During meiosis, the cytokinesis that follows telophase II results in

The body is made up of trillions of somatic cells with the capacity to divide into identical daughter cells facilitating organismal growth, repair, and response to the changing environment. This process is called “mitosis.” In the gametes, a different form of cell division occurs called “meiosis.” The outcome of meiosis is the creation of daughter cells, either sperm or egg cells, through reduction division which results in a haploid complement of chromosomes so that on joining with another sex cell at fertilization a new diploid chromosomal complement is restored in the fertilized egg.[1][2][3]

Genomic diversity and genetic variation is produced through the process of meiosis due to chromosomal recombination and independent assortment. Each daughter cell created is genetically half-identical to that of its parent cell yet distinctly different from its parent cell and other daughter cells.[4][5]

The genome is encoded by the chemical sequence of DNA nucleotides within our cells. If stretched from end to end, the DNA in one cell would span about 3 meters. In order to fit into each cell, the DNA is condensed by proteins to create “chromatin,” a complex of DNA and proteins. Somatic human cells contain 23 paired chromosomes or 46 total chromosomes. 46 is considered the “diploid” number (2n), while 23 is considered the “haploid” number (1n), or half the diploid number.[6][7]

Meiosis is important for creating genomic diversity in a species. It accomplishes this primarily through 2 processes: independent assortment and crossing over (recombination).

1. The law of independent assortment states that the random orientation of homologous chromosome pairs during metaphase I allow for the production of gametes with many different assortments of homologous chromosomes. For example, tetrads containing chromosomes 1A/1B and 2A/2B can create 2 different variations in daughter cells: 1A2A, 1A2B, 1B2A, or 1B2B. With 46 cells in the human body, about 8 million different variations can be produced.

2. Crossing over refers to a phenomenon that takes place during prophase I. When homologous chromosomes come together to form tetrads, the arms of the chromatids can swap at random, creating many more possibilities for genetic variation of the gametes.

There are 2 parts to the cell cycle: interphase and mitosis/meiosis. Interphase can be further subdivided into Growth 1 (G1), Synthesis (S), and Growth 2 (G2). During the G phases, the cell grows by producing various proteins, and during the S phase, the DNA is replicated so that each chromosome contains two identical sister chromatids (c). Mitosis contains 4 phases: prophase, metaphase, anaphase, and telophase. 

Mitosis

• Prophase: The nuclear envelope breaks down. The chromatin condenses into chromosomes.

• Metaphase: The chromosomes line up along the metaphase plate. Microtubules originating from the centrosomes at the 2 opposite poles of the cell attach to the kinetochores of each chromosome.

• Anaphase: Chromatids separate and are pulled by microtubules to opposite ends of the cell.

• Telophase: The chromosomes gather at the poles of the cell, and the cell divides via cytokinesis forming 2 daughter cells. The nuclear envelope reappears, the spindle apparatus disappears and the chromosomes de-condense back into chromatin.

The cell can now enter Interphase where it grows and replicates its DNA in preparation for division, yet again.

Meiosis goes through all 5 phases of the cell cycle twice, with modified mechanisms that ultimately create haploid cells instead of diploid. In sperm cells, the male gametes, meiosis proceeds in the following manner:

Meiosis I

• Prophase I: The nuclear envelope breaks down. The chromatin condenses into chromosomes. Homologous chromosomes containing the two chromatids come together to form tetrads, joining at their centromeres (2n 4c). This is when “crossing over” occurs, which creates genetic variation.

• Metaphase I: The tetrads line up along the metaphase plate. Microtubules originating from the centrosomes at the 2 opposite poles of the cell attach to the kinetochores of each chromosome.

• Anaphase I: Homologous chromosomes are separated by the microtubules to opposite poles of the cell.

• Telophase I: The chromosomes gather at the poles of the cell, and the cell divides via cytokinesis forming 2 daughter cells (1n 2c). The nuclear envelope reappears, the spindle apparatus disappears and the chromosomes de-condense back into chromatin.

Interkinesis/Interphase II 

There is a brief pause between each round of meiosis providing time for the cell to replenish proteins; however, there is no S phase.

Meiosis II

• Prophase II: In each of the daughter cells, a new spindle apparatus forms, the nuclear envelope breaks down, and the chromatin condenses into chromosomes again.

• Metaphase II: The chromosomes line up along the metaphase plate. Microtubules originating from the centrosomes at the 2 opposite poles of the cell attach to the kinetochores of each chromosome.

• Anaphase II: Sister chromatids separate and are pulled by the microtubules to opposite poles of the cell.

• Telophase II: The chromosomes gather at the 2 poles of the cell and the cell divides via cytokinesis forming 2 daughter cells (1n 1c) from each of the two cells from meiosis I. The nuclear envelope reappears, the spindle apparatus disappears and the chromosomes de-condense back into chromatin.

In egg cells, the female gametes, meiosis follows the same general phases with only a slight variation. During telophase I, the cytoplasm divides unequally, creating a larger daughter cell and a smaller polar body. The polar body and the daughter cell both then enter meiosis II. In telophase II, the cytoplasm of the daughter cell again divides unequally and creates a daughter cell and another polar body. In addition, the polar body from meiosis I divides and forms 2 smaller polar bodies. After meiosis is completed, there is one daughter cell (1n, 1c) and 3 polar bodies (1n 1c). The polar bodies disintegrate as they do not have enough cytoplasm and proteins to survive as gametes.

Clinically, errors in meiosis can create many life-threatening outcomes. The most common error of meiosis is nondisjunction, when chromatids fail to separate during either anaphase I or II, creating imbalances in the number of chromosomes in each daughter cell. Most imbalances are incompatible with life, but some will result in viable offspring with a spectrum of developmental disorders. These medical conditions include Down syndrome, Patau syndrome, Edwards syndrome, Klinefelter syndrome, Turner syndrome, Triple X syndrome, and XYY syndrome.

Review Questions

1.

Zelkowski M, Olson MA, Wang M, Pawlowski W. Diversity and Determinants of Meiotic Recombination Landscapes. Trends Genet. 2019 May;35(5):359-370. [PubMed: 30948240]

Arbel-Eden A, Simchen G. Elevated Mutagenicity in Meiosis and Its Mechanism. Bioessays. 2019 Apr;41(4):e1800235. [PubMed: 30920000]

Vijverberg K, Ozias-Akins P, Schranz ME. Identifying and Engineering Genes for Parthenogenesis in Plants. Front Plant Sci. 2019;10:128. [PMC free article: PMC6389702] [PubMed: 30838007]

Gheldof A, Mackay DJG, Cheong Y, Verpoest W. Genetic diagnosis of subfertility: the impact of meiosis and maternal effects. J Med Genet. 2019 May;56(5):271-282. [PMC free article: PMC6581078] [PubMed: 30728173]

Simpson B, Tupper C, Al Aboud NM. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): Jun 8, 2021. Genetics, DNA Packaging. [PubMed: 30480946]

Ishiguro KI. The cohesin complex in mammalian meiosis. Genes Cells. 2019 Jan;24(1):6-30. [PMC free article: PMC7379579] [PubMed: 30479058]

Crickard JB, Greene EC. Biochemical attributes of mitotic and meiotic presynaptic complexes. DNA Repair (Amst). 2018 Nov;71:148-157. [PMC free article: PMC6340751] [PubMed: 30195641]

The best things in life come in pairs: best friends, milk and cookies, and meiosis I and meiosis II. If you started this article by first reading about meiosis I, then you are expecting the next step in the journey of meiosis. If you haven't had the chance, go check out our article on meiosis I before delving into this next big topic!

Meiosis II is the second round of cell division in the process of meiosis, or the creation of gametes (sex cells). Following directly after meiosis I, meiosis II produces four haploid daughter cells, known as gametes.

How do we define meiosis II?

Directly after meiosis I, the two haploid daughter cells with extra chromosome copies undergo meiosis II, so that the sister chromatids, or identical chromosome copies, can be split evenly to produce four haploid daughter cells (Fig. 1). This means that after meiosis I the two daughter cells do not re-enter interphase and no duplication event occurs between meiosis I and meiosis II. Some cells may go through a brief period between these two parts of meiosis called interkinesis.

Interkinesis is a small period of rest that some cells may go through between meiosis I and meiosis II. However, no DNA duplication events occur during this time.

Stages of meiosis II

The stages that make up meiosis II are the same as those in meiosis I and mitosis, except that they contain the roman numeral "II" after each stage. They are as follows:

1. Prophase II

2. Metaphase II

3. Anaphase II

4. Telophase II and cytokinesis.

Both daughter cells produced at the end of meiosis I will go through these stages, resulting in four haploid daughter cells, or gametes. In the following explanation of each stage in detail, you will see that meiosis II shares more similarities with mitosis than meiosis I did, except for the reduced chromosome number.

Prophase II of meiosis II:

During prophase II, as in mitosis and meiosis I, the following steps occur:

• Nuclear envelope begins to dissolve.
• Centrosomes (in animal cells) migrate to opposite poles of the cells.
• The chromosomes condense to prepare for movement to opposite poles of the cell.
• Spindle fibers begin to form.

It is important to note that, in prophase II of meiosis II, crossing over does not occur. Since homologous chromosomes are now in separate cells, only sister chromatids, which include one original chromatid and its copy, remain. Therefore, crossing over would not be as beneficial in this stage of meiosis and does not occur.

Remember in animal cells, the place in which the spindle fibers or microtubules originate is called the centrosome. In plant cells, it is known as the microtubule-organizing center (MTC).

Metaphase II of meiosis II:

During metaphase II, the chromosomes align in a single line at the metaphase plate. In this stage of meiosis, the sister chromatids are preparing to be separated (Fig. 2).

Figure 2: The cells during prophase II and metaphase II of meiosis II. Hailee Gibadlo, StudySmarter Originals.

Anaphase II of meiosis II:

During anaphase II the spindle fibers, connected at the kinetochores of each chromatid, pull the chromatids to opposite cell poles. The spindle fibers not connected to a chromatid help push the centrosomes of the opposite poles. Sister chromatids are separated in this step.

Telophase II and cytokinesis:

During telophase II, the two cells are preparing to become four after the sister chromatids are separated in anaphase II and the genetic material corresponding to each cell is at opposite poles (Fig. 3). In this stage of meiosis II, the chromosomes are decondensing as the nuclear envelope reforms, making the nuclei of the future independent cells. Spindle fibers break down and the centrosomes disassemble. Finally, in telophase II, the cleavage furrow (in animal cells) begins to form as the cells prepare for cytokinesis.

The cleavage furrow is the point at which the cytoplasm starts to pinch inward in preparation for cytokinesis, which is division of the cytoplasm.

At the end of cytokinesis and telophase II of meiosis II, four haploid daughter cells remain.

Differences between meiosis II and other types of cell division

Meiosis II and meiosis I: differences

Meiosis II is the second part of meiosis and follows meiosis I. The table below highlights key differences between the two parts of meiosis (Table 1).

Table 1: Differences between meiosis I and meiosis II. Hailee Gibadlo, StudySmarter Originals.

Meiosis II and mitosis: comparison

If you've followed the whole meiosis versus mitosis comparison this far, you may notice that meiosis II has a lot more in common with mitosis than meiosis I did. That is because meiosis II does not contain any extra steps, such as crossing over or the splitting of homologous chromosomes, like in meiosis I.

Meiosis II follows the same steps as mitosis except for a few key differences:

• In meiosis II, two cells from meiosis I will undergo cell division, producing four haploid daughter cells.

  • In mitosis, one parent cell will produce two daughter cells.

• More importantly, in meiosis II, the starting cells are haploid or contain half the genetic information of the parent cell, plus a copy, meaning the four daughter cells will be haploid (chromosome number= n) and genetically different from the parent cell.

  • In mitosis, the two daughter cells are diploid (chromosome number=2n) and are genetically the same as the parent cell.

Meiosis II and you

Remember back to the first discussions we had on heredity where we talked about reproduction and its importance in passing on genetic information to the next generation. If reproduction is the mode by which genes are passed on, then meiosis functions as an important tool in reproduction.

Review our introduction to Heredity!

At the end of meiosis II, four haploid daughter cells, which are genetically different from the parent cell, are produced. This means all of the sex cells (gametes) are haploid, or half the original chromosome number (n) of the other cells in the diploid (2n) organism (somatic or body cells).

The symbol "n" denotes the chromosome number of an organism's cells.

Let's look at human cells as an example. Human cells have 23 pairs, or 46 total, chromosomes. That means the diploid chromosome number is 46 (2n=46) and the haploid chromosome number is 23 (n=23), or half the diploid chromosome number. Below, two people represent the sets of chromosomes:

The parent cell has two sets of 23 chromosomes, one set coming from mom, and one from dad, represented by the emojis:

( ) = 2 sets of 23 chromosomes, one from each parent, 2n=46.

During interphase, at the start of meiosis, duplication occurs, so 4n =92.

( ) = 4 sets, 92 chromosomes total.

During meiosis I, the homologous chromosomes are separated, so the resulting daughter cells are not diploid, but instead haploid, because corresponding chromosomes are split up. At the end of meiosis I, therefore, the daughter cells have half the number of chromosomes, plus the copies of those (n+n= 23+23).

After meiosis I:

( ) ( )= Two cells each with n+n chromosomes, in this case 23+23.

During meiosis II, sister chromatids are separated, meaning each daughter cell only has half the information of the parent cell and no copies.

After meiosis II:

( ) ( ) ( ) ( ) = Four daughter cells with half the original chromosome number (n= 23) each.

This is one example to clarify haploid, diploid, and what it means to be one or the other! Remember that this

demonstration did not take into account crossing over between homologous chromosomes during meiosis I.

Meiosis II - Key takeaways

• Meiosis II follows directly after meiosis I, there is no interphase or DNA duplication before meiosis II starts. There is a short period of rest called interkinesis that some cells may experience.
• During meiosis II the two haploid daughter cells created after meiosis I undergo another cell division to produce four haploid daughter cells, or gametes (sex cells).
• Meiosis II happens in four stages: prophase II, metaphase II, anaphase II, and telophase II plus cytokinesis.
• During anaphase II, sister chromatids are separated.
• Meiosis II is a lot like mitosis, except that instead of two identical diploid daughter cells as in mitosis, meiosis II ends with four haploid, genetically different daughter cells.

Meiosis II is the second part of meiosis and follows meiosis I. 

Below are some key differences of note: 

1. Before meiosis II there is no interphase or DNA duplication as there is before meiosis I. Sometimes there is an interkinesis phase, a small period of rest after meiosis I. 

2. Meiosis I starts with one parent diploid cell; meiosis II starts with two haploid daughter cells with copies of the haploid genome. 

3. In meiosis I, crossing over and separation of homologous chromosomes occur. In meiosis II, crossing over DOES NOT occur and sister chromatids are separated during anaphase II. 

4. At the end of meiosis II, four haploid daughter cells are produced, at the end of meiosis I, the two daughter cells are haploid but still contain copies.

During anaphase II of meiosis II, sister chromatids are separated. 

The product of meiosis II is four haploid daughter cells, or sex cells (gametes). 

At the end of telophase II, the last stage of meiosis II, the cells undergo cytokinesis, or the division of the cytoplasm to become four haploid daughter cells. The cells will become gametes, or sex cells, after the completion of meiosis II. 

Meiosis II is needed to separate sister chromatids. Meiosis I creates two haploid cells, but they still each contain a copy, hence the chromatid and its identical sister. Following meiosis II, a second cytoplasmic division takes place, creating four haploid cells that will become gametes. 

Question

During anaphase II which of the following does not happen: 

Answer

Sister chromatids line up at the metaphase plate.

Question

True or False: Crossing over does not occur during meiosis II.

Answer

Question

True or False: At the start of meiosis II, cells are haploid. 

Answer

Question

What is the short period between meiosis I and meiosis II that some cells experience called?

Answer

Question

Does DNA duplication occur during interkinesis, or after the end of meiosis I?

Answer

No! DNA duplication only occurs in interphase before the start of meiosis (I). 

Question

If humans have 46 total chromosomes and are diploid (2n) organisms, what must be the chromosome number of our gametes? 

Answer

Gametes are haploid and have 23 chromosomes (n=23). 

Question

If humans have 46 total chromosomes (2n=46), why do our sex cells each contain only 23 chromosomes?

Answer

Fertilization occurs when an egg and a sperm (both sex cells) combine to make a zygote. They each bring half the genetic material of the parent, meaning the fertilized egg (zygote) has 46 total chromosomes. 

Question

In what stage of meiosis II does the nuclear envelope begin to dissolve and the spindle fibers begin to form? 

Answer

Question

During metaphase II which of the following happens: 

Answer

Sister chromatids line up single file at the metaphase plate.

Question

What is the name of the process that follows telophase II in which division of the cytoplasm occurs? 

Answer

Cytokinesis- this process also happens at the end of the other cell division events, mitosis and meiosis I. 

Question

Which example is not a difference between meiosis I and meiosis II? 

Answer

Meiosis I starts with one haploid cell, meiosis II starts with two diploid cells.

Question

What is the cleavage furrow?

Answer

The cleavage furrow is the point at which the cytoplasm starts to pinch inward in preparation for cytokinesis, or division of the cytoplasm.