The chromosome number also remains the same, which means in the case of humans that each daughter cell produced during mitosis will have 46 total chromosomes just like in the parent cell.
The first stage of mitosis which occurs is known as prophase and is the time when chromatin condenses such that chromosomes become visible as individual structures. During this stage of division, the nucleolus also disintegrates and nuclear envelope breaks down. In order for chromosomes to move apart during a later stage, they need to attach to the spindle apparatus.
This structure consists of spindle fibers which are actually made of protein microtubules, and the structure is formed at the centrosomes of the cell. Each chromosome in the cell has a particular region usually near the center of the chromosome that is called a centromere. It is in this area of the chromosome that the kinetochore now forms. The kinetochore is actually a rather complex structure that in fact consists of more than 50 proteins which assemble in a specific fashion. Microtubules from opposite ends of the cell reach out and attach to these structures.
This is known as an amphitelic attachment of microtubule fibers of the spindle to the kinetochores of the chromosome. It is important to realize that each chromosome consists of two chromatids, each with a kinetochore to which a fiber attaches. During the next stage of mitosis, metaphase, the chromosomes align at the center of the cell on the metaphase plate.
The chromatids are attached to spindle fibers that extend the length of the cell. The chromosomes are lined up in one line at the middle of the cell in preparation for the next stage of mitosis. Anaphase is the third stage of mitosis and it involves contraction of the microtubules that make up the spindle apparatus.
As these proteins contract, it causes the fibers attached to each chromatid to actually shorten. The result of this is that the chromatids of each chromosome are pulled apart and begin to move to the opposite poles of the cell. Once these chromatids have reached the ends of the cell, the next stage of the mitotic division is ready to begin. Telophase involves these chromosomes now becoming chromatin and at the same time as this is happening the nucleoli begin to reform at each pole of the cell.
A nuclear envelope also reforms around what is to become a nucleus. At the end of this stage, there will be a fully formed nucleus at each pole of the cell. You have authorized LearnCasting of your reading list in Scitable. Do you want to LearnCast this session? This article has been posted to your Facebook page via Scitable LearnCast.
Change LearnCast Settings. Scitable Chat. 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.
Figure 2: Types of microtubules involved in mitosis During mitosis, several types of microtubules are active. The motor proteins associated with the interpolar microtubules drive the assembly of the spindle. Note the other types of microtubules involved in anchoring the spindle pole and pulling apart the sister chromatids.
Figure Detail. Cytokinesis is the physical process that finally splits the parent cell into two identical daughter cells. During cytokinesis, the cell membrane pinches in at the cell equator, forming a cleft called the cleavage furrow.
The position of the furrow depends on the position of the astral and interpolar microtubules during anaphase. The cleavage furrow forms because of the action of a contractile ring of overlapping actin and myosin filaments. As the actin and myosin filaments move past each other, the contractile ring becomes smaller, akin to pulling a drawstring at the top of a purse.
When the ring reaches its smallest point, the cleavage furrow completely bisects the cell at its center, resulting in two separate daughter cells of equal size Figure 3. Figure 3: Mitosis: Overview of major phases The major stages of mitosis are prophase top row , metaphase and anaphase middle row , and telophase bottom row. This page appears in the following eBook. Aa Aa Aa. What Are the Phases of Mitosis?
Figure 1: Drawing of chromosomes during mitosis by Walther Flemming, circa What Happens during Prophase?
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