Mitotic Phase

Mitotic Phase Definition

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.

Phases of Mitotic Cell Division

Prior to mitosis, cells undergo the 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. The interphase is divided into 3 steps: 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. There is an additional stage in which cells that won't undergo cell division are: Gap 0 (G0). Let's take a look at these four phases in more detail.

Fig. 2. As you can see, the interphase and the mitotic phase of cell division are different in both their function, but also their duration. The interphase takes much longer than the final stages of the cell division process, the mitotic stages.

Gap 0

Gap 0 (G0) is technically not part of the cell division cycle but instead is characterized by a temporary or permanent resting phase in which the cell does not undergo cell division. Usually, cells such as neurons that do not divide are said to be in the G0 phase. The G0 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.

Interphase

Gap 1 (G1) phase

During the G1 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 produces more organelles and increases its cytoplasmic volume.

Synthesis (S) phase

During this phase, the cell undergoes DNA replication where the amount of cellular DNA is doubled.

Gap 2 (G2) phase

The G2 phase is characterized by increases in cellular growth as the cell prepares to enter the mitotic phase. The mitochondria which are the cell's powerhouse also divide in preparation for cell division.

Mitotic stages

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.

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's 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 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 toward the centre 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 centre 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:

• The cohesion proteins that held the sister chromatids together break down.
• The mitotic spindles shorten, pulling the sister chromatids, now called daughter chromosomes, by the kinetochore to the poles of the cell with the centrosomes.
• Unattached microtubules elongate the cell into an oval shape, preparing the cell to split and make daughter cells during cytokinesis.

Telophase

Finally, we have the 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. 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, while cytokinesis is when the cell physically cleaves itself into two new daughter cells.

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.

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.