Define cell and how does growth take place
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Answer:
Cell growth is the process by which cells accumulate mass and increase in physical size.In some cells, size is proportional to DNA content. For instance, continued DNA replication in the absence of cell division (called endoreplication) results in increased cell size.
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Answer
A cell is a mass of cytoplasm that is bound externally by a cell membrane. Usually microscopic in size, cells are the smallest structural units of living matter and compose all living things. Most cells have one or more nuclei and other organelles that carry out a variety of tasks.
Cell growth is the process by which cells accumulate mass and increase in physical size. On average, dividing animal cells are approximately 10 to 20 μm in diameter. Terminally differentiated cells have a wide range of sizes, spanning from tiny red blood cells (∼5 μm in diameter) to motor neurons, which can grow to hundreds of micrometers in length.1 For a typical dividing cell, water accounts for about 70% of the weight of a cell, and macromolecules, such as nucleic acids, proteins, polysaccharides, and lipids constitute most of the remaining mass (∼25%—trace amounts of ions and small molecules make up the difference). The largest contribution to cellular dry mass is typically from proteins, which makes up about 18% of the total cell weight on average. There are many physical, chemical, and biological factors that affect the biosynthesis of macromolecules and therefore final cell size. Intracellular signaling networks that regulate metabolism and control macromolecule biosynthesis are particularly relevant to cancer. As discussed later, deregulation of the cellular circuitry controlling biomass accumulation is associated with a wide spectrum of human cancers.
There are many different examples in nature of how cells can grow. In some cases, cell size is proportional to DNA content. For instance, continued DNA replication in the absence of cell division (called endoreplication) results in increased cell size. Megakaryoblasts, which mature into granular megakaryocytes, the platelet-producing cells of bone marrow, typically grow this way. These cells cease division and then undergo multiple rounds of DNA synthesis, increasing from about 20 μm to approximately 100 μm in diameter as a result of the increased DNA content. It is unclear whether increased DNA content simply leads to an increase in total cellular material or whether cells actively grow to cope with the larger genome size. This growth strategy is found throughout nature in animals, plants, and single-celled organisms. By a different strategy, adipocytes can grow to approximately 85 to 120 μm by accumulating intracellular lipids. In contrast to endoreplication or lipid accumulation, some terminally differentiated cells, such as neurons and cardiac muscle cells, cease dividing and grow without increasing their DNA content. These cells proportionately increase their macromolecule content (largely protein) to a point necessary to perform their specialized functions. This involves coordination between extracellular cues from nutrients and growth factors and intracellular signaling networks responsible for controlling cellular energy availability and macromolecular synthesis.
Perhaps the most tightly regulated cell growth occurs in dividing cells, where cell growth and cell division are clearly separable processes. Dividing cells generally must increase in size with each passage through the cell division cycle to ensure that a consistent average cell size is maintained. (There are examples in the animal kingdom where cell division in the absence of growth serves an important evolutionary function, such as during the syncytial division stage of the early developing Drosophila embryo.) For a typical dividing mammalian cell, growth occurs in the G1 phase of the cell cycle and is tightly coordinated with S phase (DNA synthesis) and M phase (mitosis). The combined influence of growth factors, hormones, and nutrient availability provides the external cues for cells to grow. It is hypothesized that once dividing cells reach a threshold size, cells irreversibly commit to at least one round of division; achieving adequate size is thus a prerequisite for DNA synthesis and mitosis. Deprivation of nutrients and other growth signals, as might be the case in the nutrient (and oxygen)-starved regions of a growing tumor, may encourage normal cells to exit the cell cycle into a resting or G0 state. Therefore, mutations in signaling pathways that promote growth independently of growth factors and nutrient availability may provide tumor cells with a selective growth advantage. Efforts to identify intracellular signaling networks that control growth are therefore a mainstay of many cancer-focused research programs.
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