Meiosis

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Summary

  • Meiosis is a specialised type of cell division that produces gametes, which require a haploid genome in order to participate in sexual reproduction
  • Meiosis is fairly similar to mitosis, but the differences can be remembered with the pneumonic DISCOPUG (division, independent, synapsis, crossover, outcome, ploidy, use, genetics
  • Meiosis can be divided into two stages, meiosis II and meiosis II, which both possess prophase, metaphase, anaphase and telophase.
  • During meiosis, one diploid cell produces four daughter cells that are genetically distinct from one another
  • Meiosis is important for generating genetic diversity

Meiosis is a unique kind of cell division that produces sex cells, or gametes. In humans, these cells are egg cells (ova) and sperm cells. Unlike in mitosis, which produces two daughter cells containing 46 chromosomes (23 pairs), in meiosis, four daughter cells are produced, each carrying 23 chromosomes. Mitosis produces cells with a diploid genome (2 copies), meiosis produces cells with a haploid genome (1 copy).

Meiosis is important for two reasons. Firstly, it is required for sexual reproduction, as if the sperm and egg cells had a diploid genome, then the fertilized egg would have a quadruploid genome, which would not be viable.

The steps in meiosis are similar to the steps in mitosis. However, it undergoes an additional step of having to separate the homologous chromosomes. It also carries out steps that produce genetic diversity.

Meiosis is divided into two stages: Meiosis I and Meiosis II.

Stages of meiosis

Meiosis I

  • Interphase

Cells in between stages of meiosis are in interphase, sometimes specified as ‘premeiotic interphase’. As in mitosis, the interphase consists of G1 phase, S phase, and G2 phase. The cell must duplicate all the chromosomes during S phase. The cell then enters prophase I.

  • Prophase I

During prophase I, there are distinct differences between meiosis and mitosis. The chromosomes condense, and they exist as a pair of sister chromatids that are connected by the centromere. However, they also pair up with their homologous pair and align themselves with one another along their length. The chromosome inherited from the father (paternal) will align with the chromosome inherited from the mother (maternal).

The aligned chromosomes can then come together  and touch in an X shaped structure in a process called “synapsis” to exchange genetic material. The exchange of genetic material occurs by a process called  or “crossing over”.

The resulting crossovers are called “chiasmata”, and can be viewed under a microscope. This means sections of DNA that came from the father, now combine with sections of DNA from the mother. This is referred to as recombination.  The purpose of recombination/crossover events is to generate genetic diversity by initiating random shuffling of genetic material, generating unique combinations of alleles. This means the offspring will be genetically distinct from the parent.

Next, the meiotic spindle apparatus is formed, and the spindles attach to the kinetochore of the chromosomes from the centriole at the opposite poles of the cells.  Sometimes this stage is considered as a separate stage – prometaphase. Once the kinetochore is attached to the spindle, metaphase can begin.

  • Metaphase I

Metaphase I begins when the chromosomes line up in the centre of the nucleus at the metaphase plate. The centromere of one chromatid pair is attached to one pole via the spindle, whilst the other chromatid pair is attached to the opposite pole. The way the chromosomes line up along the equator also generates variation. The homologous pairs of chromosomes line up randomly in a process called independent assortment. This means that gametes inherit different combinations of maternal and paternal chromosomes, depending on the way they orient themselves in their pair prior to separation.

  • Anaphase I

During anaphase I, the spindles contract and pull the homologous chromosomes apart, separating them to opposite poles of the nucleus. Importantly, the sister chromatids remain attached to each other at the centromere.

  • Telophase I

Telophase I involves the formation of a nuclear envelope around each set of chromosomes, which are now separated at the opposite side of the cell. The spindle disappears, and cytokinesis (the separation of the cytoplasm and the cellular contents) occurs. This creates two daughter cells containing half the amount of DNA of the starting cells.

Meiosis II

Meiosis II begins without any further DNA replication – so the chromosome number does not change between Telophase I and Prophase II.

  • Prophase II

In prophase II, the chromosomes condense again (if they managed to decondense in telophase I) the  nuclear envelope (if one managed to form) breaks down and a new set of spindles form.

  • Metaphase II

During metaphase II, the chromatid pairs line up at the metaphase plate at the equator of the cell. This is in contrast to metaphase I, during which pairs of chromosomes align on the metaphase plate. The spindles attach to the centromeres at the kinetochore from the opposite poles.

  • Anaphase II

The sister chromatids are pulled apart at the centromere by the spindles, separating them. The sister chromatids move to the opposite sides of the cell. The sister chromatids are now referred to as chromosomes.

  • Telophase II

Finally, the two sets of chromosomes are enclosed when a new nuclear envelope forms. The cell then undergoes cytokinesis, in which the contents of the cell including cytoplasmic components are separated into two daughter cells. Since meiosis II starts with two cells, four daughter cells are produced at the end of the process. These cells are genetically unique because of the random assortment of paternal and maternal chromosomes and because of recombination that occurs when during crossover events. The haploid daughter cells become gametes – either sperm or egg cells.

Comparing meiosis and mitosis

It is easy to get mixed up between mitosis and meiosis, they are very similar processes with regards to the steps included. The differences between meiosis and mitosis can be memorized with a pneumonic from BioNinja: DISCOPUG

Differences Meiosis Mitosis
Division Two cell division One cell division
Independent Assortment Homologous chromosomes randomly assigned to different daughters in meiosis I No
Synapsis Homologous chromosomes form synapses No
Crossing-over Chromatids in homologous pairs can exchange info (prophase I) No
Outcome 4 daughter cells produced 2 daughter cells produced
Ploidy Haploid Diploid
Use Form gametes Form somatic (body) cells

 

Genetics Genetically distinct daughter cells Genetically identical daughter cells

Further reading and references:

[1]. http://biology-forums.com/gallery/2137_21_04_12_2_19_02.jpeg Image comparing meiosis to mitosis

[2]. https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/11%3A_Meiosis_and_Sexual_Reproduction/11.1%3A_The_Process_of_Meiosis/11.1B%3A_Meiosis_I Image prophase

[3]. https://cnx.org/contents/jVCgr5SL@8.17:WzgNHpon@3/The-Process-of-Meiosis Image Meiosis

[4]. http://ib.bioninja.com.au/higher-level/topic-10-genetics-and-evolu/101-meiosis/meiosis-vs-mitosis.html BioNinja pneumonic

[5]. https://www.yourgenome.org/facts/what-is-meiosis

[6]. https://opentextbc.ca/biology/chapter/7-2-meiosis/