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Many cells never have a large increase in size after they are first formed from a parental cell. Typical stem cells reproduce, double in size, then reproduce again. Most Cytosolic contents such as the endomembrane system and the cytoplasm easily scale to larger sizes in larger cells. If a cell becomes too large, the normal cellular amount of DNA may not be adequate to keep the cell supplied with RNA. Large cells often replicate their chromosomes to an abnormally high copy number or become multinucleated. Large cells that are primarily for nutrient storage can have a smooth surface membrane, but metabolically active large cells often have some sort of folding of the cell surface membrane in order to increase the surface area available for transport functions.
The relationship between cell size and cell division has been extensively studied in yeast. For some cells, there is a mechanism by which cell division is not initiated until a cell has reached a certain size. If the nutrient supply is restricted (after time t = 2 in the diagram, below) and the rate of increase in cell size is slowed, the time period between cell divisions is increased. Yeast cell size mutants were isolated that begin cell division before reaching the normal size (wee mutants). The Wee1 protein is a tyrosine kinase. It normally phosphorylates the Cdc2 cell cycle regulatory protein on a tyrosineTyrosine (from the Greek tyros for "cheese", where it was first discovered), 4-hydroxyphenylalanine or 2-amino-3(4-hydroxyphenyl)-propanoic acid is one of the 20 amino acids that are used by cells to synthesize proteins. It has a phenol side chain. It pla residue. This covalent modification of the molecular structure of Cdc2 inhibits the enzymatic activity of Cdc2 and prevents cell division. In Wee1 mutants, there is less Wee1 activity and Cdc2 becomes active in smaller cells, causing cell division before the yeast cells reach their normal size. Cell division may be regulated in part by dilution of Wee1 protein in cells as they grow larger.
The protein mTOR is a serine/threonine kinaseIn biochemistry, a kinase is a type of enzyme that transfers phosphate groups from, for example, ATP to a specified substrate or target; the process is termed " phosphorylation". Typically, the target is "activated" or "energized" by being phosphorylated. that regulates translationTranslation is an activity comprising the interpretation of the meaning of a text in one language — the source text — and the production of another, equivalent text in another language — the target text or translation''. Traditionally, translation has alw and cell division. Nutrient availability influences mTOR so that when cells are not able to grow to normal size they will not undergo cell division. The details of the molecular mechanisms of mammalian cell size control are currently being investigated.
The size of post-mitotic neurons depends on the size of the cell body, axon and dendrites. In vertebrates, neuron size is often a reflection of the number of synaptic contacts onto the neuron or from a neuron onto other cells. For example, the size of motoneuronIn vertebrates, motoneurons (also called motor neurons are efferent neurons that originate in the spinal cord and synapse with muscle fibers to facilitate muscle contraction and with muscle spindles to modify proprioceptive sensitivity. Anatomy and physios usually reflects the size of the motor unitA motor unit is a group of cells under the control of a single motor neuron; groups of motor units work together, as a single muscle. The number of cells within each unit can vary: thigh muscles can have a thousand cells in each unit, eye muscles might ha that is controlled by the motoneuron. Invertebrates often have giant neurons and axons that provide special functions such as rapid action potential propagation. Mammals also use this trick for increasing the speed of signals in the nervous system, but they can also use myelin to accomplish this, so most human neurons are releatively small.