Meiosis Telophase 1

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Nuclear Membrane

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Telophase

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Meiosis

Introduction

Mitosis is used throughout your body for the development and repair and replacement of old cells throughout your life. The purpose is producing identical daughter cells without any loss of genetic information.

sister-chromatids-prophase

Image credit: based on "The process of meiosis: Figure 1" by OpenStax College, Biology, CC BY 3.0

Meiosis, on the other hand, is a special form of cell division that produces reproductive cells. It should not be confused with mitosis, which is the division of one cell into two identical daughter cells. Much like mitosis, meiosis begins with one cell that splits into two. However, the two cells then divide into two more cells each. These four cells each have half the number of chromosomes as the parent cell. They are gametes, or sex cells. It is through the combination of a female gamete and a male gamete from another organism that sexual reproduction occurs and another, unique individual is produced.

To explain it in another way, meiosis takes us from a diploid cell (a cell with two sets of chromosomes) to a haploid cell (a cell with one set of chromosomes). When an egg (haploid cell) combine in fertilization with a sperm (haploid cell) the two haploid cells form a complete diploid set, thus making a new genome.

Meiosis is completed in several stages that occur in two processes. The first process is when the cell divides into two cells. The second process is when the cells split again to form four cells. These processes can be called meiosis 1 and meiosis II.

Meiosis Iphases-of-meiosis-1

Image Credit “Meiosis”, Khanacademy.org https://www.khanacademy.org/science/biology/cellular-molecular-biology/meiosis/a/phases-of-meiosis. NOTE: All Khan Academy content is available for free at khanacademy.org
  • Interphase – Before a cell can enter meiosis I, three things need to happen. The DNA of the cell is duplicated and two sets of chromosomes are created. Outside the nucleus, two centrosomes are formed and inside, the centriole is prepared for use in cell division. Microtubules begin to extend from the centrosome. These processes take place in interphase.
  • Prophase I – After interphase the cell is ready for meiosis. This first phase is prophase. During this phase, the chromosomes begin to condense just like in mitosis. However, in in meiosis I each chromosome partners with its homologous partner. The chromosomes are made up of two chromatids that contain identical information. The chromosomes line up so they are matching. The chromosomes are capable of sharing information; this can be referred to as recombination or crossing over. The microtubules and proteins extend across the cell. At the end of prophase, the membrane around the nucleus of the cell erodes and the chromosomes are set free.
  • Metaphase I – The pairs of chromosomes line up during this stage. This happens along the center of the cell. Each paired chromosome’s orientation is random. This random positioning helps with diversifying the cell.
  • Anaphase I – The meiotic spindle pulls the pairs of chromosomes apart. One chromosome of the pair is pulled towards one pole, and the other chromosome of the pair is pulled to the opposite pole. The sister chromatids remain attached.
  • Telophase I– During this stage, the chromosomes finishing moving to their opposite poles. When they have all moved, a complete set of chromosomes is attached to each pole. A membrane forms around each set of chromosomes, creating nuclei.
  • Cytokinesis - When both of the nuclei are formed the cell pulls itself apart, resulting in two new daughter cells. This usually happens simultaneously as Telophase I.

Meiosis II

phases-of-meiosis-2

Image Credit “Meiosis”, Khanacademy.org https://www.khanacademy.org/science/biology/cellular-molecular-biology/meiosis/a/phases-of-meiosis. NOTE: All Khan Academy content is available for free at khanacademy.org

The result of the first stage of meiosis is two daughter cells that each contain one set of 23 chromosomes. The steps of meiosis II are similar to what happened before in meiosis I, but they are happening in two cells simultaneously instead of one. Note that the cells do not replicate their DNA in this stage. These cells are also now considered haploid cells.  The division of diploid to haploid cell happened in Meiosis I. These haploid cells now are left with one chromosome from each homologue pair.  The difference is each chromosome has two sister chromatids.

  • Prophase II – The chromosomes condense, forming an X structure. The membrane around the nucleus dissolves and the chromosomes are released. The centrioles duplicate themselves and the meiotic spindle is formed. This occurs in the daughter cell.
  • Metaphase II – Since there are no tetrads in either daughter cell, Metaphase II resembles the Metaphase of mitosis except that the metaphase plate rotates 90 degrees with respect to the metaphase plate of Metaphase I. The chromosomes begin to line up in single file. The centrioles are located at the poles of the cell. The meiotic spindle attaches itself to each of the chromatids.
  • Anaphase II – This phase is almost identical to the Anaphase of mitosis, with no notable differences except the position of the metaphase plate.  This occurs in each daughter cell. The chromatids are split and divided and pulled to the two opposite poles of the cell.
  • Telophase II – Again, this phase is identical to the Telophase of mitosis, with no notable differences except the position of the metaphase plate and the addition of the cytokinesis, resulting in four gametes with distinct genetic information and a plethora of possibilities for offspring resulting from sexual reproduction. The chromosomes finish moving to the poles. The result is two full sets of chromosomes at each pole. A membrane forms around the two sets of chromosomes to form nuclei.

Summary

The cell-division cycle, inclusive of mitosis, is a universal process that occurs in every living cell in all forms of life.  This process is simpler in simple organisms and more complex in more developed organisms, but the same main phases of activity occur in every completed iteration of the process.  Meiosis, on the other hand, occurs only for reproductive cells and only in eukaryotes.  Mitosis and meiosis are cell division / replication / recombination processes that form an integral part of basic functions of life of every type of organism and are therefore essential to the operational integrity of day-to-day activity and the ongoing survival of every species. While the process of meiosis is the same in both males and females, the end results are a little different. In males, meiosis results in the production of four sperm cells. In females, the result is one egg cell and three polar bodies. The polar bodies are smaller than the egg cell and are eventually reabsorbed.

Sources

Cell and Molecular Biology by Karp 8th Ed., ISBN-13: 978-1118301791

Image Resources

Image Credit “Meiosis”, Khanacademy.org https://www.khanacademy.org/science/biology/cellular-molecular-biology/meiosis/a/phases-of-meiosis. NOTE: All Khan Academy content is available for free at khanacademy.org

Image credit: based on "The process of meiosis: Figure 1" by OpenStax College, Biology, CC BY 3.0

 

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