<< Chapter < Page Chapter >> Page >

14.1b the mitotic phase

The mitotic phase is a multistep process during which the duplicated chromosomes are aligned, separated, and move into two new, identical daughter cells.

Revisit the stages of mitosis at this site .

Mitosis

Mitosis , is divided into a series of phases—prophase, metaphase, anaphase, and telophase—that result in the division of the cell nucleus ( [link] ).

Art connection

This diagram shows the four phases of mitosis (including extra pictures of prometaphase and cytokinesis for clarity). During prophase, the chromosomes condense and become visible, spindle fibers emerge from the centrosomes, the nuclear envelope breaks down, and the nucleolus disappears. Also during prophase, the kinetochores appear at the centromeres, mitotic spindle microtubules attach to the kinetochores, and centrosomes move toward opposite poles.. These are shown as prophase and prometaphase to show all steps for extra clarification. During metaphase, the mitotic spindle is fully developed, and centrosomes are at opposite poles of the cell. Chromosomes line up at the metaphase plate and each sister chromatid is attached to a spindle fiber originating from the opposite pole. During anaphase, the cohesin proteins that were binding the sister chromatids together break down. The sister chromatids, which are now called chromosomes, move toward opposite poles of the cell. Non-kinetochore spindle fibers lengthen, elongating the cell. During telophase, chromosomes arrive at the opposite poles and begin to decondense. The nuclear envelope reforms, and cytokinesis occurs. During cytokinesis in animals, a cleavage furrow separates the two daughter cells. In plants, a cell plate separates the two cells.
Karyokinesis (or mitosis) is divided into four phases —prophase, metaphase, anaphase, and telophase. Pictures of prometaphase and cytokinesis are shown for extra clarification, but are not actually phases of mitosis. The pictures at the bottom were taken by fluorescence microscopy (hence, the black background) of cells artificially stained by fluorescent dyes: blue fluorescence indicates DNA (chromosomes) and green fluorescence indicates microtubules (spindle apparatus). (credit “mitosis drawings”: modification of work by Mariana Ruiz Villareal; credit “micrographs”: modification of work by Roy van Heesbeen; credit “cytokinesis micrograph”: Wadsworth Center/New York State Department of Health; scale-bar data from Matt Russell)

Which of the following is the correct order of events in mitosis?

  1. Sister chromatids line up at the metaphase plate. The kinetochore becomes attached to the mitotic spindle. The nucleus reforms and the cell divides. Cohesin proteins break down and the sister chromatids separate.
  2. The kinetochore becomes attached to the mitotic spindle. Cohesin proteins break down and the sister chromatids separate. Sister chromatids line up at the metaphase plate. The nucleus reforms and the cell divides.
  3. The kinetochore becomes attached to the cohesin proteins. Sister chromatids line up at the metaphase plate. The kinetochore breaks down and the sister chromatids separate. The nucleus reforms and the cell divides.
  4. The kinetochore becomes attached to the mitotic spindle. Sister chromatids line up at the metaphase plate. Cohesin proteins break down and the sister chromatids separate. The nucleus reforms and the cell divides.

During prophase     , the nuclear envelope dissociates into small vesicles, and the membranous organelles (such as the Golgi complex or Golgi apparatus, and endoplasmic reticulum), fragment and disperse toward the periphery of the cell. The nucleolus disappears (disperses). The centrosomes move to opposite poles of the cell. Microtubules that form the mitotic spindle extend between the centrosomes, pushing them farther apart as the microtubule fibers lengthen.

Also during prophase, each sister chromatid develops a protein structure called a kinetochore     in the centromeric region ( [link] ). The proteins of the kinetochore attract and bind mitotic spindle microtubules. As the spindle microtubules extend from the centrosomes, some of these microtubules come into contact with and firmly bind to the kinetochores. Once a mitotic fiber attaches to a chromosome, the chromosome will be oriented until the kinetochores of sister chromatids face the opposite poles. Eventually, all the sister chromatids will be attached via their kinetochores to microtubules from opposing poles. Spindle microtubules that do not engage the chromosomes are called polar microtubules. These microtubules overlap each other midway between the two poles and contribute to cell elongation. Astral microtubules are located near the poles, aid in spindle orientation, and are required for the regulation of mitosis.

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, General biology part i - mixed majors. OpenStax CNX. May 16, 2016 Download for free at http://legacy.cnx.org/content/col11749/1.5
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'General biology part i - mixed majors' conversation and receive update notifications?

Ask