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Meiosis

6.3 Meiosis
 
Introduction
  • Meiosis is the process of cell division that occurs in reproductive organs to produce gametes that contain half the number of chromosomes (haploid) of the parent cells (diploid).
  • Meiosis occurs in the testis (male) and ovary (female) for animals and humans.
 
The Need of Meiosis
  • Meiosis forms gametes through the process of gametogenesis and ensures that the diploid chromosome number of organisms that carry out sex reproduction is always maintained from one generation to the next.
  • Meiosis also produces genetic variation in the same species.
  • Meiosis is divided into two stages of cell division, that is meiosis I and meiosis II
    • Meiosis I comprises of prophase I, metaphase I, anaphase I and telophase I.
    • Meiosis II comprises of prophase II, metaphase II, anaphase II and telophase II.
 
Meiosis I
Prophase I
  • Chromatin shortens, thickens and forms visible chromosomes.
  • The pairing of homologous chromosomes (synapsis) forms bivalent (or known as a tetrad, that is four chromatids for each homologous chromosome).
  • The crossing over process that is an exchange of genetic material between non-identical chromatids takes place.
  • Crossing over produces a combination of genes that are new in chromosomes.
  • The point where the chromatids cross over is called chiasma.
  • At the end of prophase I, the nucleus membrane and nucleoli will start to disappear.
  • Both centrioles will move towards the opposite pole cells.
  • Spindle fibres are formed among the centrioles.
Metaphase I
  • The homologous chromosomes are arranged at the equatorial plane.
  • One chromosome from each pair of the homologous chromosome is tied to the spindle fibres from one pole cell and its homologous is tied to the spindle fibres from the opposite pole cell.
  • The sister chromatids are still tied together because the centromere has not separated.
Anaphase I
  • The spindle fibres contract and cause each homologous chromosome to separate from its homologous pair and be pulled to the opposite poles.
  • Each chromosome is still made up of a pair of sister chromatids tied to a centromere and move as one unit.
Telophase I
  • The chromosomes arrive at the opposite pole cells.
  • Each polar cell contains a number of haploid chromosomes that are made up of one set of chromosomes only.
  • The spindle fibres will then disappear.
  • Nucleoli will reappear and the nuclear membrane is formed. 
  • Telophase I is succeeded by the cytokinesis process that produces two daughter cells.
  • Both daughter cells produced are in the haploid condition.
  • The interphase for meiosis I is usually short and the DNA does not replicate.
 
Meiosis II
Prophase II
  • The nucleoli and the nuclear membrane disappear.
  • Each chromosome is made up of sister chromatids that are joined at the centromere.
  • The spindle fibres start to form in both daughter cells.
Metaphase II
  • Chromosomes are arranged at random on the equatorial plane for each daughter cell.
  • Each chromatid is tied to the spindle fibres at the centromere.
  • Metaphase II ends when the centromere separates.
Anaphase II
  • The sister chromatid centromere starts to separate.
  • The sister chromatid pair separates and moves towards the opposite poles led by the centromere.
  • Each chromatid at this stage is known as a chromosome.
Telophase II
  • Chromosomes arrive at the pole of the cell.
  • Spindle fibres disappear.
  • The nuclear membrane and the nucleoli are reconstructed.
  • The number of chromosome for each daughter cell is half the number of parent chromosomes.
  • Telophase II ends with the process of cytokinesis that produces four daughter cells that are haploid.
  • Each haploid cell contains half the number of parent cell chromosomes.
  • The genetic content is also different from the diploid parent cell.
  • The haploid cells develop into gametes.
 
 
Comparison and Contrast between Mitosis and Meiosis
The image illustrates the stages of meiosis, a type of cell division. It begins with Interphase, where DNA is copied. In Prophase I, chromosomes pair up and recombination occurs. Metaphase I shows chromosomes lining up at the equator. During Anaphase I, homologous chromosomes pull apart. Telophase I and Cytokinesis result in two diploid cells. In Prophase II, the process continues with Metaphase II, where chromosomes line up again. Anaphase II shows sister chromatids pulling apart. Finally, Telophase II and Cytokinesis lead to four haploid cells.

 

The views expressed in the notes here are those of the contributor and don’t necessarily represent the views of Pandai.

Meiosis

6.3 Meiosis
 
Introduction
  • Meiosis is the process of cell division that occurs in reproductive organs to produce gametes that contain half the number of chromosomes (haploid) of the parent cells (diploid).
  • Meiosis occurs in the testis (male) and ovary (female) for animals and humans.
 
The Need of Meiosis
  • Meiosis forms gametes through the process of gametogenesis and ensures that the diploid chromosome number of organisms that carry out sex reproduction is always maintained from one generation to the next.
  • Meiosis also produces genetic variation in the same species.
  • Meiosis is divided into two stages of cell division, that is meiosis I and meiosis II
    • Meiosis I comprises of prophase I, metaphase I, anaphase I and telophase I.
    • Meiosis II comprises of prophase II, metaphase II, anaphase II and telophase II.
 
Meiosis I
Prophase I
  • Chromatin shortens, thickens and forms visible chromosomes.
  • The pairing of homologous chromosomes (synapsis) forms bivalent (or known as a tetrad, that is four chromatids for each homologous chromosome).
  • The crossing over process that is an exchange of genetic material between non-identical chromatids takes place.
  • Crossing over produces a combination of genes that are new in chromosomes.
  • The point where the chromatids cross over is called chiasma.
  • At the end of prophase I, the nucleus membrane and nucleoli will start to disappear.
  • Both centrioles will move towards the opposite pole cells.
  • Spindle fibres are formed among the centrioles.
Metaphase I
  • The homologous chromosomes are arranged at the equatorial plane.
  • One chromosome from each pair of the homologous chromosome is tied to the spindle fibres from one pole cell and its homologous is tied to the spindle fibres from the opposite pole cell.
  • The sister chromatids are still tied together because the centromere has not separated.
Anaphase I
  • The spindle fibres contract and cause each homologous chromosome to separate from its homologous pair and be pulled to the opposite poles.
  • Each chromosome is still made up of a pair of sister chromatids tied to a centromere and move as one unit.
Telophase I
  • The chromosomes arrive at the opposite pole cells.
  • Each polar cell contains a number of haploid chromosomes that are made up of one set of chromosomes only.
  • The spindle fibres will then disappear.
  • Nucleoli will reappear and the nuclear membrane is formed. 
  • Telophase I is succeeded by the cytokinesis process that produces two daughter cells.
  • Both daughter cells produced are in the haploid condition.
  • The interphase for meiosis I is usually short and the DNA does not replicate.
 
Meiosis II
Prophase II
  • The nucleoli and the nuclear membrane disappear.
  • Each chromosome is made up of sister chromatids that are joined at the centromere.
  • The spindle fibres start to form in both daughter cells.
Metaphase II
  • Chromosomes are arranged at random on the equatorial plane for each daughter cell.
  • Each chromatid is tied to the spindle fibres at the centromere.
  • Metaphase II ends when the centromere separates.
Anaphase II
  • The sister chromatid centromere starts to separate.
  • The sister chromatid pair separates and moves towards the opposite poles led by the centromere.
  • Each chromatid at this stage is known as a chromosome.
Telophase II
  • Chromosomes arrive at the pole of the cell.
  • Spindle fibres disappear.
  • The nuclear membrane and the nucleoli are reconstructed.
  • The number of chromosome for each daughter cell is half the number of parent chromosomes.
  • Telophase II ends with the process of cytokinesis that produces four daughter cells that are haploid.
  • Each haploid cell contains half the number of parent cell chromosomes.
  • The genetic content is also different from the diploid parent cell.
  • The haploid cells develop into gametes.
 
 
Comparison and Contrast between Mitosis and Meiosis
The image illustrates the stages of meiosis, a type of cell division. It begins with Interphase, where DNA is copied. In Prophase I, chromosomes pair up and recombination occurs. Metaphase I shows chromosomes lining up at the equator. During Anaphase I, homologous chromosomes pull apart. Telophase I and Cytokinesis result in two diploid cells. In Prophase II, the process continues with Metaphase II, where chromosomes line up again. Anaphase II shows sister chromatids pulling apart. Finally, Telophase II and Cytokinesis lead to four haploid cells.

 

The views expressed in the notes here are those of the contributor and don’t necessarily represent the views of Pandai.
Chapter : Cell Division
Topic : Compare and contrast meiosis and mitosis
Form 4 Biology
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