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91.
The rate of synonymous substitution in enterobacterial genes is inversely related to codon usage bias 总被引:31,自引:14,他引:31
Genes sequences from Escherichia coli, Salmonella typhimurium, and other
members of the Enterobacteriaceae show a negative correlation between the
degree of synonymous-codon usage bias and the rate of nucleotide
substitution at synonymous sites. In particular, very highly expressed
genes have very biased codon usage and accumulate synonymous substitutions
very slowly. In contrast, there is little correlation between the degree of
codon bias and the rate of protein evolution. It is concluded that both the
rate of synonymous substitution and the degree of codon usage bias largely
reflect the intensity of selection at the translational level. Because of
the high variability among genes in rates of synonymous substitution,
separate molecular clocks of synonymous substitution might be required for
different genes.
相似文献
92.
The effects of melatonin and a related 5-methoxy-indole, 6-methoxy-1,2,3,4-tetrahydro-beta-carboline (6-MeO-THBC) were investigated in rats on the development of retinal degeneration in presence of high intensity illumination (HII). A morphometric method is used in which the degree of degeneration was evaluated by a computer-coupled graphical analyzer. Instead of measuring individual thicknesses of different retinal layers at various loci we measured large areas of retinal light microscopic sections. Thus the influence of sporadic artefactual and other fluctuations in the thickness of various layers of the retina can be essentially reduced. Continuous light produced significant degeneration of the retina and the degree of degeneration was further increased by both studied compounds and even more by 6-MeO-THBC. The role of melatonin and 6-MeO-THBC in retinal physiology is discussed. 相似文献
93.
Quantitation of autoradiographic grains in different zones of articular cartilage with image analyzer 总被引:1,自引:0,他引:1
J J Parkkinen K Paukkonen E Pesonen M J Lammi S Markkanen H J Helminen M Tammi 《Histochemistry》1990,93(3):241-245
A novel method is introduced for the estimation of grain numbers in autoradiographic sections of articular cartilage with an image analyzer. It is based on separation of grains from the underlying structures by gray level thresholding and determination of the percentage of total area occupied by grains in a relatively large measuring field. The mean grain size is used as a reference to calculate grain numbers per cell profile and per unit area of tissue in various zones of bovine articular cartilage labelled with 35S-sulphate in tissue culture. The results demonstrate considerable zonal differences as well as site related topographic variation in the rate of 35S-sulphate incorporation. The largest site-related variation in the grain counts was observed in the superficial zone, suggesting a delicate control of proteoglycan synthesis in this zone. 相似文献
94.
Molecular evolution of bacteriophages: evidence of selection against the recognition sites of host restriction enzymes 总被引:12,自引:0,他引:12
Restriction enzymes produced by bacteria serve as a defense against
invading bacteriophages, and so phages without other protection would be
expected to undergo selection to eliminate recognition sites for these
enzymes from their genomes. The observed frequencies of all restriction
sites in the genomes of all completely sequenced DNA phages (T7, lambda,
phi X174, G4, M13, f1, fd, and IKe) have been compared to expected
frequencies derived from trinucleotide frequencies. Attention was focused
on 6-base palindromes since they comprise the typical recognition sites for
type II restriction enzymes. All of these coliphages, with the exception of
lambda and G4, exhibit significant avoidance of the particular sequences
that are enterobacterial restriction sites. As expected, the sequenced
fraction of the genome of phi 29, a Bacillus subtilis phage, lacks Bacillus
restriction sites. By contrast, the RNA phage MS2, several viruses that
infect eukaryotes (EBV, adenovirus, papilloma, and SV40), and three
mitochondrial genomes (human, mouse, and cow) were found not to lack
restriction sites. Because the particular palindromes avoided correspond
closely with the recognition sites for host enzymes and because other
viruses and small genomes do not show this avoidance, it is concluded that
the effect indeed results from natural selection.
相似文献
95.
The reciprocal position of sexual organs in complementary floral morphs is central to our understanding of heterostyly. Reciprocity indices are used to quantify the spatial match between complementary sex organs, but previous indices fail to appropriately account for intra-population variation in sex organ positions. In this study, we examine how an increase in intra-population variation in sex organ heights affects reciprocity and consequently reproductive success. We formulated a reciprocity index that incorporates this variation and asked if estimates of reciprocity can predict reproductive success in naturally occurring heterostylous populations. We developed a reciprocity index that assumed pollen transfer success equalled one for a perfectly matched stigma–anther pair, and decreased to zero with increasing mismatch. We examined the relationship between intra-population variation in organ position and reciprocity, compared previously proposed indices using simulated populations and empirical data from natural populations, and tested the ability of the indices to predict reproductive success. We observed that when differences between mean complementary sex-organ heights are small, increasing intra-population variation in heights resulted in a decrease in reciprocity. However, when this difference is larger, reciprocity increased, reached a peak, and then decreased with increasing variation. Previous indices failed to capture this behavior. Seed set was positively related to reciprocity for our index. These results challenge the current understanding that increasing variation in sex-organ heights will always decrease reciprocity in heterostylous populations. This may help explain why heterostylous systems exhibit and tolerate high amounts of intra-population variation in sex organ heights. 相似文献
96.
97.