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Cell Cycle One Back gro und N o tes OBJECTIVES Students will: will investigate how the cell cycle allows for cell replication build a model of the cell cycle and examine its four phases discover how the cell cycle is regulated discover that loss of regulation of the cell cycle can lead to development of a tumor and cancer Background 1: During mitosis (prophase, metaphase, anaphase, and telophase), the replicated chromosomes are separated so that each daughter cell receives an exact copy of the parent cell’s DNA. However, mitosis is only part of the cell cycle. The complete cell cycle is an ordered set of events that results in the production of two identical cells from a single parent cell (cell division). Regulation of the cell cycle is very important. If cells divided without controlling proteins, the organization of the tissue would be lost and the function of the organ would be affected. Typically when cells come into contact with one another, interactions between the cell membranes stop the production of the proteins that move the cell into the cell division phase. This process is called contact inhibition. For example, some cells, like the lung alveolar cells, need room to expand and contract with oxygen. Therefore, these cells must only grow in a single layer. In this way, the lung alveolar cells will only divide when they are not in contact with other cells, such as when there is a hole in the alveoli. Cancer is the uncontrolled cell division of body cells, which occurs when one cell loses the ability to regulate its cell processes. This can happen after the cell’s DNA is damaged and affects the production of controlling proteins. The mutated cell, and all of its daughter cells, will follow a faster rate of division. This unregulated process allows a tumor, which is a mass of cells, to develop. A tumor may or may not be malignant (spreading). Tumors invade normal tissue and alter the tissue’s overall organization and structure. This disorganization leads to an inability of the tissue to function and ultimately affects the function of the organ. If a single cancerous cell breaks away from the tumor, it can travel in the blood to other places throughout the body. At a new site, this single cancer cell will rapidly divide, forming another tumor. This process is called metastasis and is how cancer spreads. Background 2: The length of time for a cell to complete one full cell cycle (from one phase of mitosis to the next) varies depending on cell type. For example, a cell in an embryo may go through one round of the cell cycle in one hour while other cells may take one year. How does a cell “know” when to move between the phases of the cell cycle? Control of the cell cycle occurs through specific proteins which are translated from genes in the cell’s DNA. When the cell makes these proteins, the proteins allow the cell to transition into the next phase of the cell cycle. These inducing proteins turn on the cell cycle. There are also genes that code for proteins that block the cell from moving into the next phase of the cell cycle. These inhibiting proteins stop the cell cycle. It is the balance of the inducing and inhibiting proteins that control the cell cycle. Different cell types have varying amounts of these proteins and, therefore, different lengths of time that they will stay in each phase of the cell cycle. The majority of cells spend most of their time performing their specialized function before quickly moving through mitosis and cytokinesis. In some rapidly dividing cells, mitosis and cytokinesis only last a few minutes. Background 3: Remember that regulation of the cell cycle is very important. If cells divided without controlling proteins, the organization of the tissue would be lost and the function of the organ would be affected. Production of proteins that control the cell cycle occurs when a cell is stimulated by external factors. If the gene that codes for the controlling protein is damaged or mutated, then a properly functioning protein cannot be produced. If there is no production of the controlling proteins, the cells will go through the cell cycle and divide without regulation. Each new (daughter) cell will inherit the mutated gene for this controlling protein because the cell receives an exact copy of the DNA in the original (parent) cell. This means that every daughter cell produced from the damaged cell will also divide continuously.