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meiosis

Page history last edited by Charles Forstbauer 13 years ago

Closed & totaled. 1/26/10  Mr F

Totaled 1/19/10 Mr F. 

 

In biology, 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, while in other organisms it can give rise to spores. As with mitosis, before meiosis begins, the DNA in the original cell is replicated during s-phase of the cell cycle. Two cell divisions separate the replicated chromosomes into four haploid gametes or spores.  Meiosis is essential for sexual reproduction and therefore occurs in all eukaryotes (including single-celled organisms) that reproduce sexually.

 

Meiosis Process-

 

1. Meiosis takes a cell with 2 copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid.)

     -One diploid gives you 4 haploid cells

     -Good for combining to haploid cells to create a new organism.

 

2. Meiosis scrambles the specific forms of each gene that each cell (egg or sperm) recieves.

     -This makes for alot of genetic diversity. This is accomplished through independent assortment and crossing over.

     -Genetic diversity is important for the evolution of populations and species.

     -This is why that even though a child may look like there parents, it its highly unlikely that the will look exactly the same.  

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This is a really cute video on meiosis. It acutally explains how meiosis works with a pretty catchy song and a cute video from the discovery education channel. Even though it's random hopefully the lyrics help you remember what meiosis is about.

 

Difference between Mitosis and Meiosis-

 

 This picture more clearly shows how the four gamete cells are produced in meiosis. Also it show the crossover between two chormosomes and how that ends up in the last four cells in the process.

 

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Here is a video of a professor explaining the difference between mitosis and meiosis.  

 

~The process of meiosis makes for a lot of genetic diversity through the independant assortment and crossing-over. 

~Genetic diversity is important for the evolution of populations and species. 

~Chromosomes are paired.

~Chromosomes carry genes that may be different or the same.

 

~Meiosis makes many different sex cells through independent assortment, which produces 2^n distinct gametes, where n is the number of unique assortments.

For example, in humans, n=23. 2^23= 8,000,000, meaning that there are 8 million different possibilities.

 

~Non-sister chromatids can get tangled and exchange parts, so no two outcomes are ever identical.

 

~In meiosis, the chromosomes split up, which is different from mitosis, where the chromosomes line up in the middle. 

 

Here is an animation of the meiosis process:

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Here is a quick animation of Meiosis.

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Gametogenesis is the procces of gamete formation in humans. In this process, a diploid cell of an organism will undergo a process of meitotic differentiation, a split will occur. The haploid will thus be formed and when this haploid matures enough, it will replicate its genetic information to become a diploid an undergo the same process.

Oogenesis is the path of meiosis in females. On average, it happens successfully a few hundred times in a lifetime. In humans, oogenesis begins with, and is sustained by the menstrual cycle. Approximately once every 28 days a new egg will be created and there is a peak fertilization period around the 14th day. Primary follicles are involved in the production of cells and conception. AN egg is a haploid

Spermatogenesis is the path of meiosis in males. On average, a male will produce 250,000,000 sperm in a day. Spermatogensis involves the maturation of spermatogonia into mature spermatozoa capable of reproduciton within an ovum. To preserve the identical number of chromosomes in the offspring of a given organism the sperm provides half of the genetic material needed for the child, while the ovum provieds the other half. This means, that at conception, a spermatozoa is a haploid

 

here is a link to a website that explains oogenesis and spermatogenesis:

http://www.brown.edu/Courses/BI0032/gentherp/oogIC.html

 

Haploid egg + Haploid sperm = Diploid zygote

 

XX= female/XY= male

Females have two X chromosomes, so they always contribute an X to a child. Males have both an X and a Y chromosome. If they contribute an X, the child will be a female (with two XX chromosomes. If they contribute a Y chromosome, the child will be a male (with one X and one Y chromosome).

 

This video shows the difference between meiosis and mitosis, and it offers a clear and understandable animation of the haploid nuclei.  By breaking down each part of mitosis(IPMAT) and showing how it differs from meiosis, it is easy to tell the two apart.  It also explains the crossing-over of chromosomes.

 

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This image above goes into a bit more detail about each step and shows it more clearly than the other

pictures. 

 

~This video explains meiosis in poem form.  It is similar to "twas the night before Christmas" and it has visuals to make the poem more clear. It goes into depth about the IPMAT process, and shows how the daughter nuclei divide into 4 gametes.  

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~This video goes into depth about the crossing over in meiosis.  This explains how important it is that the corresponding links of the chromosomes are linked, because this is what makes up our unique characteristics. 

 

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During meiosis, the genome of a diploid germ cell, which is composed of long segments of DNA packaged into chromosome, undergoes DNA replication followed by two rounds of division, resulting in four haploid cells. Each of these cells contains one complete set of chromosomes, or half of the genetic content of the original cell. If meiosis produces gametes, these cells must fuse during fertilization to create a new diploid cell, or zygote before any new growth can occur. Thus, the division mechanism of meiosis is a reciprocal process to the joining of two genomes that occurs at fertilization. Because the chromosomes of each parent undergo homologous recombination during meiosis, each gamete, and thus each zygote, will have a unique genetic blueprint encoded in its DNA. Together, meiosis and fertilization constitute sexuality in the eukaryotes, and generate genetically distinct individuals in populations.

 

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. Meiosis I consists of separating the pairs of homologous chromosome, each made up of two sister chromatids, into two cells. One entire haploid content of chromosomes is contained in each of the resulting daughter cells; the first meiotic division therefore reduces the ploidy of the original cell by a factor of 2.

 

Meiosis is extremely importantbecause without it, sexual reproduction would not exist. It is a process of gamete formation in which diploid germ-line cells, that is, the cells that are set aside early in animal development for sexual reproduction, yield four genetically different haploid cells. It occurs only in sex cells, which are eggs and sperms. Meiosis takes place in two stages, Meiosis I where DNA replication takes place and crossing-over occurs; and Meiosis II, which lacks DNA replication but is similar to Mitotic cell division.

 

Meiosis I and Meiosis II have four phases each. They are:

Meiosis I: prophase 1, metaphase 1, anaphase 1, and telophase 1

Meiosis II: prophase 2, metaphase 2, anaphase 2, and telophase 2

 

Meiosis involves two successive nuclear divisions, which produce four haploid cells. The meiosis I is the reduction division, meiosis II separates the chromatids, which are the daughter strands of a duplicated chromosome joined together by a centromere. In mitotic cell division, new cells genetically identical to the parent cell are produced. Meiosis is responsible for increasing genetic variation in the population.

 

 

Meiosis and Mitosis Comparison Table

 

Meiosis

Mitosis

 

Definition:

A type of cellular reproduction in which the number of chromosomes are reduced by half through the separation of homologous chromosomes in a diploid cell.

A process of asexual reproduction in which the cell divides in two producing a replica (exact copy) with an equal number of chromosomes (One cell becomes two)


 

Genetically:

different

identica


 

Produces:

four haploid daughter cells

two diploid daughter cells


 

Occurs in:

Humans, animals, plants

all organisms


 

Steps:

The steps of meiosis are Interphase, Prophase I, Metaphase I, Anaphase I, Telophase I, Prophase II, Metaphase II, Anaphase II and Telophase II.

The steps of mitosis are Interphase, Preprophase, Prophase, Prometaphase, Metaphase, Anaphase, Telophase and Cytokinesis


 

Occurrence of Crossing Over:

Yes

No


 

Creates:

Sex cells only: Female egg cells or Male sperm cells

Makes everything other than sex cells (Somatic Cells)


 

Discovered by:

Oscar Hertwig

Walther Flemming


 

Type of Reproduction:

Sexual

Asexual


 

Number of Cells produced:

4

2


 

Number of Divisions:

2 nuclear and 2 cytoplasmic divisions

1 division of the nucleus and 1 Cytokinesis


 

Chromosome Number:

Reduced by half (homologous pairs separate)

Remains the same


 

Pairing of Homologues:

Yes

No


 

Function:

sexual reproduction

Cellular (asexual) Reproduction (cell division during which the cell nucleus divides); general growth and repair of the body

 

 

 

Meiosis I separates homologous chromosomes, producing two haploid cells (23 chromosomes, N in humans), so meiosis I is referred to as a reductional division. A regular diploid human cell contains 46 chromosomes and is considered 2N because it contains 23 pairs of homologous chromosomes. However, after meiosis I, although the cell contains 46 chromatids it is only considered as being N, with 23 chromosomes, because later in anaphase I the sister chromatids will remain together as the spindle pulls the pair toward the pole of the new cell. In meiosis II, an equational division similar to mitosis will occur whereby the sister chromatids are finally split, creating a total of 4 haploid cells (23 chromosomes, N) per daughter cell from the first division. 

 

 

This table helped me a little bit more when it comes to know the differences between mitosis and meiosis:

MITOSIS MEIOSIS
In somatic cells In reproductive cells
One single division (here below A-E) of the mother cell (m) results in two daughter cells (d) mitosis mother- and daughter cell Two divisions (here below: meiosis I in A-E and meiosis II in F-H) of the mother cell (meiocyte; m) result in four meiotic products (p) Meiocyte and meiotic products
mitosis diagram meiosis diagram
A mitotic mother cell can be either haploid or diploid mitosis mother and daughter cell A meiotic mother cell (meiocyte) is always diploid Meiocyte en meiotic products
The number of chromosomes per nucleus remains the same after division Diploid (= 2n) remains 2n
Haploid (= 1n) remains 1 n
The meiotic products contain a haploid (n) number of chromosomes, in contrast to the 2 n mother cell From 2n to n
Mitosis is preceeded by a S-phase in which the amount of DNA is duplicated Example for a diploid mother cell:
DNA amount in mitosis
Only meiosis I is preceeded by a S-phase DNA amount in meiosis
Normally no pairing of homolog chromosomes x Complete pairing of all homolog chromosomes during prophase I Pairing of homolog chromosomes during prophase I
Normally no exchange of DNA (crossing-over) between chromosomes x At least one crossing-over per homolog pair of chromosomes meiosis: crossing-over
The centromeres are split during anaphase Mitose: anafase splitsing centroneren The centromeres do not separate during anaphase I, but during anaphase II Meiose: anafase II alleen splitsing centroneren
Conservative process: the genotype of the daughter cells is identical to that of the mother cells mitosis: identical genotype mother and daughter cells Meiotic products differ in their genotype from the mother cell (increase in genetic variation in the offspring) meiosis: non-identical genotype mother and daughter cells


 

The mitotic metaphase can be distinguished from the metaphase I of meiosis as no pairing of homolog chromosomes occurs in mitosis; this is not necessary, even not suitable for the mitotic process (why?). Further, homolog chromosomes separate during metaphase I of meiosis, whereas during mitosis and metaphase II the separating structures are the chromatides. The difference between the separation in mitosis and metaphase II (meiosis) is that halfing of the number of chromosomes has preceeded meiosis II, in contrast to mitosis. Also the meiotic products are genetically diverse with respect to their parent and among each other. This is not the case in mitosis

 

Gametes: A gamete is a cell that fuses with another gamete during sexual reproduction in organisms. A female is identified by havingthe larger gamete, the egg or the ovum, and the male is identified by having the smaller cell, the sperm. The egg or 'ovum' carries 2 X chromosomes while the Sperm carries both an X and Y chromosome.

 

Homologous and Non-Homologous Chromosomes:

 

Homologous chromosomes are chromosomes in a biological cell that pair (synapse) during cell division during the creation of gametes (meiosis).

Each chromosome pair contains genes for the same biological features, such as eye color, at the same locations (loci) on the chromosome.

Each chromosome pair can contain the same gene (both genes for blue eyes) or different genes (one gene for blue eyes and one gene for brown eyes) for each feature.

Non-homologous chromosomes representing all the biological features of an organism form a set, and the number of sets in a cell is called ploidy. In diploid organisms (most plants and animals), each member of a pair of homologous chromosomes is inherited from a different parent. But polyploid organisms have more than two homologous chromosomes.

Homologous chromosomes are similar in length, except for sex chromosomes in several taxa, where the X chromosome is considerably larger than the Y chromosome. These chromosomes share only small regions of homology.

 

This is the website cells alive that we used a little while ago for a web quest. It also has an interactive animation on meiosis.

http://www.cellsalive.com/meiosis.htm

 

Here is a really good website that describes the process of meiosis step by step. So just click next and it goes through pages of notes and illustrations on meiosis, so if you are not getting it completely it really helps. 

http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/main.html

 

Meiosis is a more complicated process than Mitosis is. Mitosis results in a parent cell dividing into two daughter cells, which would be exact replacates of the parent cell. Meiosis is a type of replication similar to Mitosis, but it is specific to reproduction. In Meiosis, the cell will divide twice to result in four potentially different cells, where Mitosis results in two cells.

 

 

Interphase:  Both Meiosis and Mitosis have the same process here, where it is the phase inbetween cell division with the G1, S and G2 cycles.

 

Prophase: You should know the happenings of Mitosis in prophase, but in Meiosis, it is done a little differently. This stage would actually be called prophase I. The chromosomes condense, but they also attach themselves to the membrane of the nucleus. The chromosomes are paired up and in Meiosis, sequenses of DNA are cut from the Chomosomes and are exchanged between them, which allows for an exchange of genes.

 

Metaphase: in Meiosis this is called Metaphase I, because it is the first time that this happens (out of two). in Meiosis, the chromosomes pairs line up on opposing sides of the metaphase plate (which is the middle of the cell). this is different from mitosis because all the chromosomes don't line up together.

 

Anaphase (or anaphase I): Here in Meiosis, half of the chromatid pairs more to either end of the cell. In Meiosis, the sister chromatids don't seperate here like they do in Mitosis

 

Telophase (telophase I): Chromatids arrive at different ends of the cell and new nuclear membranes form.

 

Cytokinesis: The rest of the cell divides. This process is the same for both Mitosis and Meiosis.

 

Interphase II: Only Meiosis continues from here. Mitosis is finished and the cell returns to interphase. In Meiosis for interphase II, the cell does not replicate it's chromatids.

 

Prophase II: chromosomes condense, spindle fibers form, just like in Prophase I. But they do not attach to the nuclear membrane.

 

Metaphase II: Just like in mitosis metaphase, the fibers begin to pull each of the chromosomes in different directions.

 

Anaphase II: Similar to Mitosis Anaphase, the spindle fibers pull the chromatids apart towards opposite ends of the cell

 

Telophase II: Chromatids arrive at the end of the cell and nuclear membrane starts to form.

 

Cytokinesis: The rest of the cell starts to divide into two distinct cells. at the end of this process, there are four daughter cells.

 

Meiosis is essential for sexual reproduction and therefore occurs in all eukaryotes (including single-celled organisms) that reproduce sexually. A few eukaryotes have lost the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis. Meiosis does not occur in archaea or bacteria, which reproduce via asexual processes such as binary fission.

 

Meiosis uses many of the same biochemical mechanisms employed during mitosis to accomplish the redistribution of chromosomes.There are several features unique to meiosis, most importantly the pairing and recombination between homologous chromosomes. Homologous chromosomes are chromosomes in a biological cell that pair (synapse) during cell division during the creation of gametes (meiosis). Each chromosome pair contains genes for the same biological features, such as eye color, at the same locations (loci) on the chromosome. Each chromosome pair can contain the same gene (both genes for blue eyes) or different genes (one gene for blue eyes and one gene for brown eyes) for each feature.

 

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This video does a good job of explaining what diploids are.  The visuals are good and the narrator speaks slowly.

 

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This video is really helpful in understanding how the chromosomes split.  The video is in stop motion technology, which makes it easy to understand.

 

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This video has a great visual of the process of "crossing over"

 

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This video is good because it moves slowly and shows pictures made with clay.

 

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This video helped me because it discusses diploids and haploids.  The visual is also good because it moves slowly and is slowly explained.

 

This link goes through meiosis and it's step and how it is similar and differs from mitosis:  http://www.cellsalive.com/meiosis.htm

 --heres an even better link that shows the above^ animation but slower and in more detail: http://www.johnkyrk.com/meiosis.html 

 

 

Here's a quiz that tests your knowledge on the differences between meiosis and mitosis.  It offers the correct answers so you know what you need to work on: http://www.cst.cmich.edu/USERS/BENJA1DW/bio101/tools/quiz/mitosis.htm 

 

http://www.allthingsscience.com/video/490/The-Stages-of-Mitosis << this is a really cool link to a video thats so realistic on meiosis. It shows not only meiosis but the stages it goes through and how the rest of the cell reacts.

Mitosis is a method of reproduction for single celled organisms that reproduce asexually. An identical version of the organism is created through splitting of the cell in two. Meiosis may result in millions of spermazoa and egg cells with unique genetic patterns. The mating of the two cells formed by meiosis results in a unique genetic offspring of the same species. Meiosis is a major factor in evolution, natural selection, and biodiversity. The processes of cellular division shown in mitosis and meiosis are present in all manner of life forms including humans, animals, plants, fungi, and single celled organisms and species. Essentially any cell based organism of which all organic life is based will exhibit some form of mitosis and meiosis for growth and reproduction of the individual and species.

 

This is an animation of the process of meiosis. It helped me because it broke down the system into 9 easy to remember steps and it shows clear diagrams:   http://www.biostudio.com/d_%20Meiosis.htm  

 

Here's a link that explains meiosis easily, and provides a helpful video that includes even more helpful facts about meiosis and its products: http://www.historyforkids.org/scienceforkids/biology/cells/meiosis.htm

 

To add to all the quizes that have been put up, here is one that is in Biology Corner. It is short and gives you a score and the correct answers to the ones you got wrong: http://www.biologycorner.com/bio1/qz_meiosis.html 

 

Here's a link to a game you can play to increase your knowledge of all of the activities that go on during Mitosis and Meiosism, it's really quite helpful: http://www.quia.com/hm/35138.html

 

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter3/animation__how_meiosis_works.html

^  This website is really good at explaining the whole idea of Meiosis in an easy to learn way.. 

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