Early microscopists were the first to observe these structures, and they also noted the appearance of a specialized network of microtubules during mitosis. These tubules, collectively known as the spindle, extend from structures called centrosomes — with one centrosome located at each of the opposite ends, or poles, of a cell. As mitosis progresses, the microtubules attach to the chromosomes, which have already duplicated their DNA and aligned across the center of the cell.
The spindle tubules then shorten and move toward the poles of the cell. As they move, they pull the one copy of each chromosome with them to opposite poles of the cell. This process ensures that each daughter cell will contain one exact copy of the parent cell DNA. Mitosis consists of five morphologically distinct phases: prophase, prometaphase, metaphase, anaphase, and telophase.
Each phase involves characteristic steps in the process of chromosome alignment and separation. Once mitosis is complete, the entire cell divides in two by way of the process called cytokinesis Figure 1. Walther Flemming: pioneer of mitosis research.
Nature Reviews Molecular Cell Biology 2, 72 All rights reserved. Prophase is the first stage in mitosis, occurring after the conclusion of the G 2 portion of interphase. During prophase, the parent cell chromosomes — which were duplicated during S phase — condense and become thousands of times more compact than they were during interphase. Because each duplicated chromosome consists of two identical sister chromatids joined at a point called the centromere , these structures now appear as X-shaped bodies when viewed under a microscope.
Several DNA binding proteins catalyze the condensation process, including cohesin and condensin. Cohesin forms rings that hold the sister chromatids together, whereas condensin forms rings that coil the chromosomes into highly compact forms. The mitotic spindle also begins to develop during prophase.
As the cell's two centrosomes move toward opposite poles, microtubules gradually assemble between them, forming the network that will later pull the duplicated chromosomes apart. When prophase is complete, the cell enters prometaphase — the second stage of mitosis. During prometaphase, phosphorylation of nuclear lamins by M-CDK causes the nuclear membrane to break down into numerous small vesicles.
As a result, the spindle microtubules now have direct access to the genetic material of the cell. As prometaphase ends and metaphase begins, the chromosomes align along the cell equator. Every chromosome has at least two microtubules extending from its kinetochore — with at least one microtubule connected to each pole. At this point, the tension within the cell becomes balanced, and the chromosomes no longer move back and forth.
In addition, the spindle is now complete, and three groups of spindle microtubules are apparent. Kinetochore microtubules attach the chromosomes to the spindle pole; interpolar microtubules extend from the spindle pole across the equator, almost to the opposite spindle pole; and astral microtubules extend from the spindle pole to the cell membrane.
Metaphase leads to anaphase , during which each chromosome's sister chromatids separate and move to opposite poles of the cell. Enzymatic breakdown of cohesin — which linked the sister chromatids together during prophase — causes this separation to occur.
Upon separation, every chromatid becomes an independent chromosome. Meanwhile, changes in microtubule length provide the mechanism for chromosome movement. More specifically, in the first part of anaphase — sometimes called anaphase A — the kinetochore microtubules shorten and draw the chromosomes toward the spindle poles.
Then, in the second part of anaphase — sometimes called anaphase B — the astral microtubules that are anchored to the cell membrane pull the poles further apart and the interpolar microtubules slide past each other, exerting additional pull on the chromosomes Figure 2. There, the vesicles fuse and coalesce from the center toward the cell walls; this structure is called a cell plate. As more vesicles fuse, the cell plate enlarges until it merges with the cell walls at the periphery of the cell.
Enzymes use the glucose that has accumulated between the membrane layers to build a new cell wall. The Golgi membranes become parts of the plasma membrane on either side of the new cell wall Figure 4. Figure 5 shows approximately how long a cell spends in each stage of the cell cycle:. Figure 5. The cell cycle consists of interphase and the mitotic phase. During interphase, the cell grows and the nuclear DNA is duplicated. Interphase is followed by the mitotic phase. During the mitotic phase, the duplicated chromosomes are segregated and distributed into daughter nuclei.
The cytoplasm is usually divided as well, resulting in two daughter cells. This video reviews all the steps of mitosis; seeing it all together is a great review at this stage.
Answer the question s below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times.
Use this quiz to check your understanding and decide whether to 1 study the previous section further or 2 move on to the next section. Privacy Policy. Skip to main content. Module 7: Cell Division.
Search for:. Figure 1. The cell cycle. Learning Outcomes Identify the characteristics and sub-phases of interphase Identify the characteristics and stages of mitosis Identify the characteristics of cytokinesis.
Figure 2. Stages of mitosis. Video Review: The Cell Cycle This video reviews all the steps of mitosis; seeing it all together is a great review at this stage. In this interview, we speak to Roy Smythe, CEO of SomaLogic, about their groundbreaking proteomics technology that can simultaneously measure 7, proteins. Accept Cookie Settings. Related Stories Study sheds light on the regulation of epigenetic modifications during the development Experts explain left—right asymmetry development in the embryo Brown fat protects against secondary obesity-related diseases, says study.
Sources: Collins, K. The cell cycle and cancer. Proceedings of the National Academy of Sciences , 94 7 , pp. Cell-cycle checkpoints and cancer. Nature , , pp. The Cell Cycle: A Review. Veterinary Pathology , 35 6 , pp. Written by Sarah Moore After studying Psychology and then Neuroscience, Sarah quickly found her enjoyment for researching and writing research papers; turning to a passion to connect ideas with people through writing. Download PDF Copy. Citations Please use one of the following formats to cite this article in your essay, paper or report: APA Moore, Sarah.
MLA Moore, Sarah. Chicago Moore, Sarah. Harvard Moore, Sarah. Suggested Reading. How do large proteins and drugs cross a cell membrane? Cancer invasion and migration can be suppressed by manipulating stiffness of cell membrane.
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