Cell proliferation, also known as cell growth, cell division, or cell replication is the basic process through which cells create new cells. Cells that contain an organized nucleus are classified as eukaryotic cells; and the life forms containing eukaryotic cells are classified as eukarya or eucaryotes. Cells lacking an organized nucleus, (e.g., bacterial cells), are classified as procaryotic cells, and those life forms containing prokaryotic cells are known as prokarya or procaryotes.
Cell proliferation is important to many biological processes, including the population growth of microorganic life forms and, in more complex organisms, fertilization, embryo development, body growth, and tissue repair and renovation. In higher animals and plants, reproductive cell proliferation is critical to gamete (sex cell) formation and species perpetuation. Somatic cells (i.e., non-reproductive cells) also proliferate to promote embryo development, growth, repair, and renovation of organs and body structures.
Growth processes from birth until early adulthood are an outcome of cell proliferation mechanisms. Normal growth (e.g., bone, hair, and nail growth) occurs via cell proliferation. Skin healing after a cut, or the mending of a broken bone are the result of tissue repair acomplished through cell proliferation. Less common examples of cell proliferation also exist. For example, organ-compensation overgrowth that occurs in one kidney when the other is surgically removed. In this case, renal cell proliferation (the same as kidney cell proliferation) takes place in order to increase the functional capability of the remaining kidney.
Cell proliferation may occur through two different division mechanisms: mitosis or meiosis. In eukaryotic organisms such as plants and higher animals, whose bodies are constituted by many different cell types, the vast majority of body cells are known as somatic cells or diploid cells. Somatic cells proliferate through mitosis. Unicellular microorganisms, i.e. one-cell organisms, like bacterial cells (prokaryotes) and yeast cells (eukaryotes), multiply their population also through mitosis. In mitotic division, each of the two new cells is normally provided with the same number of chromosomes as those existing in its parental cell.
Meiosis is a division process occurring in the formation of reproductive cells called gametes or germ cells. Gamete cells, also known as gametocytes, produce by meiosis either spermatocytes in males or oocytes in females. Young spermatocytes mature into spermatozoids and young oocytes become mature ovum or eggs, thus being capable of entering into union with another to form a fertilized egg cell. In meiotic division two cellular divisions occur, after the chromosomes are duplicated only once. As a diploid cell undergoes two divisions, resulting in four young gametocytes, each new cell contains only half of the original number of chromosomes of its parental cell, and is called a haploid cell.
The process of cell proliferation depends on the prior occurence of a series of internal cellular changes before either mitotic or meiotic division starts. These changes occur in a given ordely sequence known as the four phases of the cell cycle, and both mitosis and meiosis are the final steps of a given cell cycle.
In normal healthy tissues, cell cycle and cell proliferation are highly regulated events which ensure correct duplication or copying of the cellular genetic contents of each chromosome before cell division. The total genetic content of a cell is referred to as genome, and the process of making a genetic copy is referred to as DNAtranscription. DNA is comprised of genes, the basic units encoding genetic information. Each chromosome is basically a condensed DNA molecule, carrying a given set of genes, and packed by a family of proteins known as histones.
When the cell cycle escapes its regulation mechanisms or proliferation controls, the resulting new cells will carry a genetic defect that eventually can be passed away to their own off springs. These cells are denominated transformed cells. Transformed cells tend to undergo further errors during DNA transcription, and eventually may suffer a major mutation in one or more genes. As genes encode the making of specific proteins, necessary to a variety of vital functions, this ultimately implies that mutated genes will express abnormal proteins or will stop producing a given protein at all. The lack of a single protein, or the presence of an abnormal protein in a given organ or tissue can cause disease. Degenerative diseases, such as arthritis, diabetes mellitus, and cancer, are caused by errors in cell replication, as a result of errors in DNA transcription during the cell cycle. Cancer in particular, is often the final result of a progressive accumulation of DNA transcription errors occuring in transformed cells, ultimately leading to uncontrolled cell proliferation and malignancy.
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