Meiosis

1st post by nathan.olson I found a really good website with good descriptions, pictures, and some really helpful animations of the whole process. http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/main.html

2nd post by nathan.olson Another good animation that illustrates non disjunction in cells is found here. Meiosis I: http://www.biostudio.com/d_%20Meiotic%20Nondisjunction%20Meiosis%20I.htm Meiosis II: http://www.biostudio.com/d_%20Meiotic%20Nondisjunction%20Meiosis%20II.htm

2nd post by ZanaDi Meiosis: Genetic variation "reshuffling the genes" comes from several sources: The reason you look somewhat similar but yet distinctly different from your siblings and from your parents is because of genetic variation due to meiosis. Each meiosis and fertilization are like snowflakes, no two individuals are alike! 1. **Crossing Over:** The chromosomes you receive from the egg and the sperm are not the same chromosomes that your mom and dad have. They are new, 'shuffled' versions of their chromosomes and you only get ONE of them from each mom and dad. 2. **Independent assortment:** Homologues line up or "shuffle" randomly on the metaphase plate in Meiosis I. With 23 chromosomes assorting independently, there are 2^23, or about 8 million possible assortments of chromosomes inherited for every cell. 3. **Random fertilization:** The ovum has 8 million possible chromosome combinations, so does the sperm cell. 8 million x 8 million = 64 trillion possible diploid combinations in each and every zygote!

1st post by ashtonleigh Meiosis vs. Mitosis Meiosis and Mitosis often get confused with one another. Here are a few good links that explain the differences between the two; they are videos and are therefore, more easily understood since we can visualize the pictures. The second link uses everyday items and there is music with the video so it makes it easier to watch and learn more about the differences between meiosis and mitosis. http://www.youtube.com/watch?v=agIL-mBa7Q0 http://www.youtube.com/watch?v=Qy-GY-YCg3Y More info on meiosis: Because meiosis is a "one-way" process, it cannot be said to engage in a cell cycle as mitosis does. However, the preparatory steps that lead up to meiosis are identical in pattern and name to the interphase of the mitotic cell cycle. Interphase is divided into three phases: • Growth 1 (G1) phase: This is a very active period, where the cell synthesizes its vast array of proteins, including the enzymes and structural proteins it will need for growth. In G1 stage each of the chromosomes consists of a single (very long) molecule of DNA. In humans, at this point cells are 46 chromosomes, 2N, identical to somatic cells. • Synthesis (S) phase: The genetic material is replicated: each of its chromosomes duplicates, producing 46 chromosomes each made up of two sister chromatids. The cell is still considered diploid because it still contains the same number of centromeres. The identical sister chromatids have not yet condensed into the densely packaged chromosomes visible with the light microscope. This will take place during prophase I in meiosis. • Growth 2 (G2) phase: G2 phase is absent in Meiosis Interphase is followed by meiosis I and then meiosis II.

2nd post by ashtonleigh In **females**, meiosis occurs in cells known as oogonia. Each oogonium that triggers meiosis will divide two times to form a single oocyte and two polar bodies. However, before these divisions occur, these cells stop at the diplotene stage of meiosis I and are inactive within a protective shell of somatic cells called the follicle. Follicles begin growth at a steady pace in a process known as folliculogenesis and a small number enter the menstrual cycle. Menstruated oocytes continue meiosis I and delay at meiosis II until fertilization. The process of meiosis in females occurs during oogenesis and differs from the typical meiosis in that it features a long period of meiotic delay known as the Dictyate stage and lacks the assistance of centrosomes. In **males**, meiosis occurs in messenger cells known as spermatogonia that divide twice to become sperm. These cells continuously divide without delay in the seminiferous tubules of the testicles. Sperm is produced at a constant pace. The process of meiosis in males occurs during spermatogenesis.

1st post by ZanaDi In meiosis there are two nuclear divisions that occur, Meiosis I (which is considered Reduction) and Meiosis II ( which is considered Division). Meiosis produces 4 haploid cells, while mitosis produces 2 diploid cells. Meiosis I reduces the ploidy level from 2n to n (reduction) while Meiosis II divides the remaining set of chromosomes (division). Most of the differences between the process occur in Meiosis I. Here are pictures showing the differences. Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com).

​Here is another interesting link. It is from the University of Utah and it gives you different scenarios using karyotypes to predict genetic disorders and it goes into details such as what happens during normal Meiosis, and then it goes into how people can sometimes get too many or too few chromosomes, and etc. It's a nice site because it gives you some illustration videos that I thought were a good visual to actually understand the topic. I hope this is helpful for some because I know I found it quite interesting. [|Meiosis]

Here is a link to a pretty good page where it gives you a diagram and also a good description of what happens during Meiosis. It goes into some discussion about how mutations come about in Meiosis. When you go to this link click on Meiosis and it will take you right to the information! http://www.messybeast.com/genetics-basics.htm]]

2nd Post by Shannonk08 There are many genetic disorders that are caused by nondisjunction, especially nondisjunction in meiosis. Down's Syndrome is a major genetic disease caused by nondisjunction in meiosis I. Here is a link to an article about several types of genetic disorders and Down's Syndrome: []

2nd Post by mollyann12 Nondisjunction is something that occurs in both Meiosis I and Meiosis II. The following link shows nondisjunction. [] Nondisjunction can lead to trisomic and monosomic cells. In turn, this can affect individuals and often cause difficulties. 2nd post by Meredit Irsfeld (mer123) I know lots of people posted videos of meiosis I thought this one was really nice because it explained each of the steps nicely and it showed that the centromere of the chromosomes can be located in different spots not exactly in the center. Here's the link: [] The other thing I thought I'd add was abou DNA replication. DNA replication occurs during interphase, the beginning of meiosis. In meiosis each of the four gametes ends up with half of the genetic material, with different genetic codes between each of the gametes and the parent cells. This allows for unique offspring and variation. Reference: []

1st post by Meredith Irsfeld (mer123) This article I found was really interesting it talked about the maternal age affect, but also how meiosis can have problems on the paternal side which could contribute to other diseases. This one focuses on down syndrome and explaining meiosis. [] I found one more really important article talking about a study that could be linked to the cause of nondisjunction here is an excerpt from it: The cause(s) of these nondisjunctions remain(s) unknown. One of the more interesting speculations involves the correlation of successful development to the oxygenation of the egg while it is in the ovary. Oocytes of equivalent size and equivalent capacity to be fertilized differ in their ability to develop. This is apparent in clinical //in vitro// fertilization, where a certain percentage of eggs that are successfully fertilized fail to develop. Gaulden (1992) and Van Blerkom and colleagues (1997) showed that follicular fluid surrounding the mature oocytes of a single ovary differ greatly in their content of dissolved oxygen. Most had a range 3.0-5.5% oxygen, while the rest was equally divided between those follicular fluids having less than 1.5% and those in between. Oxygen content had no correlation with oocyte size, maturation, or ability to be fertilized. However, there was a very significant correlation with the ability to reach the 6-8 cell stage by 60h of incubation. Of those with reduced oxygen, only 42% reached the 6-8 cell stage; the moderately oxygenated eggs had a 66% success rate, and the well oxygenated eggs developed to this stage 79% of the time. Moreover, the eggs that reached this stage from the poorly oxygenated follicles were found to have significant defects in chromosome number, spindle arrangement, and cytoplasmic structure. Of all the eggs, 41% had chromosomal anomalies, and 92% of these embryos with chromosome anomalies came from eggs having less than 3% oxygen. For the rest of the article go to this website: []

2nd post by Ashley Zondervan i found a good video on youtube explaining meiosis, very easy to understand and nice pictures that illustrate things well: []

Posted by Ashley Zondervan Meiosis is needed for sexual reproduction and therefore occurs in all eukaryotes (including single-celled organisms) that reproduce sexually. Meiosis does not occur in archaea or bacteria, which reproduce via asexual processes.
 * meiosis** is a process of reductional division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes. Before meiosis begins, the DNA in the original cell is replicated during S-phase of the cell cycle. Two cell divisions )meiosis I and II) separate the replicated chromosomes into four haploid gametes.

2nd Post by Katie Fearing Here are some tidbits about meiosis, but first of all I want to start with a link to a website that has a good animation of meiosis: http://www.cellsalive.com/meiosis.htm __Some facts about meiosis__ - Meiosis only occurs in the sexual organs to produce gametes or sex/reproductive cells - 4 total daughter cells are produced after Meiosis II is done and they are not identical. - Human females produce 1 egg and 3 polar bodies when they undergo meiosis. Meiosis II does not complete in females until fertilization. - Each daughter cell produced has its own unique genetic make-up. This increases genetic variation and aids in the survival of every specie. - Oogenesis is meiosis in females. It is the creation of an ovum. - Spermatogenesis is meiosis in males. It is the creation of sperm. - Meiosis does not occur in single-celled organisms like bacteria.

Posted by HayLee Wax I know this is really simple, but I found it informational. There are so many different directions you can go with this topic, and you can go into great depth. To me the most simple way to understand something is if is simple and easy to understand. Hope this helps! []

Posted by Mike Hurkes One can go into great detail explaining Meiosis, although this is necessary when studying meiosis in depth, I'd like to provide a very brief, but accurate description of Meiosis I to be used as a quick reference.

__Meiosis I Prophase I:__ Chromosomes condense and organize and matched or homologous charomosomes are physically paired; segments of chromatids can cross over within each chromosome pair. __Metaphase I:__ Homologues line up at equator. __Anaphase I:__ Homologues seperateinto two groups, with each group having a mixture of maternal and parental chromosomes. __Telophase I:__ New haploid nuclei form two new daughter cells

Referenced by: Brooks, Randy. Quick Study Academic "Biology: The Basic Principles of Biology." BarCharts, Inc. 2002

Posted by Mike Hurkes I'd like to now give a brief description of Meiosis II. Again, this is an excellent reference for getting the basic concepts of what is going on during meiosis. Enjoy!

__Meiosis II Prophase II:__ Chromosomes condense. __Metaphase II:__ Chromosomes line up at equator. __Anaphase II:__ Chromatids of each chromosome are separated. __Telophase II:__ Each daughter cell from Meiosis I will form two more cells for a total of four cells

Referenced by: Brooks, Randy. Quick Study Academic "Biology: The Basic Principles of Biology." BarCharts, Inc. 2002

Posted by Nikki Fideldy I found a great video about the whole meiosis process! really easy to understand. Follow the link []

Anaphase 1 posted by Teah Fry in Anaphase 1 the chromosomes holding the chromosomes together loosen.The two homologous chromosomes are separated into separate poles. The chromosomes for either parent can go into either pole, thus creating more genetic diversity. http://www.youtube.com/watch?v=R_LUJSqeSrI this link is a demonstration of the whole process of meiosis posted by Amanda Bennett

posted by Nikki Fideldy The topic of this Wiki is Meiosis Here is a test. Just joking. Meiosis is the process that produces gametes, which are haploid non-identical cells. They are produced in the gonad organs, which result in having half the number of chromosomes as the original diploid cells.

The process of meiosis is very important in the sexual reproduction of all diploid organisms, and is the mechanism for which the diploid amount of genetic info is reduced to the haploid amount. This then leads to the formation of gametes in mammals. The process of meiosis and its stages in a simple fashion are as follows. First is the Meiotic prophase 1 the start of meiosis then is the Leptonema stage, Zygonema, Pachynema, Diplonema, Diakinesis, after these stages meiosis goes into Metaphase, Anaphase and Telaphase 1. It would be good if the next persons in this Wiki would start at the beginning of the steps of meiosis and explain them in order, as then we will all have a study guide on the steps of meiosis. You can find this information starting on page 27 0f the text book in the very bottom right paragraph.

Three events characterize the initial stage of PROPHASE I. DNA condensing, SYNAPSIS, and CROSSING OVER. Because of complexity PROPHASE I is further divided into 5 sub-stages:LEPTONEMA, ZYGONEMA, PACHYNEMA, DIPLONEMA and DIAKINESIS. Even though it is not immediately apparent, DNA of chromosomes has been replicated during the prior interphase. Druing leptonema, the interphase chromatin material begins to condense and the chromosomes become visible. During the zygonema stage the chromosomes continue to become thicker and shortened. When this stage is completed the paired homologs are now referred to as bivalents (equal to the haploid number). In the pachytene stage chromosomes co​ntinue to coil and shorten leading to synapsis (pairing of homologous chromosomes). Each bivalent contains 4 chromatids. In the diplotene stage each pair of sister chromatids begin to separate, but one or more areas remain in contact where chromatids are intertwined, called chiasma. During diakinese chromosomes pull further apart. Also the necleolus and nuclear envelope disappear and the 2 centromeres of each tetrad attach to spindle fibers. Each tetrad structure is present on the metaphase plate.

After meiotic prophase I, steps similar to those of mitosis occur. First, in **metaphase I**, the chromosomes have maximally shortened and thickened. The terminal chiasmata of each tetrad are visible and appear to be the only factor holding the nonsister chromatids together. The tetrads then interact with spindle fibers, facilitating movement to the metaphase plate. The alignment of each tetrad prior to the first anaphase is random. Next, half of each tetrad is pulled randomly to one or the other pole, and the other half moves to the opposite. During the stages of meiosis I, a single centromere does not divide. In **anaphase I**, one-half of each tetrad (dyad) is pulled toward each pole of the dividing cell. This separation process is called disjunction, which is the separation of chromosomes from one another. Nondisjunction is when errors occur and separation is not achieved. In the completion of anaphase I, a series of dyads equal to the haploid number is at each pole. Next **telephase I** reveals a nuclear membrane forming around the dyads. Then the nucleus enters into a short interphase period. If interphase occurs, the chromosomes do not replicate since they would already consist of two chromatids. Meiotic telephase is much shorter than the corresponding stage in mitosis.

After metaphase, Anaphase and Telophase I, the **second meiotic division** occurs. It is basically the same steps but it is the forming of new gamete again which is the difference from mitosis, The second meiotic division starts with Prophase II, then proceeds to metaphase, anaphase and telophase. The out come of meiotic division is 4 haploid gametes.

1st Post by Shannonk08 Some people will confuse Mitosis and Meiosis, so here is a list to help clear up the confusion: Mitosis:** - in somatic cells - one single division of the mother cell, resulting in two identical daughter cells - mother cell may be either haploid or diploid - the number of chromosomes remains the same after the division - in reproductive cells - two divisions of the mother cell, resulting in four daughter cells - mother cell must be diploid - the number of chromosomes after the divisions is half that of the mother cell
 * __Differences between Mitosis and Meiosis:__
 * Meiosis:**

Below is a diagram further comparing Mitosis and Meiosis

One problem that can result from meiosis is nondisjunction. Nondisjunction is when two sister chromatids fail to separate during anaphase. This failed separation can occur in either Meiosis I or II (or both, but such an event is very rare). As a result, some of the eggs or sperm produced have too many chromosomes. Once the affected eggs are fertilized or the affected sperm fertilize an egg, there are different medical conditions which result. One condition of this is Down Syndrome. Others include: Klinefelter's Syndrome (XXY), Turner's Syndrome (X_), Triple X Syndrome (XXX) and Edward Syndrome (trisomy 18). Nondisjunction is more likely to happen in older mothers, because their eggs have been in their body for a longer period of time and are more likely to have problems dividing. Nondisjunction causes aneuploidy, which is one or a few extra or missing chromosomes. Metaphse 1: Posted by Molly Slaby Metaphae 1 follows Prosphase 1 in Meiosis 1. During this phase, the centrioles are at opposite poles of the cell. The homologous chromosomes, or bivalents, present are tightly coiled and condensed. These chromosomes are of equal distance from the poles, or metaphase plate. Spindle fibers from one pole of the cell attach to one chromosome of each pair, or sister chromatid. Spindle fibers from the opposite pole attach to the other sister chromatid.

Anaphase: Posted by Katie Fearing Anaphase 1 is responsible for independent assortment of traits in the chromosomes because paired homologous chromosomes are randomly aligned on the metaphase plate. The chromosomes are then pulled to one pole or the other after the spindle fibers completely attach and the microtubules contract. Since the chromosomes were lined up randomly, each pole received random chromosomes. At this phase sister chromatids remain attached to each other since the entire centromere moves to one pole. This results in each pole having a complete set of chromosomes after the spindle fibers finish contracting.

Errors in Meiosis
It is estimated that from 10–20% of all human fertilized eggs contain chromosome abnormalities, and these are the most common cause of pregnancy failure (35% of the cases). These chromosome abnormalities posted by Amanda Bennett
 * arise from errors in meiosis, usually meiosis I;
 * occur more often (90%) during egg formation than during sperm formation;
 * become more frequent as a woman ages.
 * **Aneuploidy** — the gain or loss of whole chromosomes — is the most common chromosome abnormality. It is caused by **nondisjunction**, the failure of chromosomes to correctly separate:
 * homologues during meiosis I or
 * sister chromatids during meiosis II
 * Zygotes missing one chromosome ("monosomy") cannot develop to birth (except for females with a single X chromosome).
 * Three of the same chromosome ("trisomy") is also lethal except for chromosomes 13, 18, and 21 (trisomy 21 is the cause of Down syndrome).
 * Three or more X chromosomes are viable because all but one of them are inactivated

Posted by Laura Nessa: This is more indepth on the errors that are caused during meiosis.

Meiosis is process where the number of chromosomes is cut into half. Meiosis results in gametes in eukaryotic organisms and spores in prokaryotes.

Meiosis is an amazing occurrence when it happens correctly, but when things go wrong, there can be terrible consequences. The consequences can be severe or mild mental retardation, spontaneous abortion, or sexual abnormalities.

Down syndrome is one such problem. Down syndrome that occurs during meiosis occurs when there is nondisjunction during meiosis I. Nondisjunction means that during meiosis I or II, the sister chromatids don’t separate normally causing the offspring to either have an extra chromosome or a missing chromosome. In Down syndrome, homologues don’t properly separate during anaphase I. This causes two chromosomes to be released that are then joined by the mates chromosome thereby causing the condition called Trisomy 21or Down syndrome. This leads to 47 chromosomes instead of 46.

There can also be nondisjunction involving sex chromosomes. An example of this is monosomy or Turner syndrome. Turner syndrome can be caused by nondisjunction in meiosis I or II. Like Down syndrome, this is also caused by the sister chromosomes not separating properly. The mate’s chromosome is the only one that is released. Leading to only 45 chromosomes instead of 46.

Another example of nondisjunction involving sex chromosomes is an extra Y chromosome being released. This happens during meiosis II. The sister chromatids don’t separate normally leading to an extra Y chromosome being released. This is labeled 47, XYY.

Kleinfelter syndrome (47, XXY), Patau Syndrom (Trisomy 13), Edward Syndrome (Trisomy 18), and Trisomy X (47, XXX) are other examples of disorders caused by nondisjunction during meiosis I or II.

2nd post by Laura Nessa: Here is a really good link that shows what the errant chromosomes look like and what causes these errors. This site discusses Down syndrome, Sex chromosome errors, and other disorders that occur during meiosis I and II.

http://www.biology.iupui.edu/biocourses/n100/2k2humancsomaldisorders.html

Here is another site that discusses the problems caused by errors that occur during meiosis.

http://projects.cbe.ab.ca/Diefenbaker/Biology/Bio%20Website%20Final/notes/molecular_genetics/4_abnormal_meiosis.htm

1st post by Alex Yellick Mode of action. You often often hear or read this phrase in a agronomy, microbiology, pharmaceutical science, or even on the white slips of paper that come with your bottle of penicillin or zythromycin. Simply, it describes the method by which a drug reacts or inhibits cellular machinery or chemical gradients. Mode of action is being studied on meiosis in yeast and has researchpers published on the "Inhibition of Meiosis in Saccharomyces cerevisiae by Ammonium Ions: Interference of Ammonia with Protein Metabolism" by scientists at the University of Nijmeagan, Netherlands. Imagine what could be done by inhibiting meiosis. Maybe a new birth control or even a better way to control certain animal populations? A link to the paper: [].

2nd post by Alex Yellick I found a really good video depicting meiosis. [] The video I found is much better than any video on this wiki. Therefore this post is more meaningful than any other video contribution. Another video I found raps the stages of meiosis...I learn best when scientific concepts are rapped to me. Here it is: []

1st post by CKnutson55: In this post I will be listing more information about the interphase stage of meiosis. Interphase- the period of growth before a cell undergoes cell division through meiosis. 3 main phases: G1 phase: period prior to synthesis of DNA. Cell increases in mass in preparation for cell division. G1 phase is the first gap phase. S phase: period during which DNA is synthesized in most cells, there is a narrow window of time during which DNA is synthesizes. The S represents synthesis. G2 Phase: Period after DNA synthesis has occurred but prior to the start of prophase. Cell synthesizes proteins and continues to increase in size. The G2 phase is the second gap phase. In the latter part of interphase, the nucleus is bounded by a nuclear envelope and the cell's chromosomes have duplicated but are in the form of chromatin.

Link to paper: http://biology.about.com/od/meiosis/ss/meiosisstep.htm

1st post by BrianMikk: This table from the National Institutes of Heath website shows clear distinctions between Meiosis and Mitosis. Table 2.2 .
 * Mitosis and meiosis compared**

Copyright © 1999 Garland Science []
 * ~  ||~ Mitosis ||~ Meiosis ||
 * Location || All tissues || Only in testis and ovary ||
 * Products || [|Diploid] somatic cells || [|Haploid] sperm and egg cells ||
 * DNA replication and cell division || Normally one round of replication per cell division || Only one round of replication but two cell divisions ||
 * Extent of prophase || Short (~30 min in human cells) || Meiosis I is long and complex; can take years to complete ||
 * Pairing of homologs || None || Yes (in meiosis I) ||
 * Recombination || Rare and abnormal || Normally at least once in each chromosome arm ||
 * Relationship between daughter cells || Genetically identical || Different (recombination and independent assortment of homologs) ||

2nd post by BrianMikk: 

Figure 2.13 . "**Meiosis: independent assortment of maternal and paternal homologs at meiosis I produces the first level of genetic diversity** There are 223 or 8.4 million different ways of picking one chromosome from each of the 23 pairs in a [|diploid] cell. Gametes A-E show just five of the possible combinations of maternal and paternal chromosomes. This diagram ignores recombination, which introduces a second level of genetic diversity by ensuring that each individual chromosome passed on is a mixture of maternal and paternal sequences." []

First post lexinew Really interesting video that explains meiosis well. []

Second post lexinew [] This comes from a biology book and is very informative of the differences between mitosis and meiosis. Facts about meiosis: --occures in reproductive cells, whereas mitosis occurs in somatic cells --results in formation of gametes --occurs in all eukaryotes --because of meiosis, we have genetic diversity!