If a chromosome lacks a centromere, it is said acentric. The macronucleus of ciliates for example contains hundreds of acentric chromosomes. Chromosome-breaking events can also generate acentric chromosomes or acentric fragments.
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Acentric fragments are commonly generated by chromosome-breaking events, such as irradiation. Such acentric fragments are unequally distributed between the daughter cells after cell division. Acentric fragments can also be produced when an inverted segment is present in one member of a chromosome pair. In that case, a crossover event will result in one chromosome with two centromeres and an acentric fragment.
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Values of correspond to vapor pressures above the critical pressure, and are non-physical. By definition, a van der Waals fluid has a critical compressibility of 3/8 and an acentric factor of about −0.302024, indicating a small ultra-spherical molecule. A Redlich-Kwong fluid has a critical compressibility of 1/3 and an acentric factor of about 0.058280, close to nitrogen; without the temperature dependence of its attractive term, its acentric factor would be only -0.293572.
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An acentric fragment is a segment of a chromosome that lacks a centromere.Acentric Fragment, In: Sydney Brenner and Jeffrey H. Miller, Editor(s)-in-Chief, Encyclopedia of Genetics, Academic Press, New York, 2001, Page 2, , 10.1006/rwgn.2001.1750. Because the centromere is the point of attachment for the mitotic apparatus, acentric fragments are not evenly distributed to the daughter cells in cell division (mitosis and meiosis). As a result, one of the daughters will lack the acentric fragment.
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Micronuclei originating from chromosome loss events and acentric chromosome fragments can be distinguished using pancentromeric DNA probes.
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The effect proportional to the square of the electric field can exist only in crystals belonging to acentric point groups of symmetry.
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However, U-type exchange can also occur for homologous chromosomes which creates an isochromosome with homologous arms. This exchange between homologues is most likely due to homologous sequences containing low copy repeats. Regardless of the chromosome involved in U-type exchange, the acentric fragment of the chromosome is lost, thus creating a partial monosomy of genes located in that portion of the acentric chromosome.
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The acentric factor is a conceptual number introduced by Kenneth Pitzer in 1955, proven to be very useful in the description of matter. It has become a standard for the phase characterization of single & pure components. The other state description parameters are molecular weight, critical temperature, critical pressure, and critical volume (or critical compressibility). The acentric factor is said to be a measure of the non-sphericity (centricity) of molecules.
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Acentric chromosome fragments may arise in a variety of ways. One way is that disrepair of DNA double-strand breaks can lead to symmetrical or asymmetrical chromatid and chromosome exchanges as well as chromatid and chromosome fragments. If DNA damage exceeds the repair capacity of the cell, unrepaired double-stranded DNA breaks may also result in acentric chromosome fragments. Another way eccentric chromosome fragments may arise is when defects in genes related to homologous recombinational repair (ex: ATM, BRCA1, BRCA2, and RAD51) result in a dysfunctional error-free homologous recombinational DNA repair pathway and causes the cell to resort to the error-prone non-homologous end-joining (NHEJ) repair pathway, increasing the likelihood of incorrect repair of DNA breaks, formation of dicentric chromosomes, and acentric chromosome fragments.
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For the equation of state VTPR needs the critical temperature and pressure and additionally at least the acentric factor for all pure components in the considered mixture. A better quality can be achieved if the acentric factor is replaced by Twu constants which have been fitted to experimal vapor pressure data of pure components. The mixing rule uses UNIFAC which needs a variety of UNIFAC-specific parameters. Beside some model constants the most important are group interaction parameters which are fitted to experimental vapor–liquid equilibria of mixtures.
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For the equation of state PSRK needs the critical temperature and pressure, additionally at a minimum the acentric factor for all pure components in the considered mixture is also required. The integrity of the model can be improved if the acentric factor is replaced by Mathias–Copeman constants fitted to experimental vapor- pressure data of pure components. The mixing rule uses UNIFAC, which needs a variety of UNIFAC-specific parameters. Aside from some model constants, the most important parameters are the group-interaction parameters — these are obtained from parametric fits to experimental vapor–liquid equilibria of mixtures.
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Perhaps one of the famous concept- test systems is the Nielsen Bases system, which comes in different versions. Other well-known products include Decision Analyst's 'Concept Check', Acupoll's 'Concept Optimizer', Ipsos Innoquest and GFK. Examples of smaller players include Skuuber and Acentric Express Test.
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Lack of the acentric fragment in one of the daughter cells may have deleterious consequences, depending on the function of the DNA in this region of the chromosome. In the case of a haploid organism or a gamete, it will be fatal to one of the daughter cells if essential DNA is contained in the lost DNA segment. In the case of a diploid organism, the daughter cell lacking the acentric fragment will show expression of any recessive genes found in the homologous chromosome. Developmental geneticists look for cells and cell lineages lacking unpaired chromosome segments produced this way as a means of identifying essential genes for specific functions.
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Causes for this structure are mitotic loss of acentric chromosomal fragments (clastogenicity), mechanical problems from chromosomal breakage and exchange, mitotic loss of chromosomes (aneugenicity), and apoptosis. The micronucleus test in vivo is similar to the in vitro one because it tests for structural and numerical chromosomal aberrations in mammalian cells, especially in rats' blood cells.
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The Lee–Kesler method Lee B.I., Kesler M.G., "A Generalized Thermodynamic Correlation Based on Three-Parameter Corresponding States", AIChE J., 21(3), 510-527, 1975 allows the estimation of the saturated vapor pressure at a given temperature for all components for which the critical pressure Pc, the critical temperature Tc, and the acentric factor ω are known.
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A dicentric chromosome is an abnormal chromosome with two centromeres. It is formed through the fusion of two chromosome segments, each with a centromere, resulting in the loss of acentric fragments (lacking a centromere) and the formation of dicentric fragments. The formation of dicentric chromosomes has been attributed to genetic processes, such as Robertsonian translocation and paracentric inversion. Dicentric chromosomes have important roles in the mitotic stability of chromosomes and the formation of pseudodicentric chromosomes.
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The parameters of the Mathias–Copeman equation are fitted to experimental vapor-pressure data of pure components and provide a better description of the vapor pressure than the original relation. The form of the equation is chosen as it can be reduced to the original Soave form by setting the parameters c2 and c3 to zero. Additionally, the parameter c1 can be obtained from the acentric factor, using the relation : c_1 = 0.48 + 1.574 \, \omega - 0.176 \, \omega^2.
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Mechanisms of formation of neocentromeres are still unclear, but a few have been proposed. It is strongly speculated that neocentromeres form during mitosis or meiosis. For Class I, the proposed mechanism is that chromatid breakage occurs during mitosis, resulting in a chromosome fragment. That acentric chromosome fragment may segregate with the intact chromatid and result in partial tetrasomy; or, it may segregate with the complementary broken chromatid and result in partial trisomy because the broken chromatid may be saved by telomere restitution.
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Only a small part of the ancient strombolian cone remains visible, in the south, with the rest covered by the tuff ring. The ashes of this eruptive episode spread over a large region, at least until Clermont-Ferrand. A second strombolian cone appeared then, the new Pariou one, slightly acentric to the south. It covered all southern and east-southern parts of the old Pariou which remain visible and then forms a large trachyandesite flow to the east which covers the first trachybasalt flow.
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In 1972 G. Soave replaced the 1/ term of the Redlich-Kwong equation with a function α(T,ω) involving the temperature and the acentric factor (the resulting equation is also known as the Soave-Redlich-Kwong equation of state; SRK EOS). The α function was devised to fit the vapor pressure data of hydrocarbons and the equation does fairly well for these materials. Note especially that this replacement changes the definition of a slightly, as the T_c is now to the second power.
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If crossing over between a balancer chromosome and the balancer's homolog does occur during meiosis each chromatid ends up lacking some genes and carrying two copies of other genes. Recombination in inverted regions leads to dicentric or acentric chromosomes (chromosomes with two centromeres or no centromere). Progeny carrying chromosomes that are the products of recombination between balancer and normal chromosomes are not viable (they die). Dominant markers such as genes for green fluorescent protein or enzymes that make pigments allow researchers to easily recognize flies that carry the balancer chromosome.
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280x280px Micronucleus is the name given to the small nucleus that forms whenever a chromosome or a fragment of a chromosome is not incorporated into one of the daughter nuclei during cell division. It usually is a sign of genotoxic events and chromosomal instability. Micronuclei are commonly seen in cancerous cells and may indicate genomic damage events that can increase the risk of developmental or degenerative diseases. Micronuclei form during anaphase from lagging acentric chromosome or chromatid fragments caused by incorrectly repaired or unrepaired DNA breaks or by nondisjunction of chromosomes.
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In holocentric chromosome a chromosome-wide kinetochore is present (red lines) and no primary constriction is present during metaphase (M, bottom box). During anaphase (A, top box) holocentric chromatids move towards poles as linear bars parallel. If a chromosomal breakage occurs in a monocentric chromosome (bottom box), acentric chromosome fragments cannot be attached to microtubules during metaphase (M) and they are lost during anaphase (A). On the contrary, chromosome breakage of a holocentric chromosome results in chromosomal fragments that retain kinetic activity due to the chromosome-wide centromere extension and can be properly inherited.
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Lepidopteran chromosomes possess a localized kinetochore plate to which the spindle microtubules attach during cell division. The kinetochore plates are large and cover a significant portion of the chromosome length, ensuring that more radiation-induced breaks will not lead to the loss of chromosome fragments as is typical in species with monocentric chromosomes. In species with large kinetochore plates, the fragments may persist for a number of mitotic cell divisions, and can even be transmitted through germ cells to the next generation. The plates also reduce the risk of lethality caused by the formation of dicentric chromosomes, acentric fragments, and other unstable aberrations.
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Micronuclei primarily result from acentric chromosome fragments or lagging whole chromosomes that are not included in the daughter nuclei produced by mitosis because they fail to correctly attach to the spindle during the segregation of chromosomes in anaphase. These full chromosomes or chromatid fragments are eventually enclosed by a nuclear membranes and are structurally similar to conventional nuclei, albeit smaller in size. This small nucleus is referred to as a micronucleus. The formation of micronuclei can only be observed in cells undergoing nuclear division and can be clearly seen using cytochalasin B to block cytokinesis to produce a binucleated cells.
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According to X-ray data, lomonosovite structure was determined is triclinic unit cell with parameters: a = 5.44 Å, b = 7.163 Å, c = 14.83 Å, α = 99°, β = 106°, and γ = 90°, usually centrosymmetric (sp. gr. P-1) , but acentric varieties (polytype) are also reported. The crystal structure of lomonosovite is based on three-layer HOH packets consisting of a central octahedral O layer and two outer heteropolyhedral H layers. Ti- and Na centered octahedra are distinguished in the O layer, whereas the H layers are composed of Ti-centered octahedra and Si2O7 diorthogroups, (like in other heterophyllosilicates, for example lamprophyllite).
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