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Apoptosis, programmed cell death, is a hot issue to biologists and scientists. It occurs in multicellular organisms with active process. Apoptosis occurs by cell shrinkage, protein fragmentation, chromatin condensation, DNA degradation and collapse of cell. 
Cells die in two different ways: apoptosis and necrosis. Apoptosis and necrosis can be similar, but they are very different. Apoptosis is controlled process for number of cells, but necrosis is dying cells by swelling and bursting. Unlikely necrosis, apoptosis is integral process. Body weight is balanced, body, body shape, separating digits, and making right neuronal connections occur by apoptosis. 
Apoptosis has two pathways: extrinsic and intrinsic. Extrinsic pathways deliver apoptotic messages, and intrinsic pathways transmit death signals. 
Since the 1990s, research on apoptosis has been a hot issue. Apoptosis is defective process, so it might bring cancer, ischemic damage, and cell-lose disease. Usually 50 billion to 70 billion cells die each day. 
Labeling nuclear DNA fragmentation and gel electrophoresis are methods to identify apoptosis.
The word origin of apoptosis is Greek. In Greek, apoptosis means falling off. Apoptosis is a very necessary part of the life cycle of organisms. Apoptosis is an active process and plays an important role in multicellular organisms in that it regulates and adjusts cell population. 
Programmed cell death, Apoptosis, is a necessary process in both animal tissue and plant tissue development. The cells grow and produce other cells, but also die, due either to Apoptosis or cell injuries. Dead cells are used by other cells.
Different tissues use different signals for apoptosis. BMP, bone morphogenetic proteins, induces apoptosis in interdigital tissue which is in birds. 
Cell death currently remains a mystery. However, scientists have proposed two possible reasons for programmed cell death:
- The cells may no longer be needed. When a baby is in his mother’s womb, his fingers or toes are connected to each other by connective tissue. While he is growing up, the connective tissue disappears through cell death (apoptosis). If the connective tissue does were not killed by apoptosis, the baby’s fingers would remain attached as with a duck’s feet.
- Apoptosis may be a mechanism for preventing cancer. When the cells live longer, they have a genetic problem or damage. Finally the damage leads the cells to become the cancer. Without apoptosis, there is the possibility that originally normal cells become cancer. (Purves, p208)
Research in the area of cell death indicates that apoptosis is a very important process in our body.
Cell signals control process of apoptosis. These cell signals are either extracellular or intracellular.
Hormones, growth factors, nitric oxicers, and cytokines are extracellular signals, in that they cross plasma membranes. Intercellular signals are cell stress, cell suicide, heat, radiation, nutrient deprivation, and viral infection. 
Apoptosis occurs within extrinsic pathway and intrinsic pathways. Extrinsic pathway occurs by the simulation of a death receptor which is located on the plasma membrane. (Ligands are signal molecules that simulate death receptors.) Intrinsic pathway occurs by releasing of signals within mitochondria cells. (Cellular stress triggers intrinsic pathway. Cell stress is caused by DNA damage and heat shock.)  Studying and understand pathways of apoptosis will help to prevent and treat diseases.
Defective Apoptotic Pathways
Diseases appear by disorder or modification of apoptotic pathways. Disorder and modification prevent normal function of apoptosis. Because apoptosis does not notice “use-by-date”, cells live longer or replicate and produce more cells. When unnecessary cells live longer, they can be cancer or disease.
HIV, Human Immunodeficiency Virus, has gene that cannot kill infected cells by apoptosis pathway. It happens when host’s immune system becomes weak. The host’s immune system becomes weak, because of wrong pathway of apoptosis programs to kill immune cells. In other words, number of immune cells is decreased by wrong pathway of apoptosis. 
Function of Apoptosis
Even though apoptosis is programmed cell death, it plays an important role in the life of an organism. Apoptosis helps to form the overall body shape and the shape of organs. In the immune system, apoptosis plays an important role that eliminates aggressive immune cells. 
Apoptosis plays an important role in immune system. Every person has white blood cells and red blood cells. If a person has more number of white blood cells than number of red blood cells, he will have a caner. If apoptosis plays its own role (programmed cell death), he will not have the cancer (follicular large-cell lymphoma). Unfortunately the white cells do not die, but they keep dividing. By dividing white blood cells, mutation occurs. A person will eventually die from over population of white blood cells. (Purves, p401)
Cell damage or Infection
Apoptosis is an important process in that it prevents diseases and controls unnecessary cells within the body. When cells are damaged, apoptosis occurs and the cells die. If apoptosis does not occur, the damaged cells will be nutrients to other organisms. Later these organisms will grow and become diseases. Therefore, apoptosis is integral process in immune system. Apoptosis prevents cancer. When apoptosis does not occur well, the cells divide and develop more cells. Later those cells will be cancer or diseases. 
- Introduction to Apoptosis Celldeath
- Apoptosis World Apoptosis World
- Apoptosis Wikipedia
- Apoptosis The Science Creative Quarterly
- Apoptosis Nature.com
- Apoptosis Essential Science Indicators Special Topics
- Purves, Sadava, Orians, and Heller. Life: The Science of Biology. Virginia: Gordonsville, 2004