Meiosis: Production of gametes

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In the same manner that a cell prepares for mitosis by undergoing DNA replication, a cell that will form sex cells will also go through the phases of G1, S and G2. meiosis consists of cell divisions, Meiosis I and meiosis I

Meiosis displays two major differences when compared to mitosis:

• Firstly, the DNA is reorganised through the two processes of recombination (crossing over and independent assortment) producing gametes that are all genetically different. This ensures that progenies are genetically different; in that way ensuring variation in populations. 

• Secondly, meiosis halves the number of chromosomes in an ordered manner to ensure that at fertilisation the zygotic chromosome number is restored. Both recombination and the halving of the chromosome number occur during the first meiotic division, meiosis I.

Escalation of the chromosome number if meiosis did not halve the chromosome number:

 

Meiosis I

Similarly to mitosis the chromosomes spiralize, the nuclear membrane and nucleolus disappear, and the spindle appears during the first meiotic division’s prophase, prophase I. However, late in prophase 1 (pachytene) the homologous chromosomes pair (synapse) with one another forming paired structures, bivalents. Each bivalent contains two duplicated chromosomes and are thus four stranded. In human males the small homologous regions that the “X” and “Y” share permit these two chromosomes to pair. 

During metaphase I the bivalents align on the equator of the cell and attach with their centromeres to the spindle. During anaphase I the homologous chromosomes of the bivalents separate and move to opposite poles of the cell. Once the chromosomes reach the poles, they reform into two nuclei during telophase I and despiralize forming a chromatin network, after which cytokinesis occurs.  

Products to meiosis I

The two cells that result from meiosis I contain half the number of chromosomes. In humans the there are 23 chromosomes (n= 23 = x), one set, in each cell after meiosis I. In males, one cell contains an “X” and the other cell the “Y”. We say that the “X” chromosome segregated from the “Y” during the first division.

Meiosis II 

The second meiotic division is similar to mitosis, but is not preceded by DNA replication. The result of meiosis is four cells which undergo some differentiation to produce functional gametes. Some differences exist between the sexes in animals and plants that will be discussed is other articles.

Products of meiosis II

The four cells that result from meiosis II contain half the number of chromosomes. In humans two cells contain an “X” and two contain a “Y”. Because of recombination during the meiosis I (discussed in another article) the gametes are genetically different.