class=\"title\">Anaphase II<\/h2>\n<p id=\"fs-id1488854\" class=\"para\">The sister chromatids are pulled apart by the kinetochore microtubules and move toward opposite poles. There is a physical elongation of the cell.<\/p>\n<div id=\"attachment_1389\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1389\" class=\"size-large wp-image-1389\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/198\/2016\/11\/28162456\/Figure_11_04-1024x822.jpg\" alt=\"This illustration compares chromosome alignment in meiosis I and meiosis II. In prometaphase I, homologous pairs of chromosomes are held together by chiasmata. In anaphase I, the homologous pair separates and the connections at the chiasmata are broken, but the sister chromatids remain attached at the centromere. In prometaphase II, the sister chromatids are held together at the centromere. In anaphase II, the centromere connections are broken and the sister chromatids separate.\" width=\"1024\" height=\"822\" \/><\/p>\n<p id=\"caption-attachment-1389\" class=\"wp-caption-text\">Figure 4. The process of chromosome alignment differs between meiosis I and meiosis II. In prometaphase I, microtubules attach to the fused kinetochores of homologous chromosomes.\u00a0 The homologous chromosomes are arranged at the metaphase plate of the cell in metaphase I. In anaphase I, the homologous chromosomes are separated. In prometaphase II, microtubules attach to the kinetochores of sister chromatids, and the sister chromatids are arranged at the metaphase plate of the cells in metaphase II. In anaphase II, the sister chromatids are separated.<\/p>\n<\/div>\n<figure id=\"fig-ch11-01-04\" class=\"ui-has-child-figcaption\"><\/figure>\n<\/section>\n<section id=\"fs-id1470624\">\n<h2 class=\"title\">Telophase II and Cytokinesis<\/h2>\n<p id=\"fs-id1282360\" class=\"para\">The chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form around the chromosomes. Cytokinesis separates the two cells into four unique haploid cells. \u00a0 The cells produced are genetically unique due to crossing over in prophase I and the random assortment of the tetrads during metaphase I.\u00a0 The entire process of meiosis is outlined in Figure 5.<\/p>\n<div id=\"attachment_1390\" style=\"width: 740px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1390\" class=\"size-large wp-image-1390\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/198\/2016\/11\/28162535\/Figure_11_05-730x1024.jpg\" alt=\"This illustration outlines the stages of meiosis. In interphase, before meiosis begins, the chromosomes are duplicated. Meiosis I then proceeds through several stages. In prophase I, the chromosomes begin to condense and the nuclear envelope fragments. Homologous pairs of chromosomes line up, and chiasmata form between them. Crossing over occurs at the chiasmata. Spindle fibers emerge from the centrosomes. In prometaphase I, homologous chromosomes attach to the spindle microtubules. In metaphase I, homologous chromosomes line up at the metaphase plate. In anaphase I, the spindle microtubules pull the homologous pairs of chromosomes apart. In telophase I and cytokinesis, the sister chromatids arrive at the poles of the cell and begin to decondense. The nuclear envelope begins to form again, and cell division occurs. Meiosis II then proceeds through several stages. In prophase II, the sister chromatids condense and the nuclear envelope fragments. A new spindle begins to form. In prometaphase II, the sister chromatids become attached to the kinetochore. In metaphase II, the sister chromatids line up at the metaphase plate. In anaphase II, the sister chromatids are pulled apart by the shortening spindles. In telophase II and cytokinesis, the nuclear envelope forms again and cell division occurs, resulting in four haploid daughter cells.\" width=\"730\" height=\"1024\" \/><\/p>\n<p id=\"caption-attachment-1390\" class=\"wp-caption-text\">Figure 5. An animal cell with a diploid number of four (2<em>n<\/em>\u00a0=\u00a04) proceeds through the stages of meiosis to form four haploid daughter cells.<\/p>\n<\/div>\n<figure id=\"fig-ch11-01-05\" class=\"ui-has-child-figcaption\"><\/figure>\n<\/section>\n<\/section>\n<section id=\"fs-id1267128\">\n<h1 class=\"title\">Comparing Meiosis and Mitosis<\/h1>\n<p id=\"fs-id850564\" class=\"para\">Mitosis and meiosis are both forms of division of the nucleus in eukaryotic cells. They share some similarities, but exhibit distinct differences that lead to very different outcomes (Figure 6). Mitosis is a single nuclear division that results in two nuclei that are divided into two new cells. The nuclei are genetically identical to the original nucleus.\u00a0 In most plants and all animal species, it is typically diploid cells that undergo mitosis to form new diploid cells. In contrast, meiosis consists of two nuclear divisions resulting in four nuclei that are divided into four new cells. The nuclei resulting from meiosis are not genetically identical and they contain one chromosome set only.<\/p>\n<p id=\"fs-id1322104\" class=\"para\">The main differences between mitosis and meiosis occur in meiosis I, which is a very different nuclear division than mitosis. In meiosis I, the homologous chromosome pairs associate with each other, are bound together, and undergo crossing over between nonsister chromatids.\u00a0 They line up along the metaphase plate as tetrads.\u00a0 With pulling apart of the tetrad during anaphase I, the number of chromosomal sets has been reduced.\u00a0 Mitosis has not chromosomal reduction.<\/p>\n<p id=\"fs-id1418144\" class=\"para\">Meiosis II is much more analogous to a mitotic division. In this case, the duplicated chromosomes line up on the metaphase plate. During anaphase II, as in mitotic anaphase, the centromeres divide and one sister chromatid is pulled to one pole while the other sister chromatid is pulled to the other pole. If not for crossing over, the two products of each individual meiosis II division would be identical (like in mitosis). But there will always be some crossing over.\u00a0 Meiosis II is not a reduction division because although there are fewer copies of the genome. There is still one set of chromosomes, as at the end of meiosis I.<\/p>\n<div id=\"attachment_1391\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1391\" class=\"size-large wp-image-1391\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/198\/2016\/11\/28162632\/Figure_11_06-1024x945.jpg\" alt=\"This illustration compares meiosis and mitosis. In meiosis, there are two rounds of cell division, whereas there is only one round of cell division in mitosis. In both mitosis and meiosis, DNA synthesis occurs during S phase. Synapsis of homologous chromosomes occurs in prophase I of meiosis, but does not occur in mitosis. Crossover of chromosomes occurs in prophase I of meiosis, but does not occur in mitosis. Homologous pairs of chromosomes line up at the metaphase plate during metaphase I of meiosis, but not during mitosis. Sister chromatids line up at the metaphase plate during metaphase II of meiosis and metaphase of mitosis. The result of meiosis is four haploid daughter cells, and the result of mitosis is two diploid daughter cells.\" width=\"1024\" height=\"945\" \/><\/p>\n<p id=\"caption-attachment-1391\" class=\"wp-caption-text\">Figure 6. Meiosis and mitosis are both preceded by one round of DNA replication; however, meiosis includes two nuclear divisions. The four daughter cells resulting from meiosis are haploid and genetically distinct. The daughter cells resulting from mitosis are diploid and identical to the parent cell.<\/p>\n<\/div>\n<figure id=\"fig-ch11-01-06\" class=\"ui-has-child-figcaption\"><\/figure>\n<div class=\"textbox key-takeaways\">\n<section>\u00a0<\/section>\n<\/div>\n<div id=\"fs-id1244433\" class=\"note ui-has-child-title textbox shaded\">\n<h3><span class=\"title tight\">Link to Learning<\/span><\/h3>\n<p>Click through the steps of this interactive animation to compare the meiotic process of cell division to that of mitosis: <a href=\"https:\/\/www.pbs.org\/wgbh\/nova\/body\/how-cells-divide.html\" target=\"_window\" rel=\"nofollow\">How Cells Divide<\/a>.<\/p>\n<\/div>\n<\/section>\n<section id=\"fs-id1361505\">\n<h2 class=\"title\">Section Summary<\/h2>\n<p id=\"fs-id1135317\" class=\"para\">Sexual reproduction requires that diploid organisms produce haploid cells.\u00a0 These then fuse during fertilization to form diploid offspring.\u00a0 Meiosis is a series of events that arrange and separate chromosomes and chromatids into daughter cells. During the interphases of meiosis, each chromosome is duplicated. In meiosis, there are two rounds of nuclear division resulting in four nuclei and usually four daughter cells, each with half the number of chromosomes as the parent cell. The first separates homologous chromosomes, and the second separates chromatids into individual chromosomes. During meiosis, variation in the daughter nuclei can occur due to crossing over(prophase I) and random alignment of tetrads(metaphase I). The cells produced by meiosis are genetically unique.<\/p>\n<p id=\"fs-id1664568\" class=\"para\">Meiosis and mitosis share similarities, but have distinct outcomes. Mitotic divisions are single. nuclear divisions producing daughter nuclei that are genetically identical with the same number of chromosome sets as the original cell. Meiotic divisions include two nuclear divisions producing four daughter nuclei that are genetically different, having one chromosome set instead of the two sets like the parent cell. The main differences between the processes occur in the first division of meiosis.\u00a0 The second division of meiosis is more similar to a mitotic division.<\/p>\n<p><iframe src=\"https:\/\/lumenoea.herokuapp.com\/assessments\/load?src_url=https:\/\/lumenoea.herokuapp.com\/api\/assessments\/478.xml&#38;results_end_point=https:\/\/lumenoea.herokuapp.com\/api&#38;assessment_id=478&#38;confidence_levels=true&#38;enable_start=true&#38;eid=https:\/\/courses.lumenlearning.com\/nemcc-biology1v2\/chapter\/the-process-of-meiosis\/\" frameborder=\"0\" style=\"border:none;width:100%;height:100%;min-height:400px;\"><\/iframe><\/p>\n<section id=\"fs-id1207123\">\n<div class=\"textbox exercises\">\n<h3>Additional Self Check Questions<\/h3>\n<div id=\"fs-id1181228\" class=\"exercise\">\n<section>\n<div id=\"fs-id872298\" class=\"problem\">\n<p id=\"fs-id1606170\" class=\"para\">1.\u00a0 Define tetrad.<\/p>\n<\/div>\n<\/section>\n<\/div>\n<div id=\"fs-id1321696\" class=\"exercise\">\n<section>\n<div id=\"fs-id1892091\" class=\"problem\">\n<p>2.\u00a0 Name two methods of variation in cell division.<\/p>\n<p>3. \u00a0 In a comparison of the stages of meiosis to the stages of mitosis, which stages are unique to meiosis and which stages have the same events in both meiosis and mitosisAnswers<\/p>\n<\/div>\n<\/section>\n<\/div>\n<\/div>\n<\/section>\n<div>\n<div class=\"textbox exercises\">\n<div>1. A tetrad forms when homologous chromosomes pair up during synapsis.<\/div>\n<div><\/div>\n<div>2.\u00a0 Crossing over and random alignment of tetrads provides variation during cell division.<\/div>\n<div><\/div>\n<div>\n<p>3.\u00a0 All of the stages of meiosis I are unique because homologous chromosomes are separated, not sister chromatids.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<\/section>\n<\/div>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-210\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Biology. <strong>Authored by<\/strong>: Open Stax. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@9.17:1\/Biology\">http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@9.17:1\/Biology<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em><\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":18,"menu_order":19,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Biology\",\"author\":\"Open Stax\",\"organization\":\"\",\"url\":\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@9.17:1\/Biology\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"\"}]","CANDELA_OUTCOMES_GUID":"","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-210","chapter","type-chapter","status-publish","hentry"],"part":179,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/nemcc-biology1v2\/wp-json\/pressbooks\/v2\/chapters\/210","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/nemcc-biology1v2\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/nemcc-biology1v2\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/nemcc-biology1v2\/wp-json\/wp\/v2\/users\/18"}],"version-history":[{"count":24,"href":"https:\/\/courses.lumenlearning.com\/nemcc-biology1v2\/wp-json\/pressbooks\/v2\/chapters\/210\/revisions"}],"predecessor-version":[{"id":1652,"href":"https:\/\/courses.lumenlearning.com\/nemcc-biology1v2\/wp-json\/pressbooks\/v2\/chapters\/210\/revisions\/1652"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/nemcc-biology1v2\/wp-json\/pressbooks\/v2\/parts\/179"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/nemcc-biology1v2\/wp-json\/pressbooks\/v2\/chapters\/210\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/nemcc-biology1v2\/wp-json\/wp\/v2\/media?parent=210"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/nemcc-biology1v2\/wp-json\/pressbooks\/v2\/chapter-type?post=210"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/nemcc-biology1v2\/wp-json\/wp\/v2\/contributor?post=210"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/nemcc-biology1v2\/wp-json\/wp\/v2\/license?post=210"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}