During a lifetime, many of the cells that make up the body age and die. These cells must be replaced so that the body can continue functioning optimally. Reasons that cells are lost and must be replaced include the following:
Sloughing off of epithelial cells such as those lining the skin and intestines. The old, worn out cells on the surface of the tissues are constantly replaced. A special case of this is the monthly replacement of the cells lining the uterus in pre-menopausal women.
Wound healing requires that cells in the area of the damage multiply to replace those lost. Viral diseases such as hepatitis may also cause damage to organs that then need to replace lost cells.
Replacement of the cells that make up blood. Red blood cells carry oxygen to tissues. White blood cells such as B and T lymphocytes are part of the body's immune system and help to ward off infections. Most of these cells have very short lifespans and must be constantly replaced. The precursors of these cells are located in bone marrow. These precursors, or stem cells, must reproduce at a very high rate to maintain adequate amounts of the blood cells.
The process by which a cell reproduces to create two identical copies of itself is known as mitosis. The goal of mitosis is the formation of two identical cells from a single parent cell. The cells formed are known as daughter cells. In order for this to happen, the following must occur:
The genetic material, the DNA in chromosomes, must be faithfully copied. This occurs via a process known as replication.
The organelles, such as mitochondria, must be distributed so that each daughter cell receives an adequate amount to function.
The cytoplasm of the cell must be physically separated into two different cells.
As we will see, many of the features of cancer cells are due to defects in the genes that control cell division. The cell division process occurs as an orderly progression through four different stages. These four stages are collectively known as the cell cycle. The following pages describe the cell cycle in detail.
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