Meiosis is a cell division process in which haploid gamete cells are produced in a diploid organism. A diploid organism is an organism which has homologous copies of each parent chromosomes within their cells. The parents of the offspring each donate a set of chromosomes which will then equal to two sets (Xu, 2006). Humans have 46 chromosomes within their diploid cells, compared to the 23 chromosomes which can be found within haploid cells. Haploid cells can be found within the gametes of diploid organisms, otherwise known as the sperm and ovum cells or reproductive cells. Haploid cells only have one set of chromosomes because when two parent haploid cells come together, they become fertilised. This means that the offspring will gain a complete set of chromosomes and become a diploid cell or diploid organism. (Kent, 2013).
Meiosis is an incredibly significant process for maintaining life itself. MacLennan (2015) states that meiosis is essential as it allows for the reduction of chromosomes within the gamete cells to maintain the correct number of chromosomes within the offspring after fertilisation. This is to reduce chromosomal disease and maintain genetic diversity within a species. Meiosis creates genetic diversity by recombining the chromosomes genetic material. Genetic variation is then increased further when the two parent gametes come together during fertilisation, there by creating an offspring with unique DNA combinations. (Humphryes and Hochwagen, 2014).
There are two consecutive divisions that take place during meiosis and the result of the processes of meiosis is the production of four haploid daughter cells. These cells that are produced are genetically different from their parent cells and the four cells that are produced are even genetically different from each other. This is because during meiosis, a pair of homologous chromosomes can exchange genetic material before being separated. (Kent, 2013). Meiosis firstly begins with Interphase. During interphase the chromosomes and organelles within a cell begin to duplicate. This results in each chromosome consisting of two daughter chromatids. (Simon et al., 2013). Interphase is also used to store energy as ATP (Adenosine Triphosphate) which can then later be used during the meiosis process. (Kent, 2013).
The process then moves on to Prophase 1. This stage consumes around 90% of the division time as it is an extremely complex stage. During Prophase 1 the chromosomes begin to condense, the nuclear envelope disintegrates, and the spindle apparatus starts to appear. (Kent, 2013). As the chromosomes begin to condense and coil up, proteins cause the homologous chromosomes to draw together and form pairs. The results of this are structures consisting of four chromatids. Within each structure, chromatids start to swap corresponding segments. This is also known as “crossing-over”. The process of crossing over means that the genetic information present is then rearranged. This is how the cells that are produced become genetically different from their parent cells, and each other. (Simon et al., 2013).
The next step is Metaphase 1. During this stage the homologous chromosome pairs begin to line up along the equator of the cell. Spindle microtubules from one pole attach to the centromere of one chromosome, and spindle microtubules from the other pole attach to the centromere of the chromosome’s homologous pair. The centromere does not divide so the sister chromatids remain together. (Kent, 2013).
The homologous chromosomes begin to separate and migrate towards the opposite poles in which they were attached to. (Kent, 2013). The chromosomes are separated from their homologous partners, but the sister chromatids move as a single component. (Simon et al., 2013). This process is known as Anaphase 1.
The final stage of meiosis 1 is Telophase 1 and Cytokinesis. The chromosomes finally arrive at the poles of the cell. Once they arrive each pole consists of a set of haploid chromosomes, but the chromosomes are still doubled. Cytokinesis coincides with Telophase 1 and two haploid cells are produced. (Simon et al., 2013).
The process of Meiosis 2 results in the separation of the sister chromatids pairs that were separated from their homologous pairs in Meiosis 1. Meiosis 2 begins with a haploid cell that has not undertaken duplication of chromosomes during the Interphase period. This process is considerably similar to that of Meiosis 1. (Kent, 2013). A spindle forms during Prophase 2 which pushes the chromosomes into the centre of the cell. During Metaphase 2 the chromosomes align, with spindle microtubules from opposite poles attaching to the sister chromatids of each chromosome. The centromeres of the sister chromatids separate during Anaphase 2, and the sister chromatids of each pair begin to migrate towards the opposite poles. Finally, in Telophase 2 nuclei begin to form at the poles. Cytokinesis also occurs at this time, and the final result is four haploid daughter cells, each consisting of single chromosomes. (Simon et al., 2013).
The meiotic cell division process does not always run its course perfectly. When things go wrong during meiosis, the effects can be projected through chromosomal disease. Chromosomal disease can be fatal to an embryo. Qi et al, (2018) shows that during their study 42.95% of embryo miscarriage samples, out of 149 samples, all contained at least one chromosomal abnormality. Not all cases of chromosomal abnormalities are fatal, but there are a variety of problems that can occur. Errors in meiosis are referred to as nondisjunction. This means that during the meiotic cell division phase either the homologous chromosomes or sister chromatids failed to separate properly. Turner Syndrome is a condition that affects around 1 in every 2,500 female babies. The condition means that these female babies are born with only one X chromosome. Normally females are born with two X chromosomes. Because these females are lacking the second X chromosome, their ovaries normally degenerate before they are even born. This means that the females can not undergo puberty. The condition can be treated to an extent by providing oestrogen as a hormone treatment, these would stimulate secondary sexual characteristics such as enlarged breasts, but because the females do not have functioning ovaries they still would not be able to produce offspring. Other characteristics of Turner Syndrome can include; increased risks of cardiovascular disease, kidney defects and hearing loss. (Audesirk et al., 2017)
Another common chromosomal abnormality is Trisomy 21, or Down Syndrome. Trisomy 21 is a condition where by the offspring has accumulated an extra copy of chromosome 21. Children that are born with Trisomy 21 usually have very distinctive physical abnormalities. Some of these can include; a smaller mouth that can only open partially to try and accommodate for a much larger tongue, weak muscle tone and distinctive eye shape. These characteristics also go along side some degree of cognitive impairment (depending on the person) and learning difficulties. Over the years there has been an improvement of health and life longevity for people who suffer from Trisomy 21, but this increase in life longevity has also been linked to the rise in people with Trisomy 21 suffering from Alzheimer’s disease as they get older. Chromosomal abnormalities such as Trisomy 21, occur during meiotic cell division. Abnormalities can occur due to increased maternal age (Prenatal screening for Trisomy 21 is usually advised for mothers over the age of 35) but this isn’t always the case as babies who suffer from this condition have been born to mothers under the age of 35. (Diamandopoulos and Green, 2018).
In conclusion meiosis is vital for life to continue and succeed. Without the success of meiosis every species of living diploid organism would eventually become extinct, as organisms would not be able to further reproduce as a result of haploid gamete cells not being produced. Gamete cell production is essential for the natural continuation of a species. Although meiosis is vital for reproduction, it is also essential for genetic variability. Genetic variability allows new organisms to be produced with a completely unique set of DNA. Without genetic variability the risk of chromosomal disease can be much higher. But, chromosomal disease can occur in any new offspring, even with genetic variability present within their parents. With this in mind, chromosomal disease should continue to be researched to understand more as to why failures in meiosis can arise.
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