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41.
Evolution at the tip and base of the X chromosome in an African population of Drosophila melanogaster 总被引:1,自引:0,他引:1
Hitchhiking effects of advantageous mutations have been invoked to explain
reduced polymorphism in regions of low crossing-over in Drosophila. Besides
reducing DNA heterozygosity, hitchhiking effects should produce strong
linkage disequilibrium and a frequency spectrum skewed toward an excess of
rare polymorphisms (compared to the neutral expectation). We measured DNA
polymorphism in a Zimbabwe population of D. melanogaster at three loci,
yellow, achaete, and suppressor of forked, located in regions of reduced
crossing-over. Similar to previously published surveys of these genomic
regions in other populations, we observed low levels of nucleotide
variability. However, the frequency spectrum was compatible with a neutral
model, and there was abundant evidence for recombination in the history of
the yellow and ac genes. Thus, some aspects of the data cannot be accounted
for by a simple hitchhiking model. An alternative hypothesis, background
selection, might be compatible with the observed patterns of linkage
disequilibrium and the frequency spectrum. However, this model cannot
account for the observed reduction in nucleotide heterozygosity. Thus,
there is currently no satisfactory theoretical model for the data from the
tip and base of the X chromosome in D. melanogaster.
相似文献
42.
Oligogalacturonic acids (OGAs), derived from plant cell wall pectin, have
been implicated in a number of signal transduction pathways involved in
growth, development and defense responses of higher plants. This study
investigates the size range of OGAs capable of inducing ethylene synthesis
in tomato plants, and demonstrates that in contrast with many other
effects, only short chain OGAs are active. Oligomers across a range of DP
from 2-15 were separated and purified to homogeneity by QAE-Sephadex anion
exchange chromatography using a novel elution system. The OGAs were applied
to tomato plants and assayed for their ability to induce ethylene gas
release and changes in steady state levels of mRNA encoding the ethylene
forming enzyme aminocyclopropane-1-carboxylic acid oxidase (ACO). The study
demonstrated that only OGAs in the size range of DP4-6 were active both in
eliciting ACO expression and in the production of ethylene.
相似文献
43.
A change in twist of actin provides the force for the extension of the acrosomal process in limulus sperm: the false-discharge reaction 总被引:6,自引:5,他引:1
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One of the most spectacular motions is the generation of the acrosomal process in the limulus sperm. On contact with the egg, the sperm generates a 60-mum-long process that literally drills its way through the jelly surrounding the egg. This irresversible reaction takes only a few seconds. We suggested earlier that this motion is driven by a change in twist of the actin filaments comprising the acrosomal process. In this paper we analyze the so-called false discharge, a reversible reaction, in which the acrosomal filament bundle extends laterally from the base of the sperm and not anteriorly from the apex. Unlike the true discharge, which is straight, the false discharge is helical. Before extension, the filament bundle is coiled about the base of the sperm. In the coil, the bundle is not smoothly bent but consists of arms (straight segments) and elbows (corners) so that the coil looks like a 14-sided polygon. The extension of the false discharge works as follows: starting at the base of the bundle, the filaments change their twist which concomitantly changes the orientations of the elbows relative to each other; that is, in the coil, the elbows all like in a common plane, but after the change in twist, the plane of each elbow is rotated to be perpendicular to that of its neighbors. This change transforms the bundle from a compact coil into an extended left- handed helix. Because the basal end of the bundle is unconstrained, the extension is lateral. The true discharge works the same way but starts at the apical end of the bundle. The apical end, however, is constrained by its passage through the nuclear canal, which directs the extention anteriorly. Unlike the false discharge, during the true discharge the elbows are melted out, making the reaction irreversible. This study shows that rapid movement can be regenerated by actin without myosin and gives us insight into the molecular mechanism. 相似文献
44.
A G Doukas M R Junnarkar R R Alfano R H Callender V Balogh-Nair 《Biophysical journal》1985,47(6):795-798
The picosecond fluorescence kinetics and quantum yield from bovine rhodopsin were measured in the 5-40 degrees K range. The fluorescence rise and decay times are faster than our resolution of 15 ps (full width at half maximum) over this entire temperature range. The size of the observed emission was also temperature independent, and we find that the upper limit of rhodopsin's fluorescence quantum yield to be phi f approximately equal to 10(-5). Replacing all of rhodopsin's exchangeable protons with deuterons by suspending rhodopsin in D2O had no effect on either the kinetics of the emission or the value of the quantum yield. Our data provide strong confirmation of the idea that the first step in the visual process is an excited-state cis-to-trans isomerization about the C11-C12 double bond of retinal. 相似文献
45.
Adenosine 3',5'-cyclic monophosphate (cAMP) concentration and 3',5'-cyclic-nucleotide phosphodiesterase (PDE) activity were measured in skeletal muscle, heart, and liver of rats exposed to 1, 3, 5, and 7 days of cold. Cyclic nucleotide concentration increased in fast-twitch red muscle at the same time that PDE activity was decreasing. Nucleotide concentration and enzyme activity of slow-twitch red muscle were not altered by the cold exposure. The PDE activity of fast-twitch white muscle was elevated approximately 50% above control after 1 and 3 days of cold exposure. By the 5th day in the cold, white muscle PDE activity had returned to control levels and remained there through the 7th day of experimentation. cAMP concentration in hearts of cold-exposed rats was significantly (P less than 0.01) elevated above control at all time points measured. Myocardial PDE activity was elevated above control (P less than 0.05) at 1 and 3 days of cold exposure but returned to control levels by the 5th day in the cold. Hepatic cAMP and PDE activity were elevated above control at all time points analyzed. These data suggest that changes in cyclic nucleotide metabolism play a role in attaining homeostasis during acute cold exposure. 相似文献
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The influence of increasing the in vivo concentration of cyclic AMP on the activity of cyclic nucleotide phosphodiesterase (PDE) in rat heart was investigated. One, three, and five hourly injections of 5.0 mg dibutyryl (Bt2) cyclic AMP significantly increased the activity of PDE in the supernatant fraction of rat heart using 1.0 microM cyclic AMP as the assay substrate concentration. When 100 microM cyclic AMP was used in the assay reaction, increases in enzymes activity were seen following five and eight nucleotide injections. The nucleotide-induced increase in PDE activity was dose dependent. When the five-injection protocol was used, PDE activity remained elevated for at least 4 h, while activity had returned to control levels within this time when two hourly injections were used. The nucleotide stimulation of PDE activity was blocked by cycloheximide. Five hourly infections of Bt2 cyclic AMP increased PDE activity in the liver and fast-twitch red muscle. A reduction in PDE activity in fast-twitch white muscle was seen following nucleotide injections. These findings are consistent with the hypothesis that prolonged elevations in the intracellular concentration of cyclic AMP cause an elevation in myocardial PDE activity. The increased activity seems to be the result of protein synthesis. These data suggest that cyclic AMP contributes significantly in regulating its own metabolism in the rat heart. 相似文献
50.
Warren K. Palmer Therese A. Studney Sylvia Doukas 《Biochimica et Biophysica Acta (BBA)/General Subjects》1981,672(1):114-122
Numerous cellular biochemical events caused by hormones are mediated throught cyclic AMP. Although many changes occur in the cell during exercise that could be attributed to this nucleotide, little evidence is available implicating it as an important regulator of exercise metabolism. In this investigation it was found that a 60 min bout of treadmill exercise caused a 2.4-fold increase in myocardial cyclic AMP immediately following the work. Rather than the imemediate nucleotide hydrolysis that was expected, it was found that the elevated cyclic AMP level remained for approx. 24 h before returning to control levels. Cardiac glycogen fell to 30% of control after work but supercompensated 60% above control within 1 h following exercise. Therefore, cardiac cyclic AMP was elevated at a time when glycogen was being synthesized. Study of the temporal relationship between the exercise-induced increase in cyclic AMP and cyclic nucleotide phosphodiesterase indicated that the work caused an increase in the hearts' capacity to hydrolyze cyclic AMP. Measurement of heart phosphodiesterase at substrate concentrations of 1.0 and 100 μM produced significant increased in enzyme activity immediately following exercise which remained elevated for 48 h and was back to control activity 96 h following work. These data present a potentially fascinating model for the study of the dissociation between cyclic AMP, glycogenesis and elevations in phosphodiesterase activity in the heart. 相似文献