Continuity of Life: Processes


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Ben Whitten

Meiosis (Updated)

Introduction to meiosis.

What is meiosis?

Image: Meiosis Stages - Numerical Version image, Image by Ali Zifan, Sourced Under a Creative Commons 4.0 License from Wiki Commons

Meiosis can often confuse people more so than mitosis; why are there so many stages? What's the difference? One of the key differences between mitosis and meiosis is that meiosis only occurs in sex or germline cells, and results in the production of gametes (sperm or ova). There are two sets of divisions in meiosis, being called meiosis I and meiosis II. The phases of both are accompanied by the I and II suffixes, and are the same/similar to mitosis. 

  1. Prophase I 

  2. Metaphase I 

  3. Anaphase I 

  4. Telophase I 

  5. Cytokinesis I 

  6. Prophase II 

  7. Metaphase II 

  8. Anaphase II 

  9. Telophase II 

  10. Cytokinesis II

What are the steps of meiosis I?
  1. Prophase I: During this phase, chromatin threads condense to form chromosomes. Both maternal and paternal homologous (same) chromosomes are attracted to each other and pair up. It's at this stage where crossing over occurs. Each chromosome consists of two sister chromatids held together by a centromere. The nuclear membrane disintegrates along with the nucleolus. Meiotic spindle begins to form, and attaches to the chromosomes at the centromeres. The centromeres move to opposite poles of the cell. (Exactly the same as mitosis, except for the homologous chromosomes!) 

  2. Metaphase I: Maternal and paternal (homologous) chromosomes line up along the metaphase plate in pairs, which is known as independent assortment. This is because each pair is lined up on one side or the other, independent of every other pair. This results in the random assortment of chromosomes. The spindle fibres are attached to the centromeres. (Note: during Metaphase I, crossing over occurs which increases genetic variation - this is covered more in another content page!) 

  3. Anaphase I: The spindle fibres shorten, pulling on the centromere of each chromosome. One member of each pair of homologous chromosomes moves to each end of the cell (a whole chromosome is pulled). A random combination of maternal/paternal chromosomes is dragged to each pole. 

  4. Telophase I: New nuclear membranes form and the chromosomes decondense, and the spindle fibres disintegrate. 

  5. Cytokinesis I: This is the separation of the cytoplasm, as the cell splits into two cells. Technically, the daughter cells are considered haploid as they only contain one chromosome from each pair of homologous chromosomes. No more DNA replication occurs. What a mouthful! Just remember, this is extremely similar to mitosis with key differences in metaphase and anaphase, as whole chromosomes are being pulled to the poles.

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What are the steps of meiosis II?

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  1. Prophase II: Chromatin condenses to form visible chromosomes again, new spindle fibres are produce and the nuclear membrane disintegrates. 

  2. Metaphase II: Individual chromosomes line up in one line along the equator, or metaphase plate, in a random order. The spindle fibres attach to the sister chromatids at the centromeres. 

  3. Anaphase II: The centromeres of each chromosome disconnect which allows the sister chromatids to separate. The spindle fibres shorten, and individual sister chromatids move to opposite poles of the cell. 

  4. Telophase II: Chromosomes unwind, loosen and reform chromatin. Four new nuclear membranes form around the nuclei, one in each daughter cell. 

  5. Cytokinesis II: This is the physical separation of the cytoplasm. The cells separate into four new, non-identical daughter cells.

Distinguishing between mitosis and meiosis

To create ease with remembering the process of meiosis, just think of the fundamentals to mitosis. Meiosis, in simple terms, is PMAT twice over with a few differences here and there. The main difference lies in chromosome number. The chromosomes are halved each time, whereas the number of chromosomes in mitosis remains the same.