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Mitotic Phase

Mitotic Phase

The mitotic phase is the end of the cell cycle, concluding in cell division. During the mitotic phase, the DNA and cell structures that were duplicated in interphase, divide into two new daughter cells by cell division. The mitotic phase consists of two sub-phases: mitosis and cytokinesis. During mitosis, the DNA chromosomes and nuclear contents are aligned and separated. During cytokinesis, the cell pinches and separates into two new daughter cells. Below is a diagram of the whole cell cycle: interphase and the mitotic phase (Fig. 1).

Phases of mitotic cell division

There are two phases of mitotic cell division: mitosis and cytokinesis. Mitosis, sometimes called karyokinesis, is the division of the nuclear contents of the cell and has five sub-phases: Prophase, Prometaphase, Metaphase, Anaphase, and Telophase. Cytokinesis, literally meaning "cell movement", is when the cell splits itself and the cell structures in the cytoplasm are divided into two new cells. Below is a simplified diagram displaying each portion of the mitotic phase, how the DNA chromosomes condense, arrange, divide, and finally how the cell divides into the two new daughter cells

Mitotic phase stages

Prior to mitosis, cells undergo interphase in which the cell prepares for mitotic cell division. When cells undergo interphase, they are constantly synthesizing RNA, generating proteins, and growing in size. Interphase is divided into 4 steps: gap 0 (G0), Gap 1 (G1), Synthesis (S), and Gap 2 (G2). These stages occur in sequential order and are extremely important to get the cell ready for division. Let's take a look at these four phases in more detail.

Gap 0 (G0): Gap 0 is technically not part of the cell division cycle but instead characterized a temporary or permanent resting phase in which the cell does not undergo cell division. Usually cells such as neurons which do not divide are said to be in the Gap 0 phase. The gap 0 phase can also occur when cells are senescent. When a cell is senescent, it no longer divides. The number of senescent cells in the body increases as we age. Researchers are still investigating the cause of why senescent cells increase as we age but they suspect that it could be due to decreased efficiency of autophagy.

Autophagy: The process of clearing out cellular debris.

Gap 1 (G1): During the gap 1 phase, the cell grows and produces a large amount of proteins which allows the cell to almost double in size. In this phase, the cell's produce more organelles and increases its cytoplasmic volume.

Synthesis (S): During this phase, the cell undergoes DNA replication where the amount of cellular DNA is doubled.

Gap 2 (G2): The gap 2 phase is characterized by increases cellular growth as the cell prepares to enter the mitotic phase. The mitochondria which is the cell's power house also divides in preparation for cell division.

Now that interphase is completed let's move on to discuss the phases of mitosis. Below is a brief overview of the mitotic phase stages.

Mitotic phase definition

Mitosis consists of five stages: prophase, prometaphase, metaphase, anaphase, and telophase. As you review the stages of mitosis, keep in mind what happens to the major cell structures, and how the chromosomes are arranged in the cell. Interestingly, mitosis only occurs in eukaryotic cells. Prokaryotic cells, which lack a nucleus divide by a method known as binary fission. Let' go over the stages of mitosis in more detail.

Prophase

During prophase, the first stage of mitosis, the DNA chromosomes condense into sister chromatids and are now visible. The centrosomes start to separate to opposite sides of the cell, producing long strands called spindle microtubules, or mitotic spindles, as they move through the cell. These microtubules are almost like puppet strings that move the main cell components during mitosis. Lastly, the nuclear envelope surrounding the DNA begins to break down, allowing access to the chromosomes and clearing space in the cell.

Prometaphase

The next stage of mitosis is prometaphase. The key visible features of this stage of the cell cycle include DNA that is now fully condensed into their duplicated X-shaped chromosomes with sister chromatids. The centrosomes have now reached the opposite sides, or poles, of the cell. Spindle microtubules are still forming and begin to attach to the centromeres of the chromosomes at structures called kinetochores. This allows the mitotic spindles to move the chromosomes towards the center of the cell.

Metaphase

Metaphase is the easiest phase of mitosis to identify when looking at a cell. At this stage of mitosis, all of the DNA chromosomes with fully condensed sister chromatids are aligned in the center of the cell in a straight line. This line is called the metaphase plate, and this is the key feature to look for in distinguishing this stage of mitosis from others in the cell cycle. The centrosomes have fully separated to the opposite poles of the cell and the spindle microtubules are fully formed. This means that the kinetochore of each sister chromatid is attached to the centrosome on its side of the cell by the mitotic spindles.

Anaphase

Anaphase is the fourth stage of mitosis. When the sister chromatids finally separate, the DNA is divided. Many things are happening all at once. First, the cohesion proteins that held the sister chromatids together break down. Then the mitotic spindles shorten, pulling the sister chromatids, now called daughter chromosomes, by the kinetochore to the poles of the cell with the centrosomes. Lastly, unattached microtubules elongate the cell into an oval shape, preparing the cell to split and make daughter cells during cytokinesis.

Telophase

Finally, we have telophase. During this final stage of mitosis, two new nuclear envelopes begin to surround each set of DNA chromosomes, and the chromosomes themselves start to loosen into usable chromatin. Nucleoli start to form within the new nuclei of the forming daughter cells. The mitotic spindles break down completely and the microtubules will be reused for the cytoskeleton of the new daughter cells. This is the end of mitosis. However, you may often see diagrams that combine telophase and cytokinesis (like the one above). This is because these two stages often happen at the same time, but when cell biologists talk about mitosis and telophase, they only mean the separation of the chromosomes, and cytokinesis is when the cell physically cleaves itself into two new daughter cells. See Fig 2.

Cytokinesis

Cytokinesis is the second stage of the mitotic phase and often happens concurrently with mitosis. This stage is truly when cell division occurs, and two new cells are formed after mitosis has separated the sister chromatids into their daughter chromosomes.

In animal cells, cytokinesis will begin with anaphase as a contractile ring of actin filaments from the cytoskeleton will contract, pulling the cell's plasma membrane inwards. This creates a cleavage furrow. As the cell's plasma membrane is pinched inwards, the opposite sides of the cell close, and the plasma membrane cleaves into two daughter cells (Fig. 3: Animal Cell).

Cytokinesis in plant cells occurs a little differently. The cell must build a new cell wall to separate the two new cells. Preparing the cell wall begins back in interphase as the Golgi apparatus stores enzymes, structural proteins, and glucose. During mitosis, the Golgi separates into vesicles that store these structural ingredients. As the plant cell enters telophase, these Golgi vesicles are transported via microtubules to the metaphase plate. As the vesicles come together, they fuse and enzymes, glucose, and structural proteins react to build the cell plate. The cell plate continues to build through cytokinesis until it reaches the cell wall and finally splits the cell into two daughter cells.

Cytokinesis is the end of the cell cycle. The DNA has been separated and the new cells have all the cell structures they need to survive. As the cell division is completed, the daughter cells begin their cell cycle. As they cycle through the stages of interphase, they will accumulate resources, duplicate their DNA into matching sister chromatids, prepare for mitosis and cytokinesis, and eventually have their daughter cells as well, continuing the cell division.

Mitotic Phase - Key takeaways

  • The mitotic phase consists of two stages: Mitosis and Cytokinesis. Mitosis is further broken down into five phases: prophase, prometaphase, metaphase, anaphase, and telophase.

  • Mitosis is how the cell separates its DNA chromosomes during cell division, and cytokinesis is the separation of the cell into new daughter cells.

  • The main events of mitosis are chromosome condensation during prophase, chromosome arrangement via spindle microtubules during prometaphase and metaphase, sister chromatid separation during anaphase, the formation of new daughter nuclei during telophase.

  • Cytokinesis in animal cells occurs with the formation of a cleavage furrow, that pinches the cell into two daughter cells. In plant cells, a cell plate is formed and builds into a cell wall separating the daughter cells.

Frequently Asked Questions about Mitotic Phase

The first mitotic stage in figure 2 is prophase. 

Prophase, Metaphase, Anaphase, Telophase 

The cell divides into two daughter cells. 

Mitosis or somatic cell division. Mitosis only occurs in somatic cells. 

The mitotic phase is when a somatic cell divides to produce two daughter cells. 

Final Mitotic Phase Quiz

Question

What are the two sub-phases of the mitotic phase? 

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Answer

Mitosis and cytokinesis

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Question

What are the phases of mitosis in order?

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Prophase, Prometaphase, Metaphase, Anaphase, Telophase

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What is the main concern of the cell during mitosis?

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The separation of the DNA chromosomes.

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What is the main concern of Cytokinesis?

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Answer

Separating the cell into two daughter cells.

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A cell plate will form in plant cells during what phase?

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Answer

Cytokinesis

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Question

During cytokinesis in an animal cell, how does the cell split?

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Answer

The cell forms a microtubule contractile ring to make a cleavage furrow.

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Question

Fill in the blank: Metaphase is easily identifiable because the chromosomes_______

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Answer

align in the center of the cell on the metaphase plate.

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What do kinetochores do during mitosis?

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Answer

Kinetochores form on each sister chromatid, allowing the spindle microtubules to pull the chromatids apart into new nuclei.

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What organelles build the spindle microtubules?

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Centrosomes

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The cell plate forms from what organelles?

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Answer

Golgi vesicles release enzymes structural proteins and glucose.

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Mitosis is sometimes called _________ meaning "cell nuclei movement"

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Answer

Karyokinesis

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Question

What phase of mitosis do the cohesin proteins break down and the sister chromatids separate?

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Answer

Anaphase

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What protein causes the chromosomes to condense and become visible?

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Answer

Condensin

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Where are the kinetochores located?

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In the centromere of the chromosome for each sister chromatid.

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Question

What three roles do the spindle microtubules play?

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Answer

1) separate the sister chromatids during anaphase

2) The polar microtubules elongate the cell in preparation for cytokinesis

3) The astral microtubules position the other microtubules in the cell.

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Question

What happens to the membranous organelles (Nucleus, Golgi Apparatus, Endoplasmic Reticulum) during mitosis?

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Answer

They break apart into vesicles and move to the outskirts of the cell, making way for the chromosomes to separate.

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What is another name for mitosis?

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Karyokinesis, meaning cell nuclei movement.

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What role does the kinetochore play during mitosis?

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Answer

Kinetochores attach the chromosomes to the spindle microtubules, allowing the chromosomes to be moved and pulled apart by the spindles

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At the end of mitosis, how much DNA do each new nuclei possess?

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A full set of DNA, (2n/diploid)

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Why does mitosis occur?

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Answer

Replacement of dead/degrading cells

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What are the steps of mitosis? 

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Answer

Prophase

Prometaphase

Metaphase

Anaphase

Telophase

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Question

In what phase of the cell cycle does cytokinesis occur?

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Answer

Mitotic phase

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What is cytokinesis?

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Answer

Cytokinesis is the phase when cell division actually occurs through the physical separation of the cytoplasmic contents into two genetically identical daughter cells

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In what cells does cytokinesis generally occur through a cleavage?

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Animal cells

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How does a cleavage furrow form?

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During anaphase, a contractile ring made up of actin filamentsfrom the cytoskeleton will form inside the plasma membrane(where the metaphase plate was). As the actin filaments interact with myosin molecules, the contractile ring contracts, pulling the cell's equator inward, thereby forming the cleavage furrow.

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Where does the contractile ring form?

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The contractile ring forms inside the plasma membrane, where the metaphase plate was.

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What is a major difference between cytokinesis in animal cells and plant cells?

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Answer

Cytokinesis in animal cells occurs through a cleavage, while in plant cells occurs through the formation of a cell plate.

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What is abscission in cytokinesis?

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Abscission is the process by which a cleavage furrow seals itself.

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Does cytokinesis always lead to the formation of two equal daughter cells? Explain. 

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Answer

No, there are some exceptions. For instance, during asymmetric cell division in developing organisms, an axis forms in the parent cell and the mitotic spindle reorients along this axis. Then, cell fate determinants are unequally distributed in the cell so that cytokinesis results in asymmetric daughter cells with different concentrations of fate-determining molecules, inducing different developmental outcomes for each cell. 

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How is a cell plate formed?

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As the plant cell enters telophase, these Golgi vesicles are transported via microtubules to form a vesicular structure called phragmoplast at the metaphase plate. Then, the vesicles move from the center of the cell toward the cell walls where they fuse together into a structure called a cell plate.

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After the formation of a cell plate, how does a plant cell separate into two daughter cells?

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The cell plate grows until its surrounding membrane merges with the plasma membrane around the cell's perimeter. This splits the cell into two daughter cells, each with its own set of organelles, and eventually enzymes harvest the glucose that has built up between the membrane layers to build a new cell wall between the two daughter cells. 

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Where is the phragmoplast formed?

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The phragmoplast is formed at the metaphase plate before moving towards the center of the cell.

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What happens after cytokinesis?

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Answer

Cytokinesis marks the end of the cell cycle. The DNA has been separated and the new cells have all the cell structures they need to survive. As the cell division is completed, the daughter cells begin their cell cycle. 

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Question

It is essential that cytokinesis occurs  ___ the segregation of chromosomes.

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Answer

after

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Question

This refers to an organism whose cells are missing one or more chromosomes

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Aneuploid

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Question

These structures form during cell division to separate and equally partition the chromosomes between the two daughter cells. 

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Answer

Mitotic spindles

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Question

Prior to mitosis, cells undergo______ in which the cell prepares for mitotic cell division.

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Answer

Interphase

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Question

During the gap 1 phase, the cell grows and produces a large amount of proteins which allows the cell to almost double in size.

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Answer

True

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Question

Gap 0 is technically not part of the cell division cycle but instead characterized a temporary or permanent resting phase in which the cell does not undergo cell division.

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Answer

True

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Question

The process of clearing out cellular debris is_____.

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Answer

Autophagy

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