Mitosis and Meiosis

27 06 2013

A back-to-the-fundamentals question that landed in my inbox: what is the difference between mitosis and meiosis? I will answer it in the most basic way possible, to avoid getting into niggly details and processes.

Mitosis is a part of the standard cell cycle, which goes through the following stages:

  1. Interphase, the stage in which a cell is at for most of its life (G1 stage), at the end of which DNA gets duplicated (synthetic stage).
  2. Mitosis, during which the duplicated DNA is distributed equally to go to two daughter cells.
  3. Cytokinesis, in which the separation of the two daughter cells takes place.

So, the goal of mitosis is to make sure that the daughter cells contain the same genotype as their parent cell. It proceeds through four phases:

  1. Prophase: The chromatin in which DNA is packed condenses into chromosomes visible by microscope. A chromosome consists of two chromatids held together by a centromere in the center.
  2. Metaphase: The chromosomes become aligned at the center of the cell, held there by chains of microtubules, called spindle fibers, attached from the chromsome to the centrioles found at opposing ends of the cell.
  3. Anaphase: The spindle fibers attach to kinetochores on the chromsome, which act like little engines, constantly pulling the chromosomes to opposing ends of the cell. Eventually, a biochemical cascade is unleashed that splits the chromatids apart. Each chromatid goes to one end of the cell.
  4. Telophase: By this time, cytokinesis is already underway, and the cell is almost completely divided. Each end of the cell contains one complete set of DNA in the chromatid strands. They get enveloped by a nucleus, and the chromatids unwind.

The end result is that at the end of the cell division, the two daughter cells will have almost exact copies of their parent cell’s DNA. Almost every somatic cell type undergoes mitosis.

Meiosis is a very special process that is used to create gametes, cells like the sperm and the egg which have only half the genetic content of the parent cells. It’s a critical part of sexual reprodutction.

A normal animal cell is diploid, it has two sets of chromosomes, each chromosome having a homologous pair. Meiosis splits these pairs and puts them into daughter cells which are haploid (only one set of chromosomes). When syngamy occurs in sexual reproduction, the two haploid cells combine to make the resultant cell diploid again.

Meiosis has two parts to it, but we will only look at the first part here, the part when the ploidy gets halved. As in mitosis, the precursor is the synthesis stage when new DNA is formed.

  1. Prophase I: As in mitosis, the chromatin condenses into the chromosomes. However, this is where similarities end. After the chromosomes form, they pair up into their homologous pairs. This leads to the most important aspect of meiosis: homologous recombination. Chiasmata form between the chromatids of sister chromosomes that lie next to each other, and in these areas, genetic information is exchanged. The importance of this is that this leads to new alleles, meaning it’s a source of novelty for evolution to act on. This is why one of the main reasons why sex is maintained as a reproductive mode.
  2. Metaphase I: As in mitotic metaphase, this is when the chromsomes align themselves at the center of the cell and start getting pulled by kinetochores. However, in meiosis metaphase I, it isn’t chromatids that are pulled apart, it’s sister chromosomes. In other words, whole chromosomes are pulled to the opposing ends of the cell.
  3. Anaphase I: Chromosomes are pulled to opposing ends of the cell.

At the end of this, the cell undergoes cytokinesis, with each daughter cell being haploid, having only one set of chromosomes. In other words, it has only half the genetic material of the parent cell, setting the stage for crucial genetic shuffling once fertilisation occurs. Meiosis completes after a second stage, meiosis II, which is mechanically fairly identical to mitosis.

So, the end results of mitosis and meiosis are completely different:

  • Mitosis: One diploid cell → two diploid cells. Goal is cell division.
  • Meiosis: One diploid cell → four haploid cells. Goal is genetic shuffling and production of gametes.

Further Technical Reading:

Bernstein H & Bernstein C. 2010. Evolutionary Origin of Recombination during Meiosis. BioScience 60, 498-505.

Clarke PR & Zhang C. 2008. Spatial and temporal coordination of mitosis by Ran GTPase. Nature Reviews Molecular Cell Biology 9, 464–477.

Debec A, Sullivan W & Bettencourt-Dias M. 2008. Centrioles: Active Players or Passengers During Mitosis? Cellular and Molecular Life Sciences 67, 2173–2194.

Extravour C. 2009. Oogenesis: making the Mos of meiosis. Current Biology 19, R489-R491.

Nicklas RB. 1988. The forces that move chromosomes in mitosis. Annual review of biophysics and biophysical chemistry 17, 431-449.

Ravi M, Marimuthu MPA & Siddiqi I. 2008. Gamete formation without meiosis in Arabidopsis. Nature 451, 1121-1124.

Tanaka K. 2012. Regulatory mechanisms of kinetochore–microtubule interaction in mitosis. Cellular and Molecular Life Sciences 70, 559-579.

Wilkins AS & Holliday R. 2008. The Evolution of Meiosis From Mitosis. Genetics 181, 3-12.

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2 responses

16 07 2013
Marcoli

Just a comment. Some texts recognize a phase called prometaphase, which includes a few events that used to be a part of prophase.

17 07 2013
Marc

Yep, that’s the version I got in university. Not sure if they teach it in high school though, so I kept it at the traditional 4-stage version.

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