What is the cell cycle?

The National Human Genome Research Institute defines the cell cycle as “a series of events that takes place in a cell as it grows and divides.” Have you ever got a cut or a scrape and wondered how your body is able to regrow that skin to heal it? Your body is able to do this through a process called the cell cycle, more specifically the mitosis portion. The cell cycle includes two major phases, interphase, and the mitotic phase. The mitotic phase is when mitosis (cellular division) and cytokinesis happens. According to an article published by the University of Hawaii at Manoa, "Mitosis helps organisms grow in size and repair damaged tissue." The outcome of mitosis is two identical daughter cells. That means your body goes through the process of cellular division countless times just to heal a simple paper cut. Mitosis is divided into a series of distinct phases. The phases include prophase, prometaphase, metaphase, anaphase, and telophase. There are also additional elements of the cell cycle important to cellular division, interphase, cytokinesis, and G0 phase. 

Interphase

During interphase, the cell is preparing itself to begin the process of division. The cell grows and DNA (Deoxyribonucleic acid; genetic material) is replicated during interphase. Interphase is made up of three phases, G1 phase, S phase, and G2 phase. 

G1 phase is known as the growth phase. The cell is gaining the beginnings of chromosomal DNA and the proteins associated with it. During the G1 phase the cell is accumulating enough energy to carry out the S phase. 

The S phase is known as the DNA replication phase. DNA is made up of chromosomes. Each chromosome is made of two sister chromatids. During the S phase the centrosome is duplicated.  Spindle fibers will develop from the centrosome later.

 G2 phase is known as the second growth phase. The cell replenishes its energy and gathers the proteins required for chromosome manipulation. The G2 phase is the final preparation for the cell to enter mitosis.

Above: This image of the cell cycle shows that a cell spends most of its time in interphase. Only a small sliver of time is spent in the mitotic phase including mitosis and cytokinesis.

Prophase 

It is the first official phase of mitosis. During prophase the nuclear envelope starts to break into small vesicles. The Golgi apparatus and the endoplasmic reticulum fragment. The nucleolus disappears and the centrosomes begin to move to opposite poles of the cell. Sister chromatids start to coil tightly. Fun fact, at this point the sister chromatids will be visible under a light microscope.

Prometaphase

During prometaphase, the chromosomes become more condensed and form a connection with the cytoskeleton. The mitotic spindle continues to develop.  

Metaphase

This phase is called metaphase meaning middle because the chromosomes are aligned in a plane called the metaphase plate in the middle of the cell in between the two cellular poles. 

Anaphase

During anaphase, the chromosomes that are aligned in the middle split apart at the centromere into chromatids. Each chromatid is pulled toward the centrosome. The cell becomes visibly elongated at this point.  

Telophase

During telophase, the chromosomes reach opposite poles and begin to unravel. The previously mentioned mitotic spindles are broken down and will later be used to assemble the cytoskeleton components for each daughter cell. The nuclear envelopes form around the chromosomes. At this point you can start to see the distinction between what the two separate daughter cells will soon be.  

Cytokinesis

Although cytokinesis is not phase of mitosis, it is still important to mention. Cytokinesis is the physical separation of the cell into the two separate daughter cells. Cytokinesis is different between plants and animals. Animals have no cell wall, so cytokinesis begins after anaphase begins. Plants have a cell wall, so during cytokinesis the cell wall beings to crack which is called the cleavage furrow.  

Above: This is a representation of a more simplified visual aid of what exactly happens in each stage of mitosis.

G0 Phase

This phase marks when the cell is not actively preparing for division. Cells enter this phase temporarily until external signals trigger the start of G1.  

Review

To recap, interphase, prophase, prometaphase, metaphase, anaphase, telophase, cytokinesis, and even the G0 phase all contribute to how the skin on your body heals itself (mitosis is not limited to just this example). All of these phases make up the cell cycle in which mitosis occurs during the mitotic phase along with cytokinesis. The outcome being two identical daughter cells. Next time you get a scrape, just remember how complex the healing process is even on a cellular level.

References

National Human Genome Research Institute. (n.d.). Cell cycle. Genome.gov. https://www.genome.gov/genetics-glossary/Cell-Cycle

University of Hawaii at Manoa. (n.d.). Exploring our Fluid Earth. Growth, Development, and Reproduction | manoa.hawaii.edu/ExploringOurFluidEarth. https://manoa.hawaii.edu/exploringourfluidearth/biological/aquatic-plants-and-algae/growth-development-and-reproduction#:~:text=The%20process%20of%20mitosis%20generates,size%20and%20repair%20damaged%20tissue.