Cell Division and The Cell Cycle Help (page 2)
Introduction to Cell Division and the Cell Cycle
Cells have a limited life span, and many of them are destroyed accidentally. Every single day, the human body loses millions of cells! Obviously, human beings, as well as most other multicellular organisms, greatly depend upon cell division . During cell division, one cell becomes split into two cells. The original cell is called the parent cell . The two cells resulting from its division are referred to as daughter cells .
The Cell Cycle And Mitosis
Most cells within the human body go through the Cell Cycle – the entire life span of a particular cell, starting with its production from a previous parent cell, and ending with its division into two new daughter cells.
The Cell Cycle involves an orderly sequence of phases that is controlled by the DNA of the cell nucleus. Interphase is the phase occurring “between” ( inter -) cell divisions. Interphase takes up the great majority of time (about 90%) in the Cell Cycle. “What is going on during this 90% of the time?” an inquiring reader would likely ponder. Interphase provides enough time for the cell to grow large enough to eventually divide into two living daughter cells. The cell also synthesizes numerous proteins, as well as additional organelles.
Interphase begins with chromatin (kroh- MAT -in), slender strands of DNA that have a dark “color” ( chromat ) and are covered with a “protein substance” (- in ). The thin chromatin strands soon coil up and condense, creating thicker worm-like chromosomes. During interphase, the human cell makes copies of each of the 46 chromosomes in its nucleus. This creates 92 pairs of duplicated, identical chromosomes. These pairs are ready to be subdivided back into 46 single chromosomes, after the parent cell divides into two new daughter cells. The duplicated chromosome pairs line up in a vertical column in the middle or equator region of the parent cell.
After interphase, comes mitosis (my- TOH -sis). Mitosis literally means “a condition” (- osis ) of “threads” ( mit ). The “threads,” of course, are actually the thread-like chromosome pairs visible under a good microscope. The main “condition” that exists during mitosis is the division of the paired, duplicated chromosomes, into single, identical, unpaired chromosomes.
In preparation for cell division, a mitotic (my- TAH -tik) spindle is created near the nucleus. The mitotic spindle looks like an old-fashioned sewing spindle, being wider in the middle, and tapering towards both ends. (This really makes it resemble a modern fishing bobber!) The spindle is created from the orderly arrangement of cell proteins into a tapered, strand-like, pattern of microtubules. During mitosis, the duplicated chromosome pairs attach to the microtubules of the mitotic spindle. As the microtubules shorten, they pull the duplicated chromosomes apart from one another. The separated 46 chromosomes are thus moved into opposite poles or ends of the cell. Finally, a cleavage furrow appears as a narrow groove between the two pinching-off poles of the parent cell. The cleavage furrow is eventually replaced on either side by a complete new cell membrane. The ultimate result – two separate daughter cells, each with an identical set of 46 single chromosomes of their own. Eventually, each of these daughter cells enters into its own Cell Cycle, beginning with another interphase. By this means, body growth and replacement of worn or damaged cells readily occurs within our human body tissues.
Mutation and Biological Disorder in Cells - Cancer and Abnormal Organelles
Cancer: A Severe Disorder of Mitosis
In a normal mitosis, the orderly duplication and division of chromosomes, along with the division of the cytoplasm, creates two identical daughter cells. Both of these daughter cells are normal, as well. Thus, they go about doing whatever body task they have been genetically programmed to perform. Epithelial cells, for example, frequently divide and replace themselves with daughters, each daughter cell in turn performing a body covering or cavitylining function.
But what can happen if an abnormal “change” ( mut ) or mutation (mew- TAY -shun) in the genetic program occurs? One of the really bad results can be cancer! The English meaning (and the astrological sign) for cancer, of course, is the “crab”! An alternate translation for cancer is “creeping ulcer.”
Cancer is like a stubborn crab that can afflict either epithelial tissues (like the skin) or connective tissues (such as bone). When a person has a cancer, it is like a crab with pinchers, because it seems to hold on and not let go, making it very difficult to treat! Even worse, cancer has the characteristics of a creeping ulcer, because it often spreads from one affected site of the body, to many other sites. Small wonder, then, that various forms of cancer are a leading cause of death in many countries around the world.
The exact cause of cancer in human beings, unfortunately, is still unknown. But various chemical agents, called carcinogens (car- SIN -oh-jens), are suspected cancer or “crab” ( carcin ) “producers” (- gens ). Prominent among these suspected carcinogens are the poisonous or toxic chemicals present in cigarette smoke, which are thought to be the leading cause of lung cancer. The carcinogens may trigger abnormal changes (mutations) in the DNA of epithelial or connective tissue cells. The mutations create errors in the genetic program of the Cell Cycle, such that the resulting daughter cells are highly abnormal. The mutated, cancerous daughter cells have a largely unregulated, disorganized, and extremely rapid rate of mitosis. When these cancerous cells form larger tumors, the tumors interfere with the function and nutrition of normal cells. Eventually, the normal tissue cells die or are crowded out. If this process goes on long enough, the cell metabolism of the affected person may be so abnormally changed that the person dies.
Abnormal Organelles: Biological Disorder in Cells
Various normal organelles suggests the presence of Biological Order within cells. In general, such an order or pattern supports and maintains the health and survival of the cell.
What happens, however, when a cell organelle becomes abnormal or damaged? Certainly, one would expect to see some type of associated illness or cellular problem. Consider, for example, the condition called cell autolysis (aw- TAH -luh-sis) or “self breakdown” of a cell. When a particular cell is dying, dozens of its lysosomes may rupture simultaneously. This rupturing releases thousands of stored digestive enzyme molecules. When such a huge number of these powerfully dissolving enzymes are present at the same time, they break down the whole cell. Cell autolysis thereby often serves to remove dead or dying cells from otherwise healthy body tissue. Unnecessary (and perhaps disruptive) build-up of extra dead cells is prevented.
Practice problems for these concepts can be found at: Cells: The “Little Chambers” In Plants And Animals Test
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