The Cell Cycle
In this stage, the cell is preparing for the M Phase (Mitosis). The cell is continuously making RNA, producing protein, and growing. Interphase is broken into 4 parts: G0, G1, S, and G2. In G0, the cell leaves the cycle to stop its division. This period may be permanent, temporary or never happen at all. In G1, the cells grow and make RNA and protein. This phase is the midway between mitosis and the beginning of DNA replication. During G1, certain intracellular parts are replicated and the cell monitors itself in preparation for DNA replication in the S phase. The S Phase stands for synthesis and is characterized by the replication of DNA in the form of chromosomes. Through this, each cell created by cell division should have the same genetic makeup. Cell growth continues in the S Phase and so does the production of proteins associated with DNA synthesis. After the S Phase is the G2 Phase. G2, like G1, is an intermediate. G2 separates the S Phase and cell division to ensure that the S Phase has been completed correctly and monitor the cell. G2 is the last time the cell will grow before dividing.
Mitosis (M Phase)
Mitosis is the nuclear and cellular division of the cell to make two daughter cells. This phase is separated into prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis.
Prophase: Chromosomes begin to form and the nucleolus disappears. The centrioles (small blue pairs of cylinders) begin moving to opposite ends of the cell and fibers form from the centromeres. Some of these fibers (microtubules) become part of the mitotic spindle which acts as the railway for the chromosomes to travel during separation.
Prometaphase: The chromosomes attach to kinetochores on the centromeres and the chromosomes begin moving on the microtubules.
Metaphase: The spindle fibers of the mitotic spindle move the chromosomes to the middle of the cell nucleus. This line is known as the metaphase plate. This phase is necessary to ensure that each daughter cell receives one of each chromosome.
Anaphase: The chromosome pairs separate from the center of the cell nucleus and begin moving to opposite sides of the cell.
Telophase: The separated chromosomes arrive at opposite sides of the cell. New membranes form around the new nuclei. The chromosomes are no longer visible as they spread out. The spindle fibers disperse.
Cytokinesis: A ring of a protein called actin forms around the cell with two daughter nuclei and contracts in the middle of the cell in order to cut the cell into two daughter cells with their own nucleus. Cytokinesis only occurs in animal cells.
Tumor Suppressor Genes p53: The function of the p53 gene is to prevent cells with damaged DNA from dividing and passing on their faulty DNA to daughter cells. When the cell monitors itself during the cell cycle (G1 and G2), the p53 gene will stop the cell cycle if there is damaged DNA detected. Also, the p53 gene can order a cell with damaged DNA to commit cell suicide, or apoptosis. If the p53 gene is mutated, then tumors can form due to the inability of the p53 gene to regulate cell division of cells with bad DNA.
BRCA1 and BRCA2: These are two tumor suppressor genes associated with cancer. They are mutated in a high percentage of cases of breast cancer and ovarian cancer in cases before age 45. BRCA1and BRCA2 both produce tumor suppressor proteins. These proteins help repair damaged DNA and, therefore, play a role in ensuring the stability of the cell’s genetic material. When either of these genes is mutated so that its protein product is not made or does not work correctly, DNA damage may not be repaired the right way. As a result, cells are more likely to develop additional genetic problems that can lead to cancer.
ATM: The ATM gene provides instructions for making a protein that is mostly located in the nucleus of cells, where it helps control the rate at which cells grow and divide. This