39 4.13 Mitosis and Cytokinesis

Created by: CK-12/Adapted by Christine Miller

Divide and Split

Image shows a cell in anaphase of mitosis. The image is taken using immunoflourescence microscopy and components of the cell including spindle fibers and genetic material show as vivid blues and greens.
Figure 4.13.1 A cell in anaphase of mitosis.

Can you guess what the colourful image in Figure 4.13.1 represents? It shows a cell during the process of . In particular, the image shows the cell in a part of cell division called anaphase, where the is being pulled to opposite ends of the cell. Normally, DNA is located in the of most human cells. The nucleus divides before the cell itself splits in two, and before the nucleus divides, the cell’s DNA is replicated (or copied). There must be two copies of the DNA so that each daughter cell will have a complete copy of the genetic material from the parent cell. How is the replicated DNA sorted and separated so that each daughter cell gets a complete set of the genetic material? To answer that question, you first need to know more about DNA and the forms it takes.

The Forms of DNA

Diagram shows the forms that DNA takes, as a double helix, which will coil around itself, which will ultimately form a chromosome.
Figure 4.13.2 Forms of DNA.

Except when a eukaryotic cell divides, its nuclear DNA exists as a grainy material called . Only once a cell is about to divide and its DNA has replicated does DNA condense and coil into the familiar X-shaped form of a , like the one shown below.

Labelled diagram of a chromosome showing that in a chromosome with the typical "X" shape, it is comprised of two identical pieces of DNA, each called a chromatid.
Figure 4.13.3 Diagram of a chromosome showing that in a chromosome with the typical “X” shape, it is comprised of two identical pieces of DNA, each called a chromatid.

Most cells in the human body have two pairs of 23 different chromosomes, for a total of 46 chromosomes. Cells that have two pairs of chromosomes are called diploid. Because DNA has already replicated when it coils into a chromosome, each chromosome actually consists of two identical structures called . Sister chromatids are joined together at a region called a .

 

 

 

Mitosis

Diagram shows the stages of Mitosis in which DNA replicates, chromosomes align, sister chromatids separate, and then two diploid cell emerge.
Figure 4.13.4 Mitosis is the phase of the eukaryotic cell cycle that occurs between DNA replication and the formation of two daughter cells. What happens during mitosis?

The process in which the nucleus of a eukaryotic cell divides is called mitosis. During mitosis, the two sister chromatids that make up each chromosome separate from each other and move to opposite poles of the cell. This is shown in the figure below.

Mitosis actually occurs in four phases. The phases are called prophase, metaphase, anaphase, and telophase.

Prophase

Figure 4.13.5 Mitotic prophase.

The first and longest phase of mitosis is . During prophase, condenses into , and the nuclear envelope (the membrane surrounding the nucleus) breaks down. In animal cells, the  near the nucleus begin to separate and move to opposite poles of the cell. Centrioles are small organelles found only in eukaryotic cells. They help ensure that the new cells that form after cell division each contain a complete set of chromosomes. As the centrioles move apart, a spindle starts to form between them. The spindle consists of fibres made of microtubules.

Diagram shows a cell in prophase of mitosis. The nuclear envelope is breaking down, chromosomes are condensing, and spindle fibers are forming.
Figure 4.13.6 Diagram of a cell in prophase of mitosis.

 

 

Metaphase

Figure 4.13.7 Metaphase.

During , spindle fibres attach to the centromere of each pair of sister chromatids. As you can see in Figure 4.13.7, the sister chromatids line up at the equator (or center) of the cell. The spindle fibres ensure that sister chromatids will separate and go to different daughter cells when the cell divides.

Diagram shows metaphase of mitosis, in which the spindle fibers are fully formed and the chromosomes are aligned along the center of the cell.
Figure 4.13.8 Diagram showing the metaphase of mitosis.

Anaphase

Figure 4.13.9 Mitotic anaphase.

During , sister chromatids separate and the centromeres divide. The sister chromatids are pulled apart by the shortening of the spindle fibres. This is a little like reeling in a fish by shortening the fishing line. One sister chromatid moves to one pole of the cell, and the other sister chromatid moves to the opposite pole. At the end of anaphase, each pole of the cell has a complete set of chromosomes.

Image shows a eukaryotic cell in anaphase of the cell cycle, in which sister chromatids have been separated from each other and are being pulled to opposite ends of the cell by spindle fibers.
Figure 4.13.10 Diagram showing eukaryotic cell in anaphase of cell cycle.

Telophase

Figure 4.13.11 Mitotic telophase.

During , the chromosomes begin to uncoil and form chromatin. This prepares the genetic material for directing the metabolic activities of the new cells. The spindle also breaks down, and new nuclear envelopes form.

Telophase is the stage in mitosis in which the nuclear envelope starts to reform, the chromosomes decondense and the cell continues to elongate.
Figure 4.13.12 Diagram showing telophase in mitosis.

Cytokinesis

Figure 4.13.13 Mitotic cytokinesis.

Cytokinesis is the final stage of cell division. During cytokinesis, the cytoplasm splits in two and the cell divides, as shown below. In animal cells, the plasma membrane of the parent cell pinches inward along the cell’s equator until two daughter cells form. Thus, the goal of mitosis and cytokinesis is now complete, because one parent cell has given rise to two daughter cells. The daughter cells have the same chromosomes as the parent cell.

Cytokinesis is the final step in cell division, in which the cytoplasm of the two new daughter cells completely separates.
Figure 4.13.14 Diagram showing the final step in cell division: cytokinesis.

4.13 Summary

  • Until a cell divides, its nuclear exists as a grainy material called . After DNA replicates and the cell is about to divide, the DNA condenses and coils into the X-shaped form of a . Each chromosome actually consists of two , which are joined together at a .
  • Mitosis is the process during which the nucleus of a eukaryotic cell divides. During this process, sister chromatids separate from each other and move to opposite poles of the cell. This happens in four phases: prophase, metaphase, anaphase, and telophase.
  • Cytokinesis is the final stage of cell division, during which the cytoplasm splits in two and two daughter cells form.

4.13 Review Questions

  1. Describe the different forms that DNA takes before and during cell division in a eukaryotic cell.
  2. Identify the four phases of mitosis in an animal cell, and summarize what happens during each phase.
  3. Order the diagrams of the stages of mitosis:
  4. Explain what happens during cytokinesis in an animal cell.
  5. What do you think would happen if the sister chromatids of one of the chromosomes did not separate during mitosis?
  6. True or False:

4.13 Explore More

Mitosis, NDSU Virtual Cell Animations project (ndsuvirtualcell), 2012.

Nondisjunction (Trisomy 21) – An Animated Tutorial, Kristen Koprowski, 2012.

Attributions

Figure 4.13.1

Anaphase_IF by Roy van Heesbeen on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.13.2

Chromosomes by OpenClipArt-Vectors on Pixabay is used under the Pixabay License (https://pixabay.com/service/license/).

Figure 4.13.3

Chromosome/ Chromatid/ Sister Chromatid by Christine Miller is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.13.4

Simple Mitosis by Mariana Ruiz Villarreal [LadyofHats] via CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

©CK-12 Foundation Licensed under CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0) • Terms of Use • Attribution

Figure 4.13.5

Mitotic Prophase [tiny] by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.13.6

Prophase Eukaryotic Mitosis by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.13.7

Mitotic_Metaphase by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.13.8

Metaphase Eukaryotic Mitosis by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.13.9

Anaphase [adapted] by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.13.10

Anaphase_eukaryotic_mitosis.svg by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.13.11

Mitotic Telophase by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.13.12

Telophase Eukaryotic Mitosis by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.13.13

Mitotic Cytokinesis by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.13.14

Cytokinesis Eukaryotic Mitosis by Mariana Ruiz Villarreal [LadyofHats] on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

References

Koprowski, K., Cabey, R. [Kristen Koprowski]. (2012). Nondisjunction (Trisomy 21) – An Animated Tutorial. YouTube. https://www.youtube.com/watch?v=EA0qxhR2oOk&feature=youtu.be

NDSU Virtual Cell Animations project [ndsuvirtualcell]. (2012). Mitosis. YouTube. https://www.youtube.com/watch?v=C6hn3sA0ip0&t=21s

License

Icon for the Creative Commons Attribution-NonCommercial 4.0 International License

Human Biology by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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