Cell Biology of Cancer
The cell is the fundamental unit of life. It is the smallest structure of the body capable of performing all of the processes that define life. Each of the organs in the body, such as the lung, breast, colon, and brain, consists of specialized cells that carry out the organ's functions such as the transportation of oxygen, digestion of nutrients, excretion of waste materials, locomotion, reproduction, thinking, etc.
To assure the proper performance of each organ, worn out or injured cells must be replaced, and particular types of cells must increase in response to environmental changes. For example, the bone marrow increases its production of oxygen-carrying red blood cells sevenfold or greater in response to bleeding or high altitude. Certain white blood cells are produced more rapidly during an infection. Similarly, the liver or endocrine organs frequently respond to injury by regenerating damaged cells.
As stated in the previous section, reproduction of cells is a process of cell division. The division of normal cells is a highly regulated process. The cell growth, inheritance and containment is controlled by its DNA (deoxyribonucleic acid).
DNA is a highly complex molecule manufactured in the cell nucleus and serves as the cell's "brain." DNA is the blueprint for everything the cell does. In a human cell, the DNA is arranged in 46 distinct sections called chromosomes. They are arranged in pairs, 23 chromosomes from each biological parent.
Together, the 46 chromosomes contain more than 100,000 genes. A gene is a segment of DNA that determines the structure of a protein, which is needed for development and growth as well as carrying out vital chemical functions in the body. Like the chromosomes, genes are arranged in pairs — one gene from the mother and one from the father.
Each gene occupies a specific location on a chromosome. Through a number of biochemical steps, each gene tells a cell to make a different protein. Some genes instruct the cell to manufacture structural proteins, which serve as building blocks. Other genes tell the cell to produce hormones, growth factors or cytokines, which exit the cell and communicate with other cells. Still other genes tell the cell to produce regulatory proteins that control the function of other proteins or tell other genes when to turn "on" or "off." When a gene is turned on, it manufactures another complex molecule called ribonucleic acid (RNA), which contains all the information the cell needs to make new proteins.
Cells divide only when they receive the proper signals from growth factors that circulate in the bloodstream or from a cell they directly contact. For example, if a person loses blood, a growth factor called erythropoietin, which is produced in the kidneys, circulates in the bloodstream and tells the bone marrow to manufacture more blood cells.
When a cell receives the message to divide, it goes through the cell cycle, which includes several phases for the division to be completed. Checkpoints along each step of the process make sure that everything goes the way it should.
Many processes are involved in cell reproduction and all these processes have to take place correctly for a cell to divide properly. If anything goes wrong during this complicated process, a cell may become cancerous.
A cancer cell is a cell that grows out of control. Unlike normal cells, cancer cells ignore signals to stop dividing, to specialize, or to die and be shed. Growing in an uncontrollable manner and unable to recognize its own natural boundary, the cancer cells may spread to areas of the body where they do not belong.
In a cancer cell, several genes change (mutate) and the cell becomes defective. There are two general types of gene mutations. One type, dominant mutation, is caused by an abnormality in one gene in a pair. An example is a mutated gene that produces a defective protein that causes the growth-factor receptor on a cell's surface to be constantly "on" when, in fact, no growth factor is present. The result is that the cell receives a constant message to divide. This dominant "gain of function gene" is often called an oncogene (onco = cancer).
The second general type of mutation, recessive mutation, is characterized by both genes in the pair being damaged. For example, a normal gene called p53 produces a protein that turns "off" the cell cycle and thus helps to control cell growth. The primary function of the p53 gene is to repair or destroy defective cells, thereby controlling potential cancerous cells. This type of gene is called an anti-oncogene or tumor suppressor gene. If only one p53 gene in the pair is mutated, the other gene will still be able to control the cell cycle. However, if both genes are mutated, the "off" switch is lost, and the cell division is no longer under control.
Abnormal cell division can occur either when active oncogenes are expressed or when tumor suppressor genes are lost. In fact, for a cell to become malignant, numerous mutations are necessary. In some cases, both types of mutations — dominant and recessive — may occur.
A gene mutation may allow an already abnormal cell to invade the normal tissue where the cancer started or to travel in the bloodstream (metastasize) to remote parts of the body, where it continues to divide.
A normal cell can become damaged in different ways. A cell can become abnormal when part of a gene is lost (deleted), when part of a chromosome is rearranged and ends up in the wrong place (translocation), or when an extremely small defect occurs in the DNA, which results in an abnormal DNA "blueprint" and production of a defective protein occurs.
Abnormal cell division can also be caused by viruses. In this case, genes may be normal, but the protein may not function normally because the cell contains a cancer-producing virus.
How a specific cancer cell behaves depends on which processes are not functioning properly. Some cancer cells simply divide and produce more cancer cells, and the tumor mass stays where it began. Other cancer cells are able to invade normal tissue, enter the bloodstream, and metastasize to a remote site in the body.
In summary, cancer cells have defects in normal cellular functions that allow them to divide, invade the surrounding tissue, and spread by way of vascular and/or lymphatic systems. These defects are the result of gene mutations sometimes caused by infectious viruses.
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