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What is the result of meiosis?

What is the result of meiosis?


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Is the result of meiosis ONLY the sex gametes (male and female) which later meet to form a somatic cell? Sometimes I feel as if my book is hinting towards meiosis is the process where sperm cells meet oocytes?? I'm a bit confused.


In principle, Meiosis is only the process in which the haploid egg or sperm are generated. Have a look at this figure, which shows Meiosis I and II (from the Wikipedia):

During Meiosis I homologous recombination between homologous chromosomes can happen, the chromosomes are then distributed normally among the daughter cells. In Meiosis II the cells split again and here the sister chromatids are divided and the haploid gametes form.

In the oocytes the process is in principle the same, but the cells get arrested in the so-called diplotene stage of Meiosis I (picture from here):

In the diplotene stage the chromatin is decondensated and transcriptionally active, this helps maintaining a correct chromosome structure, as the diplotene stage can last up to 50 years. When the cell receives the hormonal signal to enter the fallopian tube, Meiosis I is finished. The resulting cells are not divided symmetrical, building one big oocyte and one polar body containing the second set of chromosomes. Most vertebrate cells are arrested in this stage again until they are fertilized. Then another asymmetrical division is done, leading to a haploid cell with two polar bodies. This oocyte is then fertilized.

This process can happen almost at the same time in a lot of vertebrate species, including humans. This is probably what was meant by your textbook.

For further reading I can recommend: Meiosis and Fertilization from the NIH bookshelf.


Meiosis

Meiosis
n., plural: meioses
[maɪˈəʊsɪs]
Definition: a specialized form of cell division that ultimately gives rise to non-identical sex cells
Image source: Modified by Maria Victoria Gonzaga, BiologyOnline.com, from the works of Marek Kultys (schematic diagram of meiosis), CC BY-SA 3.0.


Prophase I

Figure 1. Early in prophase I, homologous chromosomes come together to form a synapse. The chromosomes are bound tightly together and in perfect alignment by a protein lattice called a synaptonemal complex and by cohesin proteins at the centromere.

Early in prophase I, before the chromosomes can be seen clearly microscopically, the homologous chromosomes are attached at their tips to the nuclear envelope by proteins. As the nuclear envelope begins to break down, the proteins associated with homologous chromosomes bring the pair close to each other. Recall that, in mitosis, homologous chromosomes do not pair together. In mitosis, homologous chromosomes line up end-to-end so that when they divide, each daughter cell receives a sister chromatid from both members of the homologous pair. The synaptonemal complex , a lattice of proteins between the homologous chromosomes, first forms at specific locations and then spreads to cover the entire length of the chromosomes. The tight pairing of the homologous chromosomes is called synapsis . In synapsis, the genes on the chromatids of the homologous chromosomes are aligned precisely with each other. The synaptonemal complex supports the exchange of chromosomal segments between non-sister homologous chromatids, a process called crossing over. Crossing over can be observed visually after the exchange as chiasmata (singular = chiasma) (Figure 1).

In species such as humans, even though the X and Y sex chromosomes are not homologous (most of their genes differ), they have a small region of homology that allows the X and Y chromosomes to pair up during prophase I. A partial synaptonemal complex develops only between the regions of homology.

Figure 2. Crossover occurs between non-sister chromatids of homologous chromosomes. The result is an exchange of genetic material between homologous chromosomes.

Located at intervals along the synaptonemal complex are large protein assemblies called recombination nodules . These assemblies mark the points of later chiasmata and mediate the multistep process of crossover —or genetic recombination—between the non-sister chromatids. Near the recombination nodule on each chromatid, the double-stranded DNA is cleaved, the cut ends are modified, and a new connection is made between the non-sister chromatids. As prophase I progresses, the synaptonemal complex begins to break down and the chromosomes begin to condense. When the synaptonemal complex is gone, the homologous chromosomes remain attached to each other at the centromere and at chiasmata. The chiasmata remain until anaphase I. The number of chiasmata varies according to the species and the length of the chromosome. There must be at least one chiasma per chromosome for proper separation of homologous chromosomes during meiosis I, but there may be as many as 25. Following crossover, the synaptonemal complex breaks down and the cohesin connection between homologous pairs is also removed. At the end of prophase I, the pairs are held together only at the chiasmata (Figure 2) and are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible.

The crossover events are the first source of genetic variation in the nuclei produced by meiosis. A single crossover event between homologous non-sister chromatids leads to a reciprocal exchange of equivalent DNA between a maternal chromosome and a paternal chromosome. Now, when that sister chromatid is moved into a gamete cell it will carry some DNA from one parent of the individual and some DNA from the other parent. The sister recombinant chromatid has a combination of maternal and paternal genes that did not exist before the crossover. Multiple crossovers in an arm of the chromosome have the same effect, exchanging segments of DNA to create recombinant chromosomes.


Stages of Meiosis

Contrary to mitosis, meiosis [3] involves the division of diploid parental cells (both paternal and maternal) into haploid offspring with only one member of the pair of homologous chromosome from the parents.

1. Meiosis I

Just like in mitosis, a cell must first undergo through the interphase before proceeding to meiosis proper. It increases in size during G1 phase, replicates all the chromosomes during S phase, and makes all the preparations during the G2 phase. Like the usual mitosis, the first meiotic division is divided into four different stages: prophase I, metaphase I, anaphase I, and telophase I.

Meiosis I Stages (Source: Wikimedia)

Prophase I.

By far, prophase [4] I of meiosis is considered as the most complicated step in the whole process. As compared to mitotic prophase, the prophase of meiosis is definitely longer. In this stage of meiosis I, the chromosomes start to condense and pair up with its homologue. Basically, the first meiosis begins with a very long prophase that is divided into five phases: leptotene, zygotene, pachytene, diplotene, and diakinesis.

Leptotene

Leptotene is the first stage of prophase during meiosis I. This phase is characterized by the condensation of the chromosomes wherein they become visible as chromatin. It comes from the two Greek words “lepto” and “tene” which mean “thin” and “ribbon” respectively.

Zygotene

Coming from the Greek words “zygo” and “tene” which mean “union” and “thread“, zygotene is the second phase of prophase I. During this stage, homologous chromosomes begin to form an association called a synapse which results to pairs of chromosomes that has four chromatids.

Pachytene

This is the phase where the crossing over between pairs of homologous chromosomes occurs. The structure formed is referred to as the chiasmata. In contrast with leptotene (“thin thread“), the Greek word “pachy” means “thick“ thure referring to the characteristic of the chromosome in this stage.

Diplotene

In this phase, the separation of the homologous chromosomes is starting but they remain attached through the chiasmata. The word “diplo” in diplotene means “double“.

Diakinesis

Following diplotene is the final phase called diakinesis, which comes from the Greek words “dia” which means “across” and “kinesis” which means “motion“. This is when the homologous chromosomes continue to separate as the chiasmata move to the opposite ends of the chromosomes.

Metaphase I.

In this stage, the homologous pairs of chromosomes randomly align at the metaphase plate. [5] Such configuration becomes the source of genetic material as the chromosomes from the male and female parents appear similar but are not exactly identical.

Anaphase I.

The homologous chromosomes become separated as they are pulled toward the opposite ends [6] of the cell. However, the sister chromatids remain attached to their pair and do not move apart.

Telophase I.

Like in the telophase of mitosis, the chromosomes finally are separated at the different sides of the cell. In addition, the chromosomes return to their uncondensed forms as the nuclear membrane is reformed. The division of the cytoplasm (referred to as cytokinesis) occurs simultaneously with telophase I, resulting to two haploid daughter cells.

2.Meiosis II

Cells undergo through meiosis I to meiosis II without the replication of the genetic material. It is important to note that the cells that undergo meiosis II are the daughter cells produced during meiosis I. Meiosis II is shorter than meiosis I but still is divided into four stages: prophase II, metaphase II, anaphase II, and telophase II.

Meiosis II Stages (Source: Wikimedia)

Prophase II.

Prophase II is almost similar to mitotic prophase. In prophase II, the nuclear envelope disintegrates as the chromosomes condense [7] . Aside from that, the centrosomes separate with each other while the spindle fibers try to catch the chromosomes.

Metaphase II.

Just like in meiosis I, meiosis II [8] is when the chromosomes align at the metaphase plate because of the attachment of the spindle fibers to the centromeres of chromosomes. The segregation of different types of chromosome is what creates the difference between the two metaphases of meiosis.

Anaphase II.

Unlike anaphase I, which involves the separation of homologous chromosomes, anaphase II is the separation of sister chromatids. In this stage, chromosomes (each with a chromatid) are separated from each other as they move toward the opposite poles of the cell.


What is the purpose of meiosis quizlet?

The main purpose of meiosis is to create gametes, or sex cells like sperm and eggs.

Furthermore, what is the purpose of mitosis quizlet? List the two different reasons why cells need to undergo mitosis to produce new, identical cells? As organisms get bigger, growth happens in part through the creation of new cells through mitosis. Mitosis is also responsible for the replacement of cells when they die.

Simply so, what is the main function of meiosis quizlet?

To make gametes (sperm or egg cells). Chromosomes that have the same sequence of genes and the same structure. A cell that contains two complete sets of chromosomes, one from each parent.

What is the result of meiosis quizlet?

The result of meiosis is 4 gametes, or sex cells, that each contain half of the genetic information in the parent organism. A process in cell division during which the number of chromosomes decreases to half the original number.


How is Meiosis Different from Mitosis?

Mitosis is the production of two genetically identical diploid daughter cells from one diploid parent cell. Meiosis produces four genetically distinct haploid daughter cells from a single diploid parent cell. These germ cells can then combine in sexual reproduction to form a diploid zygote.

Meiosis only occurs in eukaryotic organisms which reproduce sexually, whereas mitosis occurs in all eukaryotic organisms, including those which reproduce asexually.

The table below summarizes the similarities and differences between meiosis and mitosis.

Meiosis

Mitosis

Similarities

Differences

Wrapping Up Meiosis and Biology

We now know that meiosis is the process of chromosomal reduction which allows the production of haploid germ cells necessary for sexual reproduction. Meiosis is furthermore important for its role in enabling genetic diversity and facilitating the repair of genetic defects through recombination.

The benefits that meiotic reproduction gives over mitotic reproduction are that mitotic reproduction produces identical cells, conserving the chromosomal set and the genes within, whereas meiosis allows for the expression of new traits because of the process of crossing over. Without meiosis maintaining genetic diversity within populations, organisms would not be able to adapt to suit their environment, nor evolve, nor survive catastrophic events. A population’s genetic diversity is its most reliable tool in the fight for the species’ survival.

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What happens if you have too many chromosomes?

If one of the original cells had an extra chromosome, the person will have trisomy. This is a type of aneuploidy. People with trisomies have three copies of a particular chromosome (instead of two). This means these individuals have a total of 47 chromosomes (n+1).

Trisomies are named after the chromosome pair that gets the extra chromosome. Trisomy 21 is a fairly common aneuploidy that involves an extra chromosome 21. This is also called Down syndrome. It affects about one in every 750 babies born in Canada. Children with Trisomy 21 may experience delays when learning to crawl, walk and speak. As they get older, they may have trouble with reasoning and understanding.

Two other examples are Trisomy 13 (Patau syndrome) and Trisomy 18 (Edward’s syndrome). They can both cause serious brain, heart and spinal cord defects. Many babies born with these syndromes only live a few days.


Failures in meiosis, which could result in chromosomal disease

Although the ability of an individual organism to reproduce is not essential to its survival, reproduction between different organisms is vital for the continuation of life on earth. In order for animals to be able to reproduce, they need working sexual reproductive organs which can produce gametes (ovum in females and spermatids in males) – these gametes are produced by a process called meiosis (Stauffer et al, 2018).

Meiosis is one of two types of cell division. The other type of cell division found in organisms is called mitosis, and it is the process by which a single cell divides, producing two genetically identical daughter cells. The main purpose of mitosis is for tissue growth and also to replace dead or worn out cells (Live Science, 2018). Mitosis is one of the phases of the cell cycle and it consists of five different stages. The first stage is prophase, followed by prometaphase, metaphase, anaphase and then telophase (Nature.com, 2019). There is also a further stage called cytokinesis, which begins towards the end of telophase .

Unlike mitosis, which consists of one division, meiosis involves two complex cellular divisions (Alberts et al, 2002). The first stage, (Meiosis 1), is made up of 4 stages: prophase 1 (which itself consist of five sub-phases), metaphase 1, anaphase 1 and telophase 1. Meiosis 1 is all to do with the separation of homologous chromosomes and the duplication of DNA. As well as this, the process of DNA shuffling allows genetic variation in species.

The five sub-phases of prophase 1 include leptotene, zygotene (where crossing over is initiated), pachytene, diplotene (where crossing over is completed) and finally diakinesis. Throughout these stages, the chromosomes condense and the nuclear membrane of the cell dissolves, which makes the chromosomes become gradually visible. The homologous chromosomes pair up and become bivalent, aligning with each other gene by gene. Recombination occurs and the non-sister chromatids exchange genes at corresponding segments of DNA – this produces recombinant DNA.
In metaphase 1, bivalent chromosomes line up on the metaphase plate, facing the opposite poles of the cell. Microtubules from opposing poles of the spindle fibres attach to each individual pair of homologous chromosomes. This is also the stage where independent assortment occurs (Chinnici et al, 2004). Independent assortment is where the chromosomes move around randomly to separate / opposing poles, which will eventually result in a variety of combinations of chromosomes in each gamete. This, combined with crossing over, is what causes genetic variation.
Anaphase 1 consists of the separation of the homologous chromosomes. The kinetochores retract, which pulls the chromosomes apart to the opposite poles. During this process, the sister chromatids remain associated at the centromere, which in turn results in their movement as one single unit towards the same pole that the spindle fibre is attached to.

In the final stage of meiosis 1 (telophase 1), each half of the cell now has a complete haploid set of chromosomes which have been duplicated. The nuclear membrane reforms and surrounds the two daughter nuclei that now exist. Finally, the chromosomes become less condensed.

At the same time as telophase 1 occurs, the cell is also undergoing cytokinesis. This is the process which produces the end products of meiosis 1 – two unidentical daughter cells with a complete set of chromosomes (46 chromosomes each).

The second stage of meiosis is called meiosis 2, and this is almost made up of the same stages as the first meiotic division. The stages are: prophase 2, metaphase 2, anaphase 2, telophase 2 and cytokinesis. Meiosis 2 resembles almost the exact same process as a normal mitotic division, apart from the fact that there is no chromosome division – instead of the separation of homologous chromosomes it is about the separation of the sister chromatids. Meiosis 2 begins with prophase 2 which starts immediately after interkinesis (the phase between meiosis 1 and meiosis 2). In prophase 2, the nuclear membrane dissolves again and the chromosomes become compact like in prophase 1. The main difference is that a spindle apparatus forms and each chromosome remains as a composition of two sister chromatids attached at the centromere.

The next stage of meiosis 2 is metaphase 2, which is where the chromosomes line up at the equator and microtubules from opposing poles of the spindle attach to the kinetochores of the sister chromatids. This stage is followed by anaphase 2, which is where the centromeres split, resulting in the separation of the sister chromatids. These then move to opposite poles of the cell.

Telophase 2 is where the nuclear membrane and nucleolus reappear, forming 4 haploid nuclei. These are then cleaved apart to form a tetrad of cells in cytokinesis, which produces the end result of meiosis: 4 non-identical haploid daughter cells (gametes), each cell containing 23 chromosomes. In a male, one meiotic division produces four spermatozoa cells, whereas in a female, meiosis produces one ovum and three polar bodies.

The gametes produced by meiosis (ovum and sperm cells), combine to make a zygote during sexual reproduction. The halving of the number of chromosomes in the gametes during meiosis ensures that the zygotes have the same number of chromosomes from each generation to the next. However, meiosis does not always produce the right results. In some cases, there can be failures at any stage in the meiotic divisions which can in turn lead to the offspring having a chromosomal disease. Chromosomal disorders or abnormalities can be caused by the deletion, duplication or alteration of either an entire chromosome or a large part of one. Any changes to the volume of chromosomal material, whether it be an increase or a decrease, can interfere with the normal development and function of an organism. Although each different type of failure in meiosis on individual chromosomes causes a specific set of physical symptoms, the severity of the condition can vary.

A well known example of a chromosomal disease caused by failures in meiosis is Edward’s Syndrome (Trisomy 18). This disorder is caused by the presence of all, or part of, a third copy of the 18 th chromosome. Trisomy 18 is the second most common autosomal trisomy in newborn children. The vast majority of those who suffer from Edward’s Syndrome have it as a result of maternal nondisjunction of chromosome 18 (Gaw & Platt, 2018). This means that either the homologous chromosomes fail to separate during the anaphase 1 stage of meiosis, or the sister chromatids fail to separate during the anaphase 2 stage of meiosis. Half of all babies born with Edward’ Syndrome die within less than one week of birth, and between 5% and 10% of these babies live for no longer than one year (Perlstein, n.d.).

Another example of a chromosomal disease caused by failures in meiosis is Wolf-Hirschhorn Syndrome (WHS), or 4p deletion syndrome. This disorder is caused by partial deletion of genetic material near to the end of the short arm of chromosome 4. Wolf-Hirschhorn Syndrome was the very first example of a human chromosomal deletion syndrome, however it is extremely rare in comparison with other chromosomal diseases (Lee & Van Den Veyver, 2018). Some symptoms of WHS include serious prenatal growth restriction, severe seizures and predominant or deformed facial features. Like Trisomy 18, 4p deletion syndrome can be diagnosed through a series of ultrasound findings, and its diagnosis is confirmed by certain genetic testing.

Overall, meiosis is the fundamental process in providing genetic variation, as well as ensuring that generations carry the same number of chromosomes between generations. This is critical for stable sexual reproduction through successive generations. Without meiosis occurring, no organism would be fertile and therefore there would be no continuation of life on earth. Not only is the process of meiosis vital in ensuring that there is a continuation of life, but also if there is even one tiny failure at any given point during the meiotic divisions, it can have a significant impact on the life of the offspring inheriting that gene.

Alberts, B., Johnson, A., Lewis, J. et al. (2002) Meiosis. Molecular biology of the cell. 4 th edition.


Early in meiosis I, the chromosomes can be seen clearly microscopically. As the nuclear envelope begins to break down, the proteins associated with homologous chromosomes bring the pair close to each other. The homologous chromosomes now become arranged in the center of the cell, with the ends of each pair of homologous chromosomes facing opposite poles. The orientation of each pair of homologous chromosomes at the center of the cell is random.

This randomness, called independent assortment, is the physical basis for the generation of the second form of genetic variation in offspring. Consider that the homologous chromosomes of a human are originally inherited as two separate sets, one from each parent. One set of 23 chromosomes is present in the egg donated by the mother. The father provides the other set of 23 chromosomes in the sperm that fertilizes the egg. These pairs line up at the midway point between the two poles of the cell, and their arrangement in regard to the two poles is random. Any maternally inherited chromosome may face either pole. Any paternally inherited chromosome may also face either pole. The orientation of each pair is independent of the orientation of the other 22 pairs.

In each cell that undergoes meiosis, the arrangement of the chromosomes is different. There are two possibilities for orientation (for each pair) thus, the possible number of alignments equals 2 n where n is the number of chromosomes per set. Humans have 23 chromosome pairs, which results in over eight million (2 23 ) possibilities. Other mechanisms not discussed in this class can increase the variation in each cell produced as well. Given both independent assortment and these other mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition.

Next, protein fibers in the cell pull the linked chromosomes apart. The fibers pull the chromosome to the opposite poles of the cell. At each pole, there is just one member of each pair of the homologous chromosomes, so only one full set of the chromosomes is present. This is why the cells are considered haploid—there is only one chromosome set, even though there are duplicate copies of the set because each homolog still consists of two “sister: chromatids that are still attached to each other. Cytokinesis, where the cell membrane pinches off in the center of the cell to separate it into two, now occurs, splitting each cell into two cells containing one full set of chromosomes.

Concept in Action


Why is Meiosis Important in Survival of Life?

Meiosis is a phase in sexually reproductive organisms, wherein cell-division takes place. It is of great importance, because it creates genetic diversity in the population.

Meiosis is a phase in sexually reproductive organisms, wherein cell-division takes place. It is of great importance, because it creates genetic diversity in the population.

Meiosis is a process of gamete formation in which diploid germ-line cells, i.e., the cells that are set aside early in animal development for sexual reproduction, yield four genetically different haploid cells. It occurs only in sex cells, which are eggs and sperms.

Phases

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Meiosis takes place in two stages – Meiosis I, where DNA replication takes place and crossing-over occurs and Meiosis II, which lacks DNA replication, but is similar to Mitotic cell division.

The Process

  • In meiosis, during the formation of gametes in animals and spores in plants, the chromosome number is reduced to half. These chromosomes contain the basic DNA chain.
  • During the first meiotic reduction division, the chromosomal pairs are divided so that each gamete or spore contains one of each chromosomal pair, it becomes a haploid.
  • When haploid gametes unite during fertilization, they form a zygote. Zygotes, having received one chromosome of each pair from each parent become diploid.
  • Meiosis involves two successive nuclear divisions, which produce four haploid cells. The meiosis I is the reduction division, meiosis II separates the chromatids, which are the daughter strands of a duplicated chromosome joined together by a centromere.
  • In mitotic cell division, new cells genetically identical to the parent cell are produced. Meiosis is responsible for increasing genetic variation in the population.
  • Each diploid cell, which undergoes meiosis can produce 2n different chromosomal combinations, where ‘n’ is the haploid number.
  • In humans, the number is 223, because there are 23 pairs of chromosomes. This number is greater than eight million different combinations.
  • The variation increases, because, during meiosis I, each pair of homologous chromosomes comes together.
  • In a process known as synapsis, each pair of homologous chromosomes may exchange parts.
  • The relative distance between two genes on a given chromosome can be estimated by calculating the percentage of crossing-over that takes place between them.

Tasks of Meiosis

  • Production of haploid gametes to maintain the diploid number of species, generation after generation.
  • Crossing-over, which brings together new gene combination of chromosomes.
  • A mechanism for comparing the two copies of each chromosome, provided with the purpose of error correction or repairing.

Importance

  • In meiosis, variation occurs, because each gamete (either sperm or egg) contains a mixture of genes from two different parent chromosomes in sexual reproduction. In other words, the genetic coupling of non-identical DNA takes place in meiosis.
  • It results in an offspring, which has the genetic material of two different individuals.
  • These chromosomes contain the basic DNA chain, which determines the physical and genetic characteristics of the child.
  • A new combination of genetic information is produced in the gametes. Therefore, in meiosis, the characteristics of parent chromosomes are combined with the characteristics of offspring chromosomes, which ultimately results in a new and unique set of chromosomes.
  • It enables individuals to produce physically and genetically unique offspring. Because of this, a high genetic diversity of a population is maintained.
  • With mitosis only division would have been possible and there would have been no sharing of genetic information.
  • In such a situation, there would have been only clonal populations, which would eventually suffer from diseases or natural disasters.
  • What is the explanation for the diversity in populations? How can they survive variations in the environment? The reason is meiosis. Genetic variation plays the role of a raw material for natural selection.
  • Some individuals who are favored by natural selection have greater fitness than others because of their alleles (pair of alternative forms of gene).
  • In case of animals, males that are unable to compete for mates, for example, succumb to predation or disease or fail to reproduce small and weak organisms don’t survive for long time. These are the best examples of natural selection.
  • You can also take an example of a disease to which some individuals will be at least partially resistant while others are susceptible to it.
  • A population can adapt to changes in the environment as a result of the genetic variation resulting from meiosis. However, in clonal asexual populations, organisms are not able to adapt to changes without mutations.
  • Organisms which adapt to changes in the environment, survive, while others get eliminated by natural selection. In this way, a population contains fit individuals and the process continues for generations together.
  • The diversity afforded by meiosis is beneficial for the population as a whole.

Thus, meiosis helps to create a population that is not only physically and genetically different but also one, which is perfectly fit to survive.

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Watch the video: Meiosis Updated (May 2022).


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