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Programmed cell death is also known as apoptosis, in biology, it refers to a mechanism which allows cells to self-destruct when stimulated via appropriate triggers. Apoptosis can also be triggered via mild cellular injury and various factors that may be internal or external to the cell; the damaged cells are then disposed of. Morphologically distinct from programmed cell death, apoptosis is different from the other major process of cell death such as necrosis. Apoptosis includes condensation of the nucleus and cytoplasm, followed by cellular partitioning into small fragments for disposal. In most multicellular organisms, cell number mainly results as the rate of cell production minus the rate of apoptosis.
Discovery of programmed cell death
In the year 1840, a biological use of the mechanism of planned apoptosis became famous when scientists realized that the development via a fertilized egg into an adult is not a linear process. Whereas in many cases, initial structures, like the tadpole’s tail, are superseded via entirely different adult systems, like the frog’s legs. In the early 20th century the medical importance of cell death was observed via an Australian researcher named John Foxton R. Kerr and a Scottish scientist Andrew H. Wyllie and Alastair Currie. In a paper published in the year 1972, they used the term apoptosis (which is a Greek word meaning “falling off,” just like leaves in autumn) in order to demonstrate the occurrence of apoptotic cells in human tissues.
The discovery of the developmental stage and death of each cell in the nematode Caenorhabditis elegans proved the role of programmed cell death in development. South African biologist Sydney Brenner, American biologist H. Robert Horvitz, and British biologist John E. Sulston together shared the Nobel Prize in Physiology in the year 2002 for this work.
In the majority of adult animals apoptosis can be used in the removal of cells that have become a threat to survival. Such types of cells include cancer cells or cells that are infected by bacteria or viruses. Apoptosis also removes cells which are normal but are no longer needed; these types of cells produce antibodies after the need for the antibody is over. Apoptosis can also be triggered by external stimuli, that includes nutrient removal, toxins, hormones, heat, and radiation. It is calculated that a mass of a cell equal to its body weight is removed by apoptosis each year. This observation has generated a massive research effort which is focused mainly on apoptosis.
Regulation of apoptosis
Apoptosis takes place on a cell-by-cell basis. In each affected cell, two primary phases can be seen: one of initiation and a second of execution. The resulting cell remains are processed for reuse. Both phases are complex and require an organization of multiple cellular systems, including interactions among proteins and cellular membranes. The initiation phase, or “death decision,” became of immense interest following the description of a group of proteins in mammals called the BCL-2 protein family. This protein family, which provides the framework for controlling apoptosis, takes its name from a type of cancer known as B-cell lymphoma. BCL-2, the first family member, forms the molecular basis for maintaining the lymphoma cancer cells. The BCL-2 family of proteins has at least 25 members. Many of these are called BH-3-only proteins. BH-3-only proteins function as activators or sensitizers of apoptosis and help in monitoring important cell processes for dysfunction. They also help in controlling the function of two death-initiating, or pro-apoptotic, proteins like Bax and Bak and a large number of death-preventing, or anti-apoptotic, proteins, which involves BCL-XL and BCL-2. In mammals this control takes place primarily on the membranes of mitochondria, where the mortality decision of each cell is constantly observed under the supervision of the three contesting factions of the BCL-2 protein family.
Following the intrinsic pathway, cancer cells are the cells that are infected by bacteria or virus particles, and some mutant cells can also be assigned to apoptosis. The extrinsic pathway is usually associated with cellular death receptors.
Function of Apoptosis
Apoptosis is a vital evolutionary adaptation since it allows organisms to destroy their own cells. Whenever a part of someone has become dangerous to the rest, in the case of cells with damaged DNA that could become cancerous. Apoptosis is a main killer of pre-cancerous cells, and people with mutations that prevent apoptosis from functioning correctly are much more likely to get cancer.
Multicellular organisms may also tend to lose cells which are no longer useful to the organism.
Meiosis
Before the beginning of meiosis, in S phase of the cell cycle DNA present in each chromosome is replicated so that it contains two genetically identical sister chromatids which are held together via sister chromatids cohesion. This S phase is known as “premeiotic S phase” or “meiotic S phase”. Immediately after DNA replication meiotic cells enter into a G2 like stage by the name meiotic prophase. At this time homologous chromosomes pair with each other and undergo a programmed process known as genetic recombination, where the DNA may be cut and is repaired so that it can exchange some of their genetic information. A set of recombination events results in crossovers that lead to the formation of physical links referred to as chiasmata between homologous chromosomes. In various organisms these crosslinks help each pair of homologous chromosomes to segregate away from each other during Meiosis I and thus, resulting in the formation of two haploid cells that contain half the number of chromosomes as that of the parents.
During meiosis II the cohesion that exists between two sister chromatids segregates from one another just like in mitosis. Whereas sometimes all four of the meiotic products result in the formation of gametes like sperms, spores or pollens. As the number of chromosomes is halved during meiosis the resulting gametes can fertilize to form a diploid zygote containing two copies of each chromosome, one from each of the parents. Thus sexual reproduction is enabled by the alternating cycles of meiosis and fertilization, resulting in successive generations which maintains the same number of chromosomes. Like a diploid human cell comprises 23 pairs of chromosomes along with 1 pair of sex chromosomes which is half of maternal origin and half of paternal origin. Meiosis produces only haploid gametes (i.e. sperms and ovum) having one set of 23 chromosomes. Meiosis can be seen in all sexually reproducing single celled or multicellular organisms including plants animals and fungi.
Conclusion
Programmed cell death is also known as apoptosis, in biology, it refers to a mechanism which allows cells to self-destruct when stimulated via appropriate triggers. Apoptosis can also be triggered via mild cellular injury and various factors that may be internal or external to the cell. Meiosis is very essential in life as it produces sex cells for continuation of life processes on earth. This process may take about minutes or hours depending upon the kinds of cells. It is influenced by temperature and chemicals. We hope that this article was helpful in clearing all your doubts regarding meiosis.
