Information

Do the four cells produced by meiosis each divide into four cells again?


In the beginning of meiosis, there is one cell. During meiosis, 1 cell divides into 4 cells. Does each of these 4 cells divide each into 4 more cells?


No, meiosis is not a cycle like mitosis. In mitosis, haploid or diploid cells divide to create two genetically identical cells, and this process can go on and on. On the other hand, meiosis results in 4 genetically unique daughter cells which are also haploid. If a haploid cell was to try to undergo meiosis, there would not be enough genetic material and you would end up with half-chromatids.


In species where an individual can be haploid (for example in haplodiploidy) then cells produced by meiosis do multiply by mitosis in order for the individual to grow.

From a brief scan the Wikipedia article on ploidy nicely summarises the various possibilities.


6.3: Meiosis Protocol

Almost all the cells in your body were produced by mitosis. The only exceptions are the gametes&ndash sperm or eggs &ndash which are produced by a different type of cell division called meiosis.

During fertilization, the sperm and egg unite to form a single cell called the zygote which contains all the chromosomes from both the sperm and the egg. The zygote divides into two cells by mitosis. Then, these cells each divide by mitosis. This process repeats many times to produce the cells in an embryo which develops into a baby.

1. Each cell in a normal human embryo has 23 pairs of homologous chromosomes, for a total of 46 chromosomes per cell. How many chromosomes are in a normal human zygote? Explain your reasoning.

2. What would happen if human sperm and eggs were produced by mitosis? Explain why this would result in an embryo which had double the normal number of chromosomes in each cell. A human embryo with that many chromosomes in each cell would be abnormal and would die before it could develop into a baby. So, gametes can not be made by mitosis.

3. Each human sperm and egg should have ____ chromosomes, so fertilization will produce a zygote with ____ chromosomes this zygote will develop into a healthy embryo with ____ chromosomes in each cell.

4. Each sperm and each egg produced by meiosis has only one chromosome from each pair of homologous chromosomes. When a sperm and egg unite during fertilization, the resulting zygote has ____ pairs of homologous chromosomes. For each pair of homologous chromosomes in a zygote, one chromosome came from the egg and the other chromosome came from the _______________. A cell that has pairs of homologous chromosomes is diploid. A cell that has only one chromosome from each pair of homologous chromosomes is haploid.

5. Next to each type of cell in the above chart, write:

  • The number of chromosomes in that type of cell
  • A d for diploid cells or an h for haploid cells.

How do cells divide?

There are two types of cell division: mitosis and meiosis. Most of the time when people refer to “cell division,” they mean mitosis, the process of making new body cells. Meiosis is the type of cell division that creates egg and sperm cells.

Mitosis is a fundamental process for life. During mitosis, a cell duplicates all of its contents, including its chromosomes, and splits to form two identical daughter cells. Because this process is so critical, the steps of mitosis are carefully controlled by certain genes. When mitosis is not regulated correctly, health problems such as cancer can result.

The other type of cell division, meiosis, ensures that humans have the same number of chromosomes in each generation. It is a two-step process that reduces the chromosome number by half—from 46 to 23—to form sperm and egg cells. When the sperm and egg cells unite at conception, each contributes 23 chromosomes so the resulting embryo will have the usual 46. Meiosis also allows genetic variation through a process of gene shuffling while the cells are dividing.


Frequently Asked Questions on Meiosis

What is the process of meiosis?
Answer: Meiosis is a process in which a single cell divides twice to produce four cells that contain half the amount of genetic information. a cell during meiosis? Divides twice to form four daughter cells. These four daughter cells have only half the number of chromosomes.


For whom is meiosis done?
Answer: Meiosis, on the other hand, is used in the human body for only one purpose: the gametes - the sex cells, or the production of sperm and eggs. The goal is to make daughter cells with about half the chromosomes as the starting cells.


How many stages of meiosis?
Answer: Eight steps
Meiosis I has Prophase I, Metaphase I, Anaphase I and Telophase I. Meiosis II includes Prophase II, Metaphase II, Anaphase II and Telophase II. These 8 sub-stages are often called the eight stages of meiosis.


Where does meiosis occur?
Answer : Meiosis occurs in the reproductive organs of an organism. For females, meiosis occurs in the ovary, where the egg is produced.


Why meiosis is so important?
Answer : Meiosis is important because it ensures that all organisms produced through sexual reproduction have the correct number of chromosomes. Meiosis also leads to genetic variation through the process of recombination.


What is female meiosis?
Answer: The process of meiosis in females is called oogenesis, as it produces oocytes and eventually produces mature ova (eggs). The male counterpart is spermatogenesis, the production of sperm.


What is different during meiosis?
Answer: Homologous pairs of cells are present in meiosis I and diverge into chromosomes before meiosis II. In meiosis II, these chromosomes are further separated into sister chromatids.


What are the two main functions of meiosis?
Answer: The two major functions of meiosis are to halve the DNA content and to alter the genetic material of the organism to generate genetic diversity between preterm births.


How long does meiosis occur in human women?
Answer: About 74 hours
The process of complete meiosis in human males takes about 74 hours. Spermatogenesis usually begins at the age of 12󈝹 years and continues throughout life. Several hundred million sperm cells are produced daily by healthy young adult males. There are typically 200 and 600 million sperm cells released in each ejaculation.


How does meiosis occur in humans?
Answer: Meiosis in humans is the process by which sperm cells and ovaries are produced. In the male, meiosis occurs after puberty. The diploid cells within the testis undergo meiosis to form haploid sperm cells with 23 chromosomes. A single diploid cell produces four haploid sperm cells through meiosis.


How many chromosomes are there in meiosis?
Answer: During the process of meiosis, the number of chromosomes decreases from 46 (23 pairs) to 23. Because somatic cells have half their total chromosomes, they are called haploid (n). A human egg or sperm has 23 chromosomes, one of which is X or Y.


What is the simple explanation of meiosis?
Answer: Meiosis is a process in which a single cell divides twice to produce four cells that contain half the amount of genetic information. These cells are our sex cells - sperm in men, eggs in women.


Can haploid cells undergo meiosis?
Answer: In short, you haploid cells cannot undergo meiosis because you cannot divide a cell with 23 chromosomes and this DNA, while mimicking a haploid cell, translates into a daughter cell, which is the haploid cells. With the same genome preventing genetic variation.


What would happen without meiosis?
Answer: Without meiosis, organisms will not be able to breed effectively. If organisms do not undergo mitosis, they will not be able to grow and replace the damaged cells. They are two of the most important cellular processes in existence.


At what age does meiosis occur?
Answer: In men, meiosis occurs after puberty. The diploid cells within the testis undergo meiosis to form haploid sperm cells with 23 chromosomes. A single diploid cell produces four haploid sperm cells through meiosis. In women, meiosis begins during the embryonic stage when a series of diploid cells enter meiosis.


What are the 5 differences between mitosis and meiosis?
Answer: Two daughter cells arise after mitosis and cytoplasmic division, while four daughter cells arise after meiosis. Daughter cells resulting from mitosis are diploid, whereas cells arising from meiosis are haploid.


Why do cells divide 4 times in meiosis instead of twice?

Meiosis:
DNA replicates then cell divides
The 2 new cells divide again without replication
Each of the 4 daugher cells has half the normal number of chromosomes (compared to a body cell). These are called gametes (or sex cells)

A daugher cell just means the cell created by replication. ie. one cell splits into two daughter cells

In meiosis division happens twice so that each cell can have half the number of chromosomes. This is because when two gametes join together in fertilisation, they create a cell with a full set of chromosomes, which eventually divides into an embryo

Haha, I'm just studying this right now. What a coincidence.
There are TWO DIVISIONS in meiosis NOT FOUR.
There are FOUR CELLS PRODUCED AS A RESULT OF MEIOSIS.

This is because meiosis produces gametes (sex cells) in the ovaries and testes which have half the usual number of 46 chromosomes, so that's 23 chromosomes. So, when the sperm fertilises the egg cell (note that sperm and egg cells are gametes) the zygote which is the name for a fertilised egg cell, will now combine both 23 chromosomes from the sperm and 23 from the ovaries to produce 46 chromosomes. Which ALL BODY CELLS CONTAIN.

It may be a bit confusing at first, but, just go over it and feel free to ask any more questions! ^-^

They need to multiply so that they can swap sections of DNA with other chromosomes to give variation in the half set of DNA in gametes, otherwise if the same two parents had a child, the two children would both be genetically identical.


I think this should help answer your question, because if they only divided once to form two gametes, there'd be no way to swap the dna sections to give variation in the DNA included in each gamete

Ask me specific questions about what I said if it doesn't make sense to you btw

(Original post by Nayzar)
In very simple terms:

Mitosis:
DNA Replicates then cell divides
Each daughter cell has normal number of chromosomes

Meiosis:
DNA replicates then cell divides
The 2 new cells divide again without replication
Each of the 4 daugher cells has half the normal number of chromosomes (compared to a body cell). These are called gametes (or sex cells)

A daugher cell just means the cell created by replication. ie. one cell splits into two daughter cells

In meiosis division happens twice so that each cell can have half the number of chromosomes. This is because when two gametes join together in fertilisation, they create a cell with a full set of chromosomes, which eventually divides into an embryo

(Original post by tereziscool)
Haha, I'm just studying this right now. What a coincidence.
There are TWO DIVISIONS in meiosis NOT FOUR.
There are FOUR CELLS PRODUCED AS A RESULT OF MEIOSIS.

This is because meiosis produces gametes (sex cells) in the ovaries and testes which have half the usual number of 46 chromosomes, so that's 23 chromosomes. So, when the sperm fertilises the egg cell (note that sperm and egg cells are gametes) the zygote which is the name for a fertilised egg cell, will now combine both 23 chromosomes from the sperm and 23 from the ovaries to produce 46 chromosomes. Which ALL BODY CELLS CONTAIN.

It may be a bit confusing at first, but, just go over it and feel free to ask any more questions! ^-^

(Original post by longshot100)
They need to multiply so that they can swap sections of DNA with other chromosomes to give variation in the half set of DNA in gametes, otherwise if the same two parents had a child, the two children would both be genetically identical.


I think this should help answer your question, because if they only divided once to form two gametes, there'd be no way to swap the dna sections to give variation in the DNA included in each gamete

Ask me specific questions about what I said if it doesn't make sense to you btw

They are different because the exchange of genetic material is random. This by the way happens in one single parent, so when we talk about maternal and paternal chromosomes we mean the chromosomes coming from the resulting baby's grandparents if that makes sense.

Also apart from this variation being produced this two divisions thing might be necessary to line up the chromosomes correctly so that you actually get all 23 chromosomes in each cell. But as I said there is a lot we don't know about how meiosis works.

The swapping of sections means that there is more variation in gametes. The same happens in the other parent.

Therefore if the same two humans reproduce again, the gametes that fuse will not be the same as the ones that fused to make the first child, you see?

So imagine that you had one kid with your partner, and your gametes didn't switch DNA sections during meiosis. All of your gametes would have a more limited combination of DNA sections if they didn't swap sections.

So let's say you decide to have another kid with the same partner. Without proper meiosis, the kid would have a good chance of being identical (or very similar) to the first, whereas if you had proper meiosis, it would be very hard to be extremely genetically similar.

Because of the difference, in the harsh world out there, if the first kid wasn't fit enough, at least the second one would survive. If both were the same, neither would survive.

TLDR- the way meiosis occurs means there is more variation in gametes, which means more variation in offspring.
More variation in offspring= better chance of some of your offspring surviving = better chance of the race surviving due to variation.

Sorry if I ended up repeating or paraphrasing some of the things I said before.

(Original post by longshot100)
The swapping of sections means that there is more variation in gametes. The same happens in the other parent.

Therefore if the same two humans reproduce again, the gametes that fuse will not be the same as the ones that fused to make the first child, you see?

So imagine that you had one kid with your partner, and your gametes didn't switch DNA sections during meiosis. All of your gametes would have the same combination of DNA, or at most, two different combinations.
So let's say you decide to have another kid with the same partner. Without proper meiosis, the kid would be identical to the first, whereas if you had proper meiosis, it would be different.

Because of the difference, in the harsh world out there, if the first kid wasn't fit enough, at least the second one would survive. If both were the same, neither would survive.

TLDR- the way meiosis occurs means there is more variation in gametes, which means more variation in offspring.
More variation in offspring= better chance of some of your offspring surviving = better chance of the race surviving due to variation.

Sorry if I ended up repeating or paraphrasing some of the things I said before.

Oh yeah, I never thought about if that. Thanks. I'm pretty sure I get it now

(Original post by longshot100)
The swapping of sections means that there is more variation in gametes. The same happens in the other parent.

Therefore if the same two humans reproduce again, the gametes that fuse will not be the same as the ones that fused to make the first child, you see?

So imagine that you had one kid with your partner, and your gametes didn't switch DNA sections during meiosis. All of your gametes would have the same combination of DNA, or at most, two different combinations.
So let's say you decide to have another kid with the same partner. Without proper meiosis, the kid would be identical to the first, whereas if you had proper meiosis, it would be different.

Because of the difference, in the harsh world out there, if the first kid wasn't fit enough, at least the second one would survive. If both were the same, neither would survive.

TLDR- the way meiosis occurs means there is more variation in gametes, which means more variation in offspring.
More variation in offspring= better chance of some of your offspring surviving = better chance of the race surviving due to variation.

Sorry if I ended up repeating or paraphrasing some of the things I said before.


7.2 Meiosis

Sexual reproduction requires fertilization , a union of two cells from two individual organisms. If those two cells each contain one set of chromosomes, then the resulting cell contains two sets of chromosomes. The number of sets of chromosomes in a cell is called its ploidy level. Haploid cells contain one set of chromosomes. Cells containing two sets of chromosomes are called diploid. If the reproductive cycle is to continue, the diploid cell must somehow reduce its number of chromosome sets before fertilization can occur again, or there will be a continual doubling in the number of chromosome sets in every generation. So, in addition to fertilization, sexual reproduction includes a nuclear division, known as meiosis, that reduces the number of chromosome sets.

Most animals and plants are diploid, containing two sets of chromosomes in each somatic cell (the nonreproductive cells of a multicellular organism), the nucleus contains two copies of each chromosome that are referred to as homologous chromosomes. Somatic cells are sometimes referred to as “body” cells. Homologous chromosomes are matched pairs containing genes for the same traits in identical locations along their length. Diploid organisms inherit one copy of each homologous chromosome from each parent all together, they are considered a full set of chromosomes. In animals, haploid cells containing a single copy of each homologous chromosome are found only within gametes. Gametes fuse with another haploid gamete to produce a diploid cell.

The nuclear division that forms haploid cells, which is called meiosis, is related to mitosis. As you have learned, mitosis is part of a cell reproduction cycle that results in identical daughter nuclei that are also genetically identical to the original parent nucleus. In mitosis, both the parent and the daughter nuclei contain the same number of chromosome sets—diploid for most plants and animals. Meiosis employs many of the same mechanisms as mitosis. However, the starting nucleus is always diploid and the nuclei that result at the end of a meiotic cell division are haploid. To achieve the reduction in chromosome number, meiosis consists of one round of chromosome duplication and two rounds of nuclear division. Because the events that occur during each of the division stages are analogous to the events of mitosis, the same stage names are assigned. However, because there are two rounds of division, the stages are designated with a “I” or “II.” Thus, meiosis I is the first round of meiotic division and consists of prophase I, prometaphase I, and so on. Meiosis I reduces the number of chromosome sets from two to one. The genetic information is also mixed during this division to create unique recombinant chromosomes. Meiosis II , in which the second round of meiotic division takes place in a way that is similar to mitosis, includes prophase II, prometaphase II, and so on.

Interphase

Meiosis is preceded by an interphase consisting of the G1, S, and G2 phases, which are nearly identical to the phases preceding mitosis. The G1 phase is the first phase of interphase and is focused on cell growth. In the S phase, the DNA of the chromosomes is replicated. Finally, in the G2 phase, the cell undergoes the final preparations for meiosis.

During DNA duplication of the S phase, each chromosome becomes composed of two identical copies (called sister chromatids) that are held together at the centromere until they are pulled apart during meiosis II. In an animal cell, the centrosomes that organize the microtubules of the meiotic spindle also replicate. This prepares the cell for the first meiotic phase.

Meiosis I

Early in prophase 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 tight pairing of the homologous chromosomes is called synapsis . In synapsis, the genes on the chromatids of the homologous chromosomes are precisely aligned with each other. An exchange of chromosome segments between non-sister homologous chromatids occurs and is called crossing over . This process is revealed visually after the exchange as chiasmata (singular = chiasma) (Figure 7.3).

As prophase I progresses, the close association between homologous chromosomes begins to break down, and the chromosomes continue to condense, although the homologous chromosomes remain attached to each other at chiasmata. The number of chiasmata varies with the species and the length of the chromosome. At the end of prophase I, the pairs are held together only at chiasmata (Figure 7.3) 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 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, it will carry some DNA from one parent of the individual and some DNA from the other parent. The recombinant sister chromatid has a combination of maternal and paternal genes that did not exist before the crossover.

The key event in prometaphase I is the attachment of the spindle fiber microtubules to the kinetochore proteins at the centromeres. The microtubules assembled from centrosomes at opposite poles of the cell grow toward the middle of the cell. At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome attached at one pole and the other homologous chromosome attached to the other pole. The homologous chromosomes are still held together at chiasmata. In addition, the nuclear membrane has broken down entirely.

During metaphase I, the homologous chromosomes are arranged in the center of the cell with the kinetochores 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 sexually reproducing organism are originally inherited as two separate sets, one from each parent. Using humans as an example, 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. In metaphase I, these pairs line up at the midway point between the two poles of the cell. Because there is an equal chance that a microtubule fiber will encounter a maternally or paternally inherited chromosome, the arrangement of the tetrads at the metaphase plate is random. Any maternally inherited chromosome may face either pole. Any paternally inherited chromosome may also face either pole. The orientation of each tetrad is independent of the orientation of the other 22 tetrads.

In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations depends on the number of chromosomes making up a set. There are two possibilities for orientation (for each tetrad) 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. This number does not include the variability previously created in the sister chromatids by crossover. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition (Figure 7.4).

To summarize the genetic consequences of meiosis I: the maternal and paternal genes are recombined by crossover events occurring on each homologous pair during prophase I in addition, the random assortment of tetrads at metaphase produces a unique combination of maternal and paternal chromosomes that will make their way into the gametes.

In anaphase I, the spindle fibers pull the linked chromosomes apart. The sister chromatids remain tightly bound together at the centromere. It is the chiasma connections that are broken in anaphase I as the fibers attached to the fused kinetochores pull the homologous chromosomes apart (Figure 7.5).

In telophase I, the separated chromosomes arrive at opposite poles. The remainder of the typical telophase events may or may not occur depending on the species. In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I.

Cytokinesis, the physical separation of the cytoplasmic components into two daughter cells, occurs without reformation of the nuclei in other organisms. In nearly all species, cytokinesis separates the cell contents by either a cleavage furrow (in animals and some fungi), or a cell plate that will ultimately lead to formation of cell walls that separate the two daughter cells (in plants). 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. However, although the sister chromatids were once duplicates of the same chromosome, they are no longer identical at this stage because of crossovers.

Concepts in Action

Review the process of meiosis, observing how chromosomes align and migrate, at this site.

Meiosis II

In meiosis II, the connected sister chromatids remaining in the haploid cells from meiosis I will be split to form four haploid cells. In some species, cells enter a brief interphase, or interkinesis , that lacks an S phase, before entering meiosis II. Chromosomes are not duplicated during interkinesis. The two cells produced in meiosis I go through the events of meiosis II in synchrony. Overall, meiosis II resembles the mitotic division of a haploid cell.

In prophase II, if the chromosomes decondensed in telophase I, they condense again. If nuclear envelopes were formed, they fragment into vesicles. The centrosomes duplicated during interkinesis move away from each other toward opposite poles, and new spindles are formed. In prometaphase II, the nuclear envelopes are completely broken down, and the spindle is fully formed. Each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles. In metaphase II, the sister chromatids are maximally condensed and aligned at the center of the cell. In anaphase II, the sister chromatids are pulled apart by the spindle fibers and move toward opposite poles.

In telophase II, the chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form around the chromosomes. Cytokinesis separates the two cells into four genetically unique haploid cells. At this point, the nuclei in the newly produced cells are both haploid and have only one copy of the single set of chromosomes. The cells produced are genetically unique because of the random assortment of paternal and maternal homologs and because of the recombination of maternal and paternal segments of chromosomes—with their sets of genes—that occurs during crossover.

Comparing Meiosis and Mitosis

Mitosis and meiosis, which are both forms of division of the nucleus in eukaryotic cells, share some similarities, but also exhibit distinct differences that lead to their very different outcomes. Mitosis is a single nuclear division that results in two nuclei, usually partitioned into two new cells. The nuclei resulting from a mitotic division are genetically identical to the original. They have the same number of sets of chromosomes: one in the case of haploid cells, and two in the case of diploid cells. On the other hand, meiosis is two nuclear divisions that result in four nuclei, usually partitioned into four new cells. The nuclei resulting from meiosis are never genetically identical, and they contain one chromosome set only—this is half the number of the original cell, which was diploid (Figure 7.6).

The differences in the outcomes of meiosis and mitosis occur because of differences in the behavior of the chromosomes during each process. Most of these differences in the processes occur in meiosis I, which is a very different nuclear division than mitosis. In meiosis I, the homologous chromosome pairs become associated with each other, are bound together, experience chiasmata and crossover between sister chromatids, and line up along the metaphase plate in tetrads with spindle fibers from opposite spindle poles attached to each kinetochore of a homolog in a tetrad. All of these events occur only in meiosis I, never in mitosis.

Homologous chromosomes move to opposite poles during meiosis I so the number of sets of chromosomes in each nucleus-to-be is reduced from two to one. For this reason, meiosis I is referred to as a reduction division . There is no such reduction in ploidy level in mitosis.

Meiosis II is much more analogous to a mitotic division. In this case, duplicated chromosomes (only one set of them) line up at the center of the cell with divided kinetochores attached to spindle fibers from opposite poles. During anaphase II, as in mitotic anaphase, the kinetochores divide and one sister chromatid is pulled to one pole and the other sister chromatid is pulled to the other pole. If it were not for the fact that there had been crossovers, the two products of each meiosis II division would be identical as in mitosis instead, they are different because there has always been at least one crossover per chromosome. Meiosis II is not a reduction division because, although there are fewer copies of the genome in the resulting cells, there is still one set of chromosomes, as there was at the end of meiosis I.

Cells produced by mitosis will function in different parts of the body as a part of growth or replacing dead or damaged cells. They may even be involved in asexual reproduction in some organisms. Cells produced by meiosis in a diploid-dominant organism such as an animal will only participate in sexual reproduction.


What is Meiosis?

Meiosis [1] is a type of cell division that involves the reduction in the number of the parental chromosome by half and consequently the production of four haploid daughter cells. This process is very essential in the formation of the sperm and egg cells necessary for sexual reproduction. When the haploid sperm and egg fuse, the resulting offspring acquires the restored number of chromosomes.

Meiosis is highly ubiquitous among eukaryotes as it can occur in single-celled organisms like yeast as well as multi-cellular ones like humans.

The process of meiosis is very essential in ensuring genetic diversity [2] through sexual reproduction.

In humans, two distinct types of daughter cells are produced by males and females (sperm and egg cells respectively).


  • Maintaining chromosome number in organisms: In humans, each cell typically contains 46 chromosomes organized into 23 pairs. To maintain the chromosome number generation after generation, the gametes formed from the meiotic division should contain half the number of chromosomes (23 chromosomes) as the parent cell. When the sex cells fuse to form a zygote, the usual chromosome number of 46 chromosomes is restored in the new individual. If the chromosomal reduction process is not maintained, it could cause genetic abnormality in the child.
  • Creates genetic diversity: The exchange of genetic information between the pair of homologous chromosomes allows genetic variation among the population. These variations form the basis of the evolutionary process.
  • Repairs genetic defects: The process of mixing chromosomes in meiosis, commonly known as recombination, helps repair genetic abnormalities in individuals produced through meiosis. When one of the parents has a genetic defect, recombination through meiosis can replace that abnormality in the next generation, allowing the formation of a healthy individual.

Ans. Since meiosis occurs through the union of sex cells or gametes, it cannot happen asexually.

Ans. Non-disjunction is an error in meiosis when a pair of chromosomes has failed to separate at anaphase. In such cases, both the chromosome pair has passed on to the same daughter cell.


Mitosis

Mitosis is a form of eukaryotic cell division that produces two daughter cells with the same genetic component as the parent cell. Chromosomes replicated during the S phase are divided in such a way as to ensure that each daughter cell receives a copy of every chromosome. In actively dividing animal cells, the whole process takes about one hour.

The replicated chromosomes are attached to a 'mitotic apparatus' that aligns them and then separates the sister chromatids to produce an even partitioning of the genetic material. This separation of the genetic material in a mitotic nuclear division (or karyokinesis) is followed by a separation of the cell cytoplasm in a cellular division (or cytokinesis) to produce two daughter cells.

In some single-celled organisms mitosis forms the basis of asexual reproduction. In diploid multicellular organisms sexual reproduction involves the fusion of two haploid gametes to produce a diploid zygote. Mitotic divisions of the zygote and daughter cells are then responsible for the subsequent growth and development of the organism. In the adult organism, mitosis plays a role in cell replacement, wound healing and tumour formation.

Mitosis, although a continuous process, is conventionally divided into five stages: prophase, prometaphase, metaphase, anaphase and telophase.

Prophase

Prophase occupies over half of mitosis. The nuclear membrane breaks down to form a number of small vesicles and the nucleolus disintegrates. A structure known as the centrosome duplicates itself to form two daughter centrosomes that migrate to opposite ends of the cell. The centrosomes organise the production of microtubules that form the spindle fibres that constitute the mitotic spindle. The chromosomes condense into compact structures. Each replicated chromosome can now be seen to consist of two identical chromatids (or sister chromatids) held together by a structure known as the centromere.

Prometaphase

The chromosomes, led by their centromeres, migrate to the equatorial plane in the mid-line of the cell - at right-angles to the axis formed by the centrosomes. This region of the mitotic spindle is known as the metaphase plate. The spindle fibres bind to a structure associated with the centromere of each chromosome called a kinetochore. Individual spindle fibres bind to a kinetochore structure on each side of the centromere. The chromosomes continue to condense.

Metaphase

The chromosomes align themselves along the metaphase plate of the spindle apparatus.

Anaphase

The shortest stage of mitosis. The centromeres divide, and the sister chromatids of each chromosome are pulled apart - or 'disjoin' - and move to the opposite ends of the cell, pulled by spindle fibres attached to the kinetochore regions. The separated sister chromatids are now referred to as daughter chromosomes. (It is the alignment and separation in metaphase and anaphase that is important in ensuring that each daughter cell receives a copy of every chromosome.)

Telophase

The final stage of mitosis, and a reversal of many of the processes observed during prophase. The nuclear membrane reforms around the chromosomes grouped at either pole of the cell, the chromosomes uncoil and become diffuse, and the spindle fibres disappear.

Cytokinesis

The final cellular division to form two new cells. In plants a cell plate forms along the line of the metaphase plate in animals there is a constriction of the cytoplasm. The cell then enters interphase - the interval between mitotic divisions.


Meiosis in humans [ edit | edit source ]

In females, meiosis occurs in precursor cells known as oogonia that divide twice into oocytes. These stem cells stop at the diplotene stage of meiosis I and lay dormant within a protective shell of somatic cells called the follicle. Follicles begin growth at a steady pace in a process known as folliculogenesis, and a small number enter the menstrual cycle. Menstruated oocytes continue meiosis I and arrest at meiosis II until fertilization. The process of meiosis in females is called oogenesis.

In males, meiosis occurs in precursor cells known as spermatogonia that divide twice to become sperm. These cells continuously divide without arrest in the seminiferous tubules of the testicles. Sperm is produced at a steady pace. The process of meiosis in males is called spermatogenesis.