Meiosis Cell Division

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Overview

Meiosis is reductional division, which takes place in reproductive cell. It is divided into meiosis I and meiosis II. Meiosis I is divided into prophase I, metaphase I, anaphase I, telophase I and mitosis into prophase II, metaphase II, anaphase II, and telophase II. This note has detail information on different stages of meiosis cell division.
Meiosis Cell Division

It is reductional division, which takes place in the reproductive cell. It is a longer process in comparison to mitosis. It completes in the following steps:

Meiosis I:

It is further divided into:

  • Prophase I: It is the longest stage of cell division, which is further divided into five sub-stages:
    • Leptotene: The nucleus increases in size and prepares the cell for division. Chromosomes are coiled with each other and they are thelong thread like in structure. Such chromosomes are called () like chromosome.
    • Zygotene: In this stage, the chromatid threads having similar structure, function etc. but donated by opposite sex (i.e. mother and father) starts to make their pair, such pair of the chromosome are called homologous chromosome. In this stage, the centromere starts to appear which divides the chromosome into two arms, such chromosome is called condensed chromosome. The process of formation of thehomologous chromosome is called synapse.
    • Pachytene: The chromosome splits lengthwise except centromere and the resulting chromosomes are called tetrad chromosomes. In late pachytene, the crossing over process will initiate.
    • Diplotene: In this stage, the exchange of genetic materials between paired chromosomes occurs. This process is called crossing over. Crossing over is important for evolution as it helps to bring variation. The point where the paired chromosomes attach to each other during crossing over it is called chaismata.
    • Diakinesis: Centriole divides into two and starts to move towards opposite part of the cell. Chromosomes are much thick and condensed. In late diakinesis except centriole chromosomes and cell membrane, other parts start to disappear.

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  • Metaphase I: Centrioles reach to the opposite poles of the cell. Chromosomes are arranged in theequatorial plane of the cell in two rows. Spindlefibre appears.

    http://www.quia.com/files/quia/users/lmcgee/genetics/meiosis_stages/metaphase-I.gif

  • Anaphase I:: Each paired chromosome starts to move towards the opposite poles of the cell by the help of contraction of spindlefibre and the repulsion between chromosomes. Chromosomes appear in the shape of I, J,U and V.

    http://www.phschool.com/science/biology_place/biocoach/images/meiosis/meana1.gif

  • Telophase I:: The paired chromosomes reach to the opposite poles of the cell. They are also surrounded by a nuclear membrane. Hence diploid daughter nuclei are formed. The cellularconstriction becomes more andmore deep and finally divides the cell into two equal halves.

    http://bio1151.nicerweb.com/Locked/media/ch13/13_08meiosis_I-telophase.jpg

Meiosis II

The two daughter cells that are produced at the end of telophase I undergo meiosis II. The process of meiosis II includes:

  • Prophase II: The centriole divides in two and starts to move towards the opposite pole of the cell. Chromosomes are shorter, thicker and distinct.
  • Metaphase II: Centrioles have reached their opposite poles of the cell. Chromosomes are arranged in an equatorial plane of the cell. Spindle fibre appears.
  • Anaphase II: Each chromosome splits lengthwise through centromere and produces sister chromatids. Each chromatid, start to move towards the opposite pole of the cell by the contraction of spindle fibre and repulsion between them.
  • Telophase II: Chromatids reach to opposite poles of the cell. They are surrounded by the nuclear membrane. Hence, four haploid daughter cells are produced at the end of meiosis II. The cellularconstriction divides the cell into four equal parts. All cellular parts reappear.

    http://www.bio.miami.edu/dana/pix/meiosis.jpg

Importance Of meiosis cell division:

  • Variation: Crossing over that starts in pachytene and ends at diplotene of prophase I help to exchange the genetic material between chromosomes that creates variations among offspring.
  • Sexual reproduction: Meiosis cell division occurs only in reproductive cells and produces gametes. So, it helps to carry out sexual reproduction.
  • Genetic stability: The offsprings, which are produced after sexual reproduction, are similar to their parents in many respects. Therefore, meiosis helps to keep genetic stability.
Things to remember
  • Meiosis is reductional division, which takes place in reproductive cell. It is divided into meiosis I and meiosis II.
  • Meiosis I is divided into prophase I, metaphase I, anaphase I, telophase I and mitosis into prophase II, metaphase II, anaphase II, and telophase II.
  • Prophase is further divided leptotene, zygotene, pachytene, diplotene, and diakinasis.
  • Meiosis creates variation in offspring and helps in reproduction and genetic stability.
  • It includes every relationship which established among the people.
  • There can be more than one community in a society. Community smaller than society.
  • It is a network of social relationships which cannot see or touched.
  • common interests and common objectives are not necessary for society.
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Phases of Meiosis
Questions and Answers
The process in which a diploid cell divides to form four haploid cells that are never identical is called meiosis cell division.
The sub-phases of prophase I of meiotic cell division are:
  1. Leptotene
  2. Zygotene
  3. Pachytene
  4. Diplotene
  5. Diakinesis
The five important features of prophase I are:
  1. Homologous pairs are formed.
  2. Chiasmata are formed between the non-sister chromatids of a pair.
  3. Crossing over takes place. d. In animal cells, centrioles divide and move towards opposite poles. e. Spindle fibers begin to appear from opposite poles and nucleons and nuclear membrane disappear.
The following incidents occur during this stage.
  1. The homologous pairs lie at the equatorial plane of the cell.
  2. The chromosomes are held at the centromere by the spindle fibers of respective poles.

The two incidents during metaphase I are:
i. Due to contraction of spindle fibers, homologous chromosomes separate.
ii. The homologous move towards opposite poles.
The two incidents during telophase II are:
i. Chromosomes form groups at opposite poles.
ii. Nuclear membrane reappears around each set of chromosomes.

The significances of meiotic cell division are:
i. It helps to maintain the definite number of chromosomes for each species.
ii. It helps in bringing variation among individual due to crossing over.
iii. It is responsible for the transmission of characteristics from parents to offspring.

Synapsis is the process by which paternal and maternal chromosomes come together to form a homologous pair. It occurs during Zygotene of phrases I of meiosis.
During Zygotene of prophase I, chromosomes from homologous pair, i.e. one chromosome from father and one from mother come together to form a pair by synapsis.
Any organisms results due to the fusion of male and female gametes during sexual reproduction. During fertilization the chromosomes of male and female gametes are added up in the offspring. In order to maintain the number of chromosomes from generation to generation in a species it is necessary for gametes to have half the number of chromosomes. If the gametes had the same number of chromosomes as the mother cell, then during sexual reproduction, the number of chromosomes would double in each generation.
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