The Cell Cycle
09/10/2010 § 1 Comment
The last biology class we had on Thursday was dedicated to learning about the Cell Cycle that all eukaryotic cells go through. During the class, we discussed the five different phases of the cell cycle, learning that three of them are part of the Interphase, which contains the first three stages of the cycle. The interphase is the time of a cell’s life when the cell and its organelles do all their usual functions to contribute to the cell’s sustainment and life. We also learned about the checkpoints that are actually chemical switches that control the stops and starts of a phase during the cycle. Finally, we learned about the disease of cancer and what happens when a person has a tumor anywhere in their body.
The first phase of the cell cycle is the Growth 1 phase, more commonly referred to as the G1 phase. This is a major stage in a the cell’s life, where it spends its time growing and where each organelle performs its specialized job to contribute to the cell’s survival. This is also the phase where proteins are made, but this fact is irrelevant as our proteins are constantly making proteins to keep our bodies going. Some cells continue to grow here and proceed to the next four stages and reproduce but others—like brain cells—stay in this stage and don’t divide, therefore remaining in a phase called G1-sub-0.
The next phase is the Synthesis phase, referred to as the S phase. This is the stage where the cell begins to copy its DNA, which are in the nucleotides, which are in the chromatids, which make up a chromosome. At first, one chromosome is made up of only one chromatid, only one strand. After the synthesis phase, however, a copied chromosome is made up of two chromatid strands connected at the middle by a centromere. The new chromatid strand is the copied version of the original chromatid.
[Synthesis happens when enzymes split the weak bonds of the DNA strands and opens it up easily like a zipper. Then new and ‘empty’ DNA strands float over to copy the original information onto themselves. These new and ‘empty’ DNA strands come from the very food we put into our bodies. The information is copied efficiently because of the coding in our DNA. The letters of the code match up and copy the exact replica of the DNA. (Letter matching is Adenosine-Thymine, Guanine-Cytosine). All of this work takes up about eight hours, which makes sleep very important so that the copying of our DNA is done correctly.]
The next and final stage of the interphase is the Growth 2 phase, or G2. This stage involves even more growth, production and more proteins-being-made to prepare for the final two stages of reproduction. In comparing this whole cycle to stage production and performing a musical, the G2 phase would be preparing the stage and getting the final touches on the costume, polishing up the lines, and such. The performance is basically almost ready.
The fourth stage of the cell cycle is mitosis. In this phase, the centromere splits, leaving the two chromatids free. The nucleus divides and makes two nuclei, ready for each daughter cell. Just to be clear, mitosis only divides the nucleus of the original cell and doesn’t divide the cell just yet. The fourth phase of the cell cycle include nuclear division to ensure that the two daughter cells will get the same and accurate amount of DNA and information.
Finally, the last stage of the cell cycle is cytokinesis. Here is where the cell finally divides itself, producing two daughter cells as a result. Proteins that have situated themselves to opposite ends of the cell reach out for a chromatid each and drags it to its own side of the cell. As a personal analogy that helps me remember this stage, I imagine many Mr. Fantastic’s (Reed Richards from the Fantastic Four Marvel comic book series) saving civilians. He (the protein) reaches out for a civilian (a chromatid) with each hand and pulls them to safety (one side of the cell). During this process, the cytoplasm of the cell also divides, finally making the two daughter cells that are ready to perform their own cycle.
Amongst all this reproduction and dividing and pulling civilians to safety, there are checkpoints that happen to regulate the amount of cells that are dividing and reproducing—a that fact becomes very important later when I discuss what happens in cancer. Checkpoints are there to check the cell’s progress during the cycle to act as tests for the cell to pass. If the cell doesn’t pass the test, it means it made a mistake and there’s something wrong with its work and progress. From there, enzymes can come and fix the problem or the cell will be instructed to self-terminate itself and die. This happens so that the mistakes the cell made does not get passed on to the next generation of cells and only the successful and ‘good’ cells get to keep going. This is a strictly efficient system that assures the health of the daughter cells that will be produced.
There are three major checkpoints that we learned in class. The first checkpoint happens before G1 ends, and makes sure that the cell is large and healthy enough; prepared appropriately for the next stage, Synthesis. The next checkpoint occurs during G2, when the enzymes come and repair whatever needs fixing. Here, the DNA’s replications are checked whether or not they can proceed to the next stage. The final checkpoint occurs at the end of mitosis and tells the cell (like a streetlight would) that it can proceed to cytokinesis and then to Growth 1.
Lastly, we learned about how the disease cancer works. Cancer is one of those diseases that continues to stump doctors to this day and is a commonly known world-wide disease. Cancer occurs when there is a loss of control during the cell cycle and division and a cell no longer contributes to the survival of the body. Cells that have become cancerous no longer have any maintenance over the amount of cells that keep dividing and reproducing. Because of this, the cells eventually accumulate too much and invade the blood veins, releasing more cancerous cells into the flow of blood, which enables them to move to other parts of the body, spreading the cancer. Cancer is caused by mutated genes that fail to copy correctly during the cell cycle. Some of these mutations include 1) point mutation, when the genes instruct the cell to make a different protein and do the wrong thing for the body, 2) gene amplification, when multiple copies of genes are made, meaning multiple copies of proteins are made and even more cells grow and divide, and 3) translocation, when genes move to a different place in the DNA strands and instruct the cell to produce extra proteins and cause a constant stream of proteins which probably won’t do well for the cell.
We also learned about proto-oncogenes and tumor suppressors that both prevent the overproduction of new cells. When a proto-oncogene becomes simply an oncogene because of mutation, then it can no longer do normal cell division. When a tumor suppressor is mutated, it can no longer ‘suppress’ tumors that can grow in the body meaning a tumor will grow in the body.
So now we know how the cell actually reproduces. How a eukaryotic cell reproduces, to be specific. But why they have to reproduce this way, we don’t particularly know yet, and why there must be sex, we still kind of don’t know.