Meiosis describes a specific process of cell division by which gametes are made. In this process, we begin with a cell with double the normal amount of DNA, and we will end up with 4 non-identical haploid daughter gametes, after two divisions.
There are six stages within each of the divisions, namely prophase, prometaphase, metaphase, anaphase, telophase and cytokinesis. In this article, we will look at the stages of meiosis and consider its significance in disease.
In meiosis I, homologous chromosomes are separated into two cells such that there is one chromosome (consisting of two chromatids) per chromosome pair in each daughter cell.
Prior to prophase, chromosomes replicate to form sister chromatids. There are initially four chromatids (c) and two chromosomes (n) for each of the 23 chromosome pairs (4c, 2n). The nuclear envelope disintegrates and the chromosomes begin to condense. Spindle fibres appear which will be important for successful division of the chromosomes.
To further increase the genetic diversity, homologous chromosomes exchange parts of themselves such that one chromosome contains both maternal and paternal DNA. This process is known as crossing over, and the points at which this occurs on a chromosome are referred to as chiasmata.
Now the spindle fibres attach to the chromosomes at a points along the chromosomes called centromeres. While this is happening the chromosomes continue to condense.
Next, maternal and paternal versions of the same chromosome align along the equator of the cell. These are the homologous chromosomes. A process called independent assortment occurs – this is when maternal and paternal chromosomes line up randomly align themselves on either side of the equator. This is turn determines to which gamete chromosomes are allocated to, which leads to genetic diversity among offspring.
Here each of the homologous chromosomes get pulled towards opposite poles of the cell as the spindle fibres retract to divide the DNA between the two cells which will be formed.
Telophase I and Cytokinesis I
During telophase I, the nuclear envelope reforms and spindle fibres disappear. In Cytokinesis I, the cytoplasm and cell divides resulting in two cells that are technically haploid – there is one chromosome and two chromatids for each chromosome (2c, n).
Prophase II and Prometaphase II
These stages are identical to their counterparts in meiosis I.
Now chromosomes line up in single file along the equator of the cell. This is in contrast to Metaphase I where chromosomes lined up in homologous pairs.
Next, sister chromatids are pulled to opposite poles of the equator.
This is the same as Telophase I.
Again, the cytoplasm and cell divides producing 2 non-identical haploid daughter cells, but as this is happening in both cells produced by meiosis I, the net product is 4 non-identical haploid daughter cells, each comprising one chromosome consisting of one chromatid (1c, 1n). These are gametes.
Clinical Relevance – Aneuploid, Non-disjunction and Anaphase lag
There are many conditions associated with errors in meosis, such as aneuploidy where there is the loss or gain of whole chromosomes.
Non-disjunction describes the failed separation of chromosomes during anaphase, so either chromosomes (meiosis I) or chromatids (meiosis II) move to the same pole of the cell. This leaves one gamete short of genetic information, and one with additional genetic information. This can leave cells short of a chromosome such as in Turner’s syndrome (45,X) or with an additional chromosome as in Klinefelter’s syndrome (47,XXY).
Anaphase lag can occur where chromosomes are left behind due to defects in the spindle fibres or attachment to chromosomes. This differs from non-disjunction as neither cell receives the chromosome/chromatid, leaving both daughter cells short of genetic information.