Meiosis is the type of cell division that takes place to produce sex cells correctly. This division occurs mainly in two phases, meiosis I and meiosis II, and both are fundamental in gametogenesis (gamete formation). Depending on whether the formation of eggs or spermatozoa is involved, we distinguish between ovogenesis (or oogenesis) and spermatogenesis.
If there is an alteration of meiosis, the distribution of chromosomes will not be correct and a chromosomal abnormality will occur. This may be the reason for implantation failures, repeated miscarriages or fetal anomalies such as, for example, having a trisomy of chromosome 21.
Provided below is an index with the 7 points we are going to expand on in this article.
- 1.
- 1.1.
- 2.
- 2.1.
- 2.2.
- 2.3.
- 3.
- 3.1.
- 3.2.
- 4.
- 4.1.
- 4.2.
- 4.3.
- 4.4.
- 4.5.
- 5.
- 6.
- 7.
What does cell division consist of?
Cell division is the process by which, from a mother cell, two daughter cells with the same genetic information are obtained. Thanks to it, from a single cell (the zygote), a complete human being can be formed, with millions of different cells specialized in different functions.
Despite this specialization, it is important to note that all cells in the body have the same genetic information. The only exception is eggs and spermatozoa, which only have half, as we will see below.
This information is contained in chromosomes, which are located in the nucleus of cells. The reason why a skin cell is different from a liver cell is that, of all that common information, some of it is used and some of it is not. That is, the genes expressed in each cell are different and that is what makes them not equal to each other.
Diploidy
Somatic cells (i.e. all cells in our body, except sex cells) have all the information duplicated: they are diploid cells. The number of chromosome pairs is characteristic of each species. In humans, all somatic cells have 23 pairs of chromosomes, one of which is the sex chromosome pair (XX in females, XY in males).
Each chromosome is structurally the same as its partner: its homologous chromosome. However, although they have the same structure and the information they contain codes for the same characteristics or functions, their genetic information is different, since one comes from the father's sperm and the other from the mother's egg. Thus, an embryo will have a chromosome number 1 and a chromosome number 1' (one prime) and these have the same shape, but different content.
For example, a chromosome may have a gene involved in determining a particular physical trait, such as eye color. Its paired or homologous chromosome will also have information relating to that characteristic. However, this information does not have to be coincident: one may have the gene for blue eyes and the other for green eyes. Whether you finally have one color or the other will depend, among other things, on which of the two is dominant, which in this case would be green.
Importance of meiosis
As we have already mentioned, sex cells (eggs and sperm) are the only cells that are different from the rest of the body's cells in terms of chromosomal load, since they have half the genetic information. The process by which the reduction of information in gametes occurs is called meiosis.
This division occurs mainly in two phases: meiosis I and meiosis II. The first of these is very similar to mitosis, which is the type of division that occurs in all somatic cells in our body. Each of these phases, in turn, has four different stages (prophase, metaphase, anaphase and telophase) and, between meiosis I and II, a brief interphase occurs.
In the formation of an embryo, information from two gametes is fused. This makes it necessary that, for the creation of these cells, the genetic information is reduced by half, since otherwise, the embryos would exponentially increase their number of chromosomes.
Therefore, the biological function of meiosis is that the female and male gametes contain half as many chromosomes as the rest of the cells in the body (somatic cells) so that, when fertilization occurs, the resulting embryo has the correct genetic load and is viable.
This halving ensures that, if done correctly, all offspring have the same number of chromosomes: one half coming from their father and the other half from their mother.
Ovogenesis
In ovogenesis, eggs originate from a diploid mother cell, with 22 chromosome pairs and an XX pair. At the end of cell division, from each stem cell (ovogonia), four different cells arise that have half the information of the initial stem cell. Therefore, in the eggs there will no longer be two chromosomes regulating each characteristic, but only one, with no duplicated content.
In the case of women, in addition to the formation of eggs, the production of follicles will also take place in a process known as folliculogenesis. Follicles are responsible for surrounding, protecting and nourishing the oocyte. For more information on this topic, we recommend you visit the following article: What is folliculogenesis and what are its stages?
Spermatogenesis
In the formation of spermatozoa, the same occurs as in the previous case. Thus, if a somatic cell of a male has this chromosomal content: 1,1',2,2',3,3',4,4',5,5',6,6',7,7',8,8'..., the spermatozoa derived from it can be, for example, 1,2',3',4,5,6'....
This means that the genetic content of the gametes is formed randomly, with one member of each original pair of chromosomes. When joined with the reduced information from the oocyte, they will form a new set of 23 pairs, each pair consisting of one chromosome from the father and one from the mother.
You can learn more about the process of sperm formation at the following link: What is spermatogenesis?
Fertilization
Thanks to the explained mechanism, at fertilization, an egg with half the information fuses with a sperm that has also reduced its genetic information to half. Thus, the embryo resulting from this fertilization will have one chromosome from the father (2 or 2') and one chromosome from the mother (2 or 2'). For this reason, each characteristic will be regulated by two chromosomes again, with similar structure but with different information.
We invite you to continue reading the following article for more information: What is human fertilization and what are its stages?
Relationship of meiosis to fertility
As we have seen, meiosis is essential for reproduction, since it ensures the correct number of chromosomes in the offspring, compatible with life. However, sometimes this process is not carried out correctly and these alterations can compromise the viability of these individuals.
If the division of chromosomes is not carried out equally between the two daughter cells, problems may arise, since one of them will have more chromosomes and the other will have fewer. This means that the genes containing these chromosomes will also be unbalanced and, therefore, there will be functions that cannot be performed correctly.
For example, there may be proteins involved in development that are not produced in sufficient quantity or in excess, both of which are detrimental.
If meiosis is impaired, it can severely compromise fertility. For this reason, it is advisable to analyze the karyotype of couples who are going to undergo assisted reproduction treatments. In this way, it is possible to diagnose if the sterility is due to chromosomal causes and to indicate the best treatment.
Aneuploidies
When we speak of aneuploid cells, we refer to cells that have one or more extra chromosomes. They are caused by errors in cell division, either in mitosis or meiosis. If they occur during meiosis, the embryos resulting from fertilization of these gametes will also be aneuploid.
However, aneuploidy can also occur in embryonic developmental divisions. Morphologically, some of these embryos are of good quality. This means that, in in vitro fertilization (IVF) treatments, they cannot be detected without invasive preimplantation genetic diagnosis (PGD) methods.
To perform a PGD, one should undergo IVF as the main treatment. If you are looking for a clinic to get started, we recommend that you generate your individual Fertility Report now. It is a useful, simple tool that, in just 3 steps, will give you a list of the clinics that have passed our rigorous selection process. You will receive an email in your inbox with a report that contains tips and recommendations to get started.
Aneuploidies are one of the main causes of miscarriage, especially those occurring in the first trimester of pregnancy.
Therefore, in patients with recurrent miscarriages, PGD is indicated to increase the probability of success. Although most pregnancies with aneuploid fetuses do not carry to term, there are some trisomies (i.e. with an extra chromosome) that are compatible with life:
- Patau syndrome: trisomy of chromosome 13.
- Edwards' syndrome: trisomy of chromosome 18.
- Down syndrome: trisomy of chromosome 21.
If the aneuplodies affect the sex chromosomes (XXY, XXX, XYYY...), they can also be compatible with life. However, the resulting individuals will have their fertility compromised, as they are less likely to produce gametes with the correct genetic load.
Embryonic Ploidy
It can also happen that the error in gametogenesis occurs when an entire chromosome set is distributed, leaving a diploid gamete and another without chromosomes. In these cases, if fertilization occurs, the resulting embryos will also have one set of chromosomes too many or too few.
These embryos will not be viable and many of them will stop their development early. However, some triploid embryos (with three sets of chromosomes) are able to continue to evolve and even give rise to pregnancy, but are not compatible with life.
Therefore, it is very important in IVF treatments to evaluate whether fertilization has occurred correctly on day 1 of development, since this is the only time when these alterations can be detected. Embryos that are not diploid should be discarded.
FAQs from users
What is the importance of the phases of meiosis in reproduction?
Meiosis is a type of cell division that, in the case of humans, allows the formation of gametes, i.e., both the egg and the sperm. Therefore, meiosis allows sexual reproduction, although this involves fertilization or union of the egg and sperm. In this case, fertilization is not considered meiosis.
Is it recommended to perform a PGD whenever there is a risk of aneuploidy?
Yes, it’s the only technique that allows embryos with normal chromosomal dotation, without chromosomal aneuploidies.
How do I know if I have problems with meiosis?
If you are a man, by chromosomal analysis of a seminal sample it is possible to evaluate if there is a problem in this process.
The most common test is the FISH of spermatozoa.
In the case of females, it is more difficult to evaluate, since obtaining the eggs is a more expensive process. If meiosis is suspected to be altered and IVF treatment is performed, PGD can be performed to improve success rates.
If I have Down syndrome, can I have children?
The chances of a person with Down syndrome having offspring are lower, but it can happen. In addition, the risk that the offspring will also have Down syndrome is high.
Is gametogenesis the same as meiosis?
Gametogenesis and meiosis are different but closely related processes, since it is not possible to have gametogenesis without meiosis. Gametogenesis is the process by which new gametes are formed and, as we have already mentioned, they need to have half the chromosomal load of somatic cells. This reduction of information is carried out by meiosis, which is a type of cell division.
However, in gametogenesis there are also stages in which meiosis does not occur, but mitosis, which consists of a somatic mother cell giving rise to two somatic daughter cells.
Suggested for you
As mentioned above, one option to detect possible aneuploidy in IVF treatments is to perform a complementary PGD technique. If you wish to obtain more information about it, we advise you to read this article: What is preimplantation genetic diagnosis or PGD?
If you are interested in knowing more about the meaning of being fertile, then you will like this article: What does 'being fertile' mean and what factors influence fertility?
We make a great effort to provide you with the highest quality information.
🙏 Please share this article if you liked it. 💜💜 You help us continue!
References
Ewelina Bolcun-Filas, Mary Ann Handel. Meiosis: the chromosomal foundation of reproduction. Biol Reprod. 2018 Jul 1;99(1):112-126. doi: 10.1093/biolre/ioy021 (View)
Kai Xu, Ying Yang, Gui-Hai Feng, Bao-Fa Sun, Jun-Qing Chen, Yu-Fei Li, Yu-Sheng Chen, Xin-Xin Zhang, Chen-Xin Wang, Li-Yuan Jiang, Chao Liu, Ze-Yu Zhang, Xiu-Jie Wang, Qi Zhou, Yun-Gui Yang, Wei Li. Mettl3-mediated m6A regulates spermatogonial differentiation and meiosis initiation. Cell Res. 2017 Sep;27(9):1100-1114. doi: 10.1038/cr.2017.100. Epub 2017 Aug 15 (View)
Marta Wasielak-Politowska, Paweł Kordowitzki. Chromosome Segregation in the Oocyte: What Goes Wrong during Aging. Int J Mol Sci. 2022 Mar 7;23(5):2880. doi: 10.3390/ijms23052880 (View)
Mónica Pradillo, Juan L Santos. Genes involved in miRNA biogenesis affect meiosis and fertility. Chromosome Res. 2018 Dec;26(4):233-241. doi: 10.1007/s10577-018-9588-x (View)
Nina Reis Soares, Marcelo Mollinari, Gleicy K Oliveira, Guilherme S Pereira, Maria Lucia Carneiro Vieira. Meiosis in Polyploids and Implications for Genetic Mapping: A Review. Genes (Basel). 2021 Sep 27;12(10):1517. doi: 10.3390/genes12101517 (View)
FAQs from users: 'What is the importance of the phases of meiosis in reproduction?', 'Is it recommended to perform a PGD whenever there is a risk of aneuploidy?', 'How do I know if I have problems with meiosis?', 'If I have Down syndrome, can I have children?' and 'Is gametogenesis the same as meiosis?'.
Authors and contributors
Hi there! I’m interested in genetics and I need some help to clarify the concept about meiosis… Okay, let’s go:
As far as I’m aware, in a female baby all primordial follicles are arrested in prophase of meiosis I, so I undersand they have 46 4N, isn’t it? I don’t get this concept very well as it’s rather complicated. I hope you’re experts on genetics so please help!