Cell Division and The Cell Cycle Help
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.
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