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Meiosis is defined as the process of cell division that results in the formation of a haploid “daughter” cell with the same haploid chromosomal number as the diploid “parent” (“original”) cell. After meiosis, the haploid cell would have just one portion of the parent cell’s multiple homologous chromosomal pairs.
What is Meiosis?
Meiosis is important because it decreases the number of chromosomes to half and allows for genetic diversity through genetic recombination and independent assortment.
Meiosis creates four haploid cells that may grow into potential gametes, resulting in a new individual with the complete number of genes when fertilisation occurs, preserving chromosomal number integrity over generations while increasing genetic variety and variability in population forms.
Meiosis comes from the Greek term meioun, which means “to diminish” (less).
The generation of gametes (egg cells or sperm cells) or spores is the primary purpose of meiotic division. The meiosis process occurs in the human body to reduce the number of chromosomes in a normal cell (46 chromosomes) to 23 chromosomes in eggs and sperm. As a result, the number of chromosomes in meiosis is cut in half.
As a result, when the two haploid cells merge during fertilisation, the number of chromosomes in the generated cell is restored as somatic cells (each with 46 chromosomes).
Meiosis is split into two stages: meiosis 1 and meiosis 2. Each component contains four stages (prophase, metaphase, anaphase, and telophase), which are identical to the four phases of mitosis.
The most complex portion of meiosis (i.e., meiosis I) is the initial part of the meiotic division. Because it comprises five substages: leptotene, zygotene, pachytene, diplotene, and diakinesis, prophase I takes roughly half the time it takes for meiosis. The chromosomes’ activity and structure change at each step, providing insight into prophase I’s intricacy.
Three major characteristics distinguish meiosis from mitosis:
1. Reverse recombination occurs during meiosis I. (also known as chiasma development or crossing across)
2. Meiosis I is characterised by the pairing of homologous chromosomes.
3. In Meiosis I, the cohesive sister chromatids are released in a two-step procedure.
On the mitotic spindle, these characteristics enable homologous segregation. The two sister homologous chromosomes separate during meiosis II. For the diverse events occurring in each meiosis stage, these varied chromosomal behaviours are detailed below. It’s also worth noting that these events are intertwined.
This indicates that distinct processes during chromosomal pairing, including reciprocal recombination, crossing-over, and chiasma formation, are linked. Hence, the only effective recombination mechanism during meiosis I prophase will be the one that provides proper homologous chromosome segregation at meiosis I.
The initial number of chromosomes is halved after the two subsequent chromosomal divisions. Meiosis I begins when the S phase is completed and the replication of the parent chromosome produces identical chromatids.
The chromosomes begin to couple together and eventually partition into two distinct cells. The chromatids, on the other hand, stay intact, and at the conclusion of meiosis I, each of the newly produced daughter cells will have one of the homologous chromosomes with two chromatids.
Meiosis II occurs after Meiosis I. After that, the two chromatids will split and form two daughter cells. As a result, four daughter haploid cells are formed at the end of meiosis II, each carrying one copy of each chromosome.
The pairing of homologous chromosomes after DNA replication not only allows for the segregation of meiotic chromosomes but also aids in the recombination of maternal and paternal chromosomes. This chromosomal pairing takes place during the prophase of meiosis I.
The first meiotic division (or meiosis I) and the second meiotic division (or meiosis II) are the two nuclear divisions that occur during meiosis (or meiosis II). The 4 key phases of meiosis are prophase, metaphase, anaphase, and telophase. They are also classified as I or II depending on whether they occur in meiosis I or II.
Consider this: if gametes (eggs and sperm) were generated only by mitotic division, rather than meiosis, the gametes would have the same number of chromosomes as diploid somatic cells. As a result, when the gametes unite during fertilisation, the resultant zygote will have four homologous chromosomal sets and will be tetraploid.
This “doubled chromosome content” situation will be passed down to future generations, resulting in chromosomal abnormalities. Throughout generations, the chromosomal number has fragmented and unkept. This is a chromosomal anomaly, to be sure.
Gametes are generated by the process of meiosis to keep the number of chromosomes constant in each generation. During the production of gametes, meiotic cell division reduces the number of chromosomes to haploid.
One cycle of chromosomal DNA replication precedes two rounds of nuclear division in meiosis. As a result of the parent cell’s meiotic division, four daughter nuclei (each of which is present in a new daughter cell) are generated.
Only a haploid number of chromosomes are found in each daughter cell nucleus. Gametes haploid cells are formed in two rounds: Meiosis I and II, with just one cycle of DNA replication (at the S phase of interphase).
Meiosis vs Mitosis
The two primary types of cell division are meiosis and mitosis. The distinctions between them are in mitosis only one nuclear division takes place and in meiosis two divisions. Meiosis produces haploid cell and they are sex cells, whereas in mitosis diploid cell are formed and are the somatic cells.
The end-product of mitosis is two daughter cells and in mitosis four daughter cells. Asexual reproduction takes place in mitosis and sexual reproduction in meiosis, thus crossing over takes place whereas in mitosis no crossing over is seen.
Phases of Meiosis
Meiosis is the process through which a parent diploid cell divides into four haploid cells. There really are two main phases of meiosis: meiosis I and meiosis II. Meiosis 1 is the first stage of meiotic division, often known as the reduction division of meiosis. This is due to the fact that the number of chromosomes is cut in half at this stage, resulting in the creation of haploid chromosomes.
In a process or occurrence known as a synapse, each pair of chromosomes comes close together to exchange a portion of their genetic material. This happens in the early phases of meiosis 1, especially during prophase I.
The homologous pairs of chromosomes come very close together and bond closely to each other during prophase 1 of meiosis I, and they virtually behave as one single unit. This unit is known as a bivalent or tetrad (indicating that each chromosome consists of two sister chromatids, so the sum of bivalent is four chromatids).
After aligning at the spindle equator, the bivalent separates into two pieces, allowing each chromosome to travel to the spindle pole on the opposing side. As a result, following meiosis I, each newly formed daughter nucleus is haploid, as it only carries one bivalent chromosome.
Meiosis II, the second stage of the meiosis cell cycle, is similar to mitosis in that it results in the formation of two daughter cells when the two chromatids separate.
As a result, meiosis I is the stage during which the meiosis cycle’s distinctive activities take place. Nonetheless, each step of the meiotic division, such as prophase, metaphase, anaphase, and telophase, is split in a manner that mimics the mitotic division.
The prophase of the first meiotic division, on the other hand, is far more complex and time-consuming than the prophase of mitosis. The prophase of the second meiotic division, on the other hand, is simpler and shorter.
Meiosis in a Nutshell
A cell’s chromosomes are replicated before it enters meiosis (during the interphase). The chromosomes condense along the nucleus’s centre during meiosis I and pair with their homologues during crossing over. The chromosomal pairs then split and travel to opposing ends of the cell.
For the first time, the cell splits into two cells. Both cells will go through meiosis II, when they will divide into two cells, each carrying one of each detached chromosome’s sister strands (chromatids), resulting in four genetically distinct haploid cells.
Meiosis I occurs after interphase, when the chromosomes duplicate during the S phase of the cell cycle. During the early stages of prophase, I, the chromosomes condense. The cell’s two centrosomes go to the cell’s two opposing poles to prepare it for nuclear division.
Pairs of chromatids make up homologous chromosomes. After pairing, these chromosomes form bivalents in order to align at the spindle equator during metaphase I.
During anaphase, homologous chromosomes are separated from one another, but the two sister chromatids remain connected.
i. Prophase I
The most difficult phase of meiosis I is prophase I, which is split into five stages: leptotene, zygotene, pachytene, diplotene, and diakinesis.
The chromatin fibres condense into thread-like fibres at the Leptotene stage, resembling the established structure at the onset of mitosis.
The fibres are more condensed at the zygotene stage, allowing them to be recognised as distinct chromosomes. The bivalents arise when the pairs of chromosomes get firmly linked together as a result of synapsis.
The development of bivalent is crucial in the process of crossing over, which involves the exchange of DNA segments holding genetic material between two nearby chromosomes. During the pachytene stage, this procedure occurs. For gene recombination, the matching regions of chromosomes exchange genetic information.
During the pachytene stage, chromosomes are compressed to roughly a fifth of their original length. The two chromosomes of each bivalent split from one another at the centrosome during the diplotene stage. Chiasmata, which are links found when two homologous chromosomes swap DNA segments, keep the two chromosomes together.
Diplotene is characterised by the resumption of transcription, the de-condensation of chromosomes, and the temporary halt of meiosis. When the chromosomes are re-condensed to their maximal level of compaction at the start of prophase I’s last step, diakinesis, the centrosomes move even faster.
Only the chiasmata keep the chromosomes together. Spindles develop, nucleoli vanish, and the nuclear envelope vanishes at this point. Meiosis prophase 1 ends and meiosis metaphase 1 begins when the meiotic spindle begins to develop and the nucleoli begin to disintegrate.
ii. Metaphase I
After attaching to the microtubules with their kinetochores, the bivalents migrate to the equator of the spindle during this phase. This bivalent migration to the cell’s equator generally occurs only during meiosis I and not during mitotic division.
Each bivalent includes four chromatids, which means each bivalent also contains four kinetochores. These kinetochores appear to be in close proximity to one another, as though they are a single unit facing the same cell pole.
Each kinetochore can be attached to the microtubules of the spindle pole on the opposite side using this configuration. This arrangement is the initial step in preparing the chromosomes for separation during the anaphase that follows.
The paternal and maternal chromosomes are aligned on one pole of the cell at this time, and each newly created daughter cell will get a combination of paternal and maternal chromosomes during their migration to the opposing poles during anaphase.
iii. Anaphase I
The movement of homologous chromosomes to the spindle poles with the help of their kinetochore is the first step in anaphase. One of the key distinctions between meiosis and mitosis is this stage.
During mitosis, the sister chromatids are pushed to opposing poles, causing them to split. The two sister chromatids stay connected during meiosis, and following separation, the homologous chromosomes migrate toward the spindle poles.
As a result, by the end of meiotic anaphase I, each spindle pole contains a haploid number of chromosomes. During mitosis, chromatid separation is accomplished by cleaving the two sister chromatids using an active enzyme called separase.
To counteract separase’s activity during meiosis, the cell generates shugoshin, a protein that inhibits chromatid separation by shielding the centrosomal region of the chromosome where the cleavage occurs.
iv. Telophase I
Telophase 1 is the last phase of meiosis I, and it is marked by the movement of chromosomes to the spindle poles. Before cytokinesis, a nuclear membrane might have been created around chromosomes, resulting in two daughter cells with haploid sets of chromosomes. After the start of meiosis II, the chromosomes usually condense.
Results of Meiosis I
By the completion of meiosis, I, cytokinesis has aided in the development of two haploid nuclei cells. Each haploid cell’s chromosomes will be made up of two chromatids joined at the centromere.
Meiosis II Stages
Interphase meiosis occurs between the conclusion of meiosis I and the start of meiosis II. This stage is not connected with DNA replication since each chromosome already has two chromatids that have previously been replicated by the DNA synthesis process before the start of meiosis I.
In a nutshell, DNA is duplicated once before meiosis begins. Meiosis II, also known as second mitotic division, serves a similar goal as mitosis in that two new chromatids are orientated in two new daughter cells. As a result, the second meiotic division is also known as the meiotic division of separation.
i. Prophase II
Prophase II is the stage that occurs after meiosis I or interkinesis, and it is marked by the breakdown of the nuclear envelope and nucleolus, as well as the thickness and shortening of the chromatids, and centrosome replication and migration to the polar side. Prophase II is less complicated and shorter than prophase I, and it resembles the mitotic prophase in appearance.
Prophase II, on the other hand, differs from prophase I in that chromosomal crossing occurs only during prophase I and not during prophase II. At the completion of prophase II, metaphase II begins.
ii. Metaphase II
Metaphase II of meiotic division is identical to metaphase II of mitotic division, except that metaphase II has half the number of chromosomes and is distinguished by chromosomal alignment in the cell’s centre.
iii. Anaphase II
It is the stage after metaphase II, during which the sister chromatids split and migrate towards the cell poles. Anaphase II is similar to mitotic anaphase in that both involve chromatid separation. The shortening of the kinetochore causes sister chromatids to migrate to the cell’s two ends.
iv. Telophase II
Telophase II is the final stage of meiosis, in which four haploid cells are generated from the two cells produced during meiosis I. The newly formed cells’ nuclear membranes are fully established, and the cells are entirely separated at the conclusion of this phase.
However, sperm in humans and other animals are not completely functional at the conclusion of telophase II because they require the development of flagella to operate correctly.
Results of Meiosis II
After telophase II and cytokinesis, four haploid cells are formed, each of which has just one of the two homologous pairs of chromosomes. The genetic information from the maternal and paternal chromosomes is mixed in the haploid cells formed. These cells have a role in both the genetic variety of individuals within the same species and the evolution of animals.
Meiosis is found in the life cycles of many creatures, including fungi, plants, algae, animals, and humans.
Meiosis can generate spores or gametes depending on the species, with gametes (sperm cells and egg cells) being produced in humans and other animals, while spores are produced in plants and algae.
Meiosis in Humans and Other Animals
In humans and other animals, meiosis generates haploid gametes. It is an important aspect of gametogenesis. Gametogenesis is the biological process of producing gametes, as the name suggests.
There are two types of gametogenesis in humans and other animals: spermatogenesis (the creation of male gametes, such as sperm cells) and oogenesis (the formation of female gametes, such as eggs) (formation of the female gamete, i.e., ovum or egg cell).
A diploid oocyte produces four haploid gamete cells during oogenesis. Only one cell survives to become an egg, while the other three become polar bodies.
This effect is caused by the oocyte’s uneven division during meiosis, in which one of the produced cells obtains the majority of the parent cell’s cytoplasm while the other generated cells degenerate, resulting in an increase in the concentration of nutrients in the formed egg. During prophase I of meiosis, the egg cell develops the majority of its specialised activities.
After meiosis and post-meiotic processes, such as spermiogenesis, when the sperm cell grows by obtaining a functioning flagellum and discarding much of its cytoplasm to form a compacted head, the sperm gets its specialised characteristics in order to develop into a functional gamete.
Meiosis is a process that happens throughout an organism’s reproductive period. However, in humans, meiotic division happens at various times. For example, in males, it begins during puberty and continues throughout their lives.
Females’ main oocytes will be stopped at prophase I by the time they reach adolescence, and they will proceed through the next phases of meiosis. However, each primary oocyte will cease at metaphase II of meiosis II when it develops into a secondary oocyte at ovulation.
Meiosis will only continue and finish during conception. Meiosis will stop if the secondary oocyte is not fertilised, and the arrested secondary oocyte will dissolve. Menstruation will start soon.
Meiosis in Plants and Algae
Plants and algae are multicellular creatures that produce both haploid and diploid cells throughout their lives. The haploid spores are generated by meiosis in this phenomenon known as alternation of generations. In plants and algae, this is also known as sporic meiosis.
The produced spores germinate and proceed through mitotic division, resulting in a haploid plant or algae. Because gametes are generated by mitotic division of already existing haploid cells, the haploid form is referred to as a gametophyte.
During fertilisation, the gametes unite to generate the diploid type of cells. Meiosis creates the spores from the diploid form. As a result, the diploid form is referred to as the sporophyte.
Meiosis in Fungi
In their life cycle, fungi have both asexual and sexual stages. The mycelium, for instance, may go through both sexual and asexual phases.
When haploid mycelia reach the sexual phase, they undergo plasmogamy (the union of two protoplasts) and karyogamy (the fusion of two protoplasts) (the fusion of two haploid nuclei). The development of the diploid zygote occurs after karyogamy.
The zygote develops into a stalked sporangium, which via meiosis produces haploid spores. Meiospores are the spores generated by meiosis, as opposed to mitospores, which are formed by mitosis. The sporangium will produce haploid spores (reproductive cells), each of which will germinate into a new mycelium.
Thus, in fungi, meiosis is the third step in the sexual phase’s sequential phases, which begin with plasmogamy and end with karyogamy. Meiosis is necessary for the fungus to return to its haploid stage.
Errors in Meiosis
Meiosis is prone to mistakes, which might have an impact on a person’s capacity to reproduce. Human longevity is severely harmed by abnormal meiosis. Infertility and the production of genetically unbalanced gametes can both be caused by errors in the meiosis stages.
Meiotic errors are the primary cause of congenital malformations as well as mental abnormalities in new born infants caused by genetic damage.
In more than 30% of human oocyte pachytene, errors in chromosomal pairing and recombination are present, resulting in a condition known as asynapsis, in which homologous chromosome pairing fails.
Failure of chromosomal pairing in yeast can cause cell death by triggering the cell’s checkpoints. A similar phenomenon may be seen in human germ cells. As a result of the rise in oocytes with chromosomal pairing faults, the number of germ cells will be depleted, resulting in early menopause in women.
Infertility results from mistakes in the phases of meiosis of spermatocyte development, as the quantity of functional sperm generated decreases.
Because a man generates around 300-400 million sperm each day, but a woman produces about 300-400 oocytes over her lifetime, depletion in the number of germ cells is more substantial in females than in males.
When chromosomal pairs fail to cross over properly during metaphase I, the unpaired chromosomes segregate randomly, increasing the chance of producing aneuploid gametes with an unbalanced number of chromosome copies. Furthermore, due to unsuccessful crossing-over, spermatocytes may be destroyed by apoptosis or necrosis.
Biological Importance of Meiosis
Because the balance between the number of chromosomes that are doubled during fertilisation and the halving of chromosomes during gamete production is maintained, meiosis and mitosis are two critical phases of the cell cycle for any organ that reproduces sexually.
A sexually reproducing organism’s cell cycle is divided into two distinct phases: haploid and diploid.
The haploid phase of meiosis begins with gamete creation and concludes with the development of a zygote during fertilisation, whereas the diploid phase begins with the formation of a zygote by the fusing of two gametes and terminates with meiotic cell division during gamete formation.
To complete the life cycle of sexually reproduced creatures such as humans and animals, meiotic division creates four haploid cells from one diploid cell.
The chromosomal DNA doubles in the parent diploid cell before meiosis, and four haploid nuclei are produced as a result of two subsequent diploid nucleus divisions.
Meiosis is physiologically significant since it is responsible for sexually reproduced organisms’ genetic variety, when the chromatids of two homologous chromosomes synapse and exchange portions of their genetic content during prophase I.
Meiosis is important because it decreases the number of chromosomes by half and allows for genetic diversity through genetic recombination and independent assortment.
Meiosis creates four haploid cells that may grow into potential gametes when fertilisation occurs, resulting in a new individual with the complete number of genes when fertilisation occurs, preserving chromosomal number integrity over generations while increasing genetic variety and variability in population forms.