全文获取类型
收费全文 | 240篇 |
免费 | 35篇 |
国内免费 | 2篇 |
专业分类
277篇 |
出版年
2021年 | 3篇 |
2018年 | 3篇 |
2017年 | 4篇 |
2016年 | 3篇 |
2015年 | 7篇 |
2014年 | 9篇 |
2013年 | 9篇 |
2012年 | 9篇 |
2011年 | 13篇 |
2010年 | 6篇 |
2009年 | 8篇 |
2008年 | 13篇 |
2007年 | 7篇 |
2006年 | 11篇 |
2005年 | 8篇 |
2004年 | 6篇 |
2003年 | 14篇 |
2002年 | 5篇 |
2001年 | 8篇 |
2000年 | 5篇 |
1999年 | 4篇 |
1998年 | 2篇 |
1997年 | 3篇 |
1996年 | 6篇 |
1995年 | 2篇 |
1993年 | 2篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1976年 | 6篇 |
1975年 | 5篇 |
1974年 | 10篇 |
1973年 | 6篇 |
1971年 | 3篇 |
1970年 | 4篇 |
1969年 | 7篇 |
1968年 | 2篇 |
1966年 | 2篇 |
1936年 | 3篇 |
1901年 | 1篇 |
排序方式: 共有277条查询结果,搜索用时 15 毫秒
51.
The Mongolian gerbil is being increasingly used as a laboratory animal and as a pet. Both chinchillas and gerbils are used as animal models for otitis media and other otic research. Previously, only incomplete information was available regarding the indigenous bacterial flora of the lower intestinal tracts of these coprophagic animals. Using the strict anaerobic methodology of the Virginia Polytechnic Institute Anaerobe Laboratory, we studied the predominant bacterial flora of the cecum and fecal pellets of the gerbil and the chinchilla and the bacterial flora of digesta pellets in the proximal colon. We found species of the following anaerobic genera in high dilutions of gerbil fecal pellets: Bifidobacterium, Clostridium, Propionibacterium, Lactobacillus, and Bacteroides. Only lactobacilli were found in high dilutions of digesta from the upper colon, although the cecum yielded Peptostreptococcus, Bifidobacterium, Clostridium, Lactobacillus, Propionibacterium, and Bacteroides species from high dilutions of cecal contents. The facultatively anaerobic and aerobic flora isolated consisted of species of Bacillus, Streptococcus, Staphylococcus, Acinetobacter, Alcaligenes, Escherichia, Pasteurella, and Pseudomonas plus several unidentifiable organisms. Species of Bifidobacterium, Bacteroides, Eubacterium, and anaerobic Lactobacillus were isolated from chinchillas. 相似文献
52.
53.
C.R. Worthington G.F. Elliott 《International journal of biological macromolecules》1996,19(4):287-294
The step-size distance in muscle contraction is obtained using the step-size distance equation z = u/n, where z is the step-size distance, u is the actin filament velocity and n is the ATPase rate of splitting. In a previous study a step-size distance of about 17 Å at no load was determined for intact frog muscle. Some properties of the step-size distance equation are described. We have now made estimates of the step-size distance z for a variety of muscles using existing physiological and biochemical data in the literature. The estimates are listed in Tables 1 and 2. We find that the step-size distances are clustered in the range 13–17 Å for nearly all muscles. 相似文献
54.
55.
Intraspecific nuclear DNA variation in Drosophila 总被引:18,自引:6,他引:12
We have summarized and analyzed all available nuclear DNA sequence
polymorphism studies for three species of Drosophila, D. melanogaster (24
loci), D. simulans (12 loci), and D. pseudoobscura (5 loci). Our major
findings are: (1) The average nucleotide heterozygosity ranges from about
0.4% to 2% depending upon species and function of the region, i.e., coding
or noncoding. (2) Compared to D. simulans and D. pseudoobscura (which are
about equally variable), D. melanogaster displays a low degree of DNA
polymorphism. (3) Noncoding introns and 3' and 5' flanking DNA shows less
polymorphism than silent sites within coding DNA. (4) X-linked genes are
less variable than autosomal genes. (5) Transition (Ts) and transversion
(Tv) polymorphisms are about equally frequent in non-coding DNA and at
fourfold degenerate sites in coding DNA while Ts polymorphisms outnumber Tv
polymorphisms by about 2:1 in total coding DNA. The increased Ts
polymorphism in coding regions is likely due to the structure of the
genetic code: silent changes are more often Ts's than are replacement
substitutions. (6) The proportion of replacement polymorphisms is
significantly higher in D. melanogaster than in D. simulans. (7) The level
of variation in coding DNA and the adjacent noncoding DNA is significantly
correlated indicating regional effects, most notably recombination. (8)
Surprisingly, the level of polymorphism at silent coding sites in D.
melanogaster is positively correlated with degree of codon usage bias. (9)
Three proposed tests of the neutral theory of DNA polymorphisms have been
performed on the data: Tajima's test, the HKA test, and the
McDonald-Kreitman test. About half of the loci fail to conform to the
expectations of neutral theory by one of the tests. We conclude that many
variables are affecting levels of DNA polymorphism in Drosophila, from
properties of nucleotides to population history and, perhaps, mating
structure. No simple, all encompassing explanation satisfactorily accounts
for the data.
相似文献
56.
Caccone A; Moriyama EN; Gleason JM; Nigro L; Powell JR 《Molecular biology and evolution》1996,13(9):1224-1232
Drosophila melanogaster belongs to a closely related group of eight species
collectively known as the melanogaster subgroup; all are native to
sub-Saharan Africa and islands off the east coast of Africa. The
phylogenetic relationships of most species in this subgroup have been well
documented; however, the three most closely related species, D. simulans,
D. sechellia, and D. mauritiana, have remained problematic from a
phylogenetic standpoint as no data set has unambiguously resolved them. We
present new DNA sequence data on the nullo and Serendipity-alpha genes and
combine them with all available nuclear DNA sequence data; the total data
encompass 12 genes and the ITS of rDNA. A methodological problem arose
because nine of the genes had information on intraspecific polymorphisms in
at least one species. We explored the effect of inclusion/exclusion of
polymorphic sites and found that it had very little effect on phylogenetic
inferences, due largely to the fact that 82% of polymorphisms are
autapomorphies (unique to one species). We have also reanalyzed our
previous DNA-DNA hybridization data with a bootstrap procedure. The
combined sequence data set and the DNA-DNA hybridization data strongly
support the sister status of the two island species, D. sechellia and D.
mauritiana. This at least partially resolves what had been a paradox of
parallel evolution in these two species.
相似文献
57.
58.
Sterol content and squaline-2(3)-epoxide-lanosterol cyclase activity in human foetal brain during early and mid-gestation 总被引:1,自引:0,他引:1
—Cholesterol, desmostcrol and squalene-2(3)-epoxide-lanostcrol cyclase were assayed in various regions of human foetal brain from 10 to 22 weeks gestation. The cerebellum, thalamus and hypothalamus, and the ‘remainder’ of the brain were analysed. The proportion of desmosterol to cholesterol was highest in the thalamus and hypothalamus and lowest in the cerebellum. It is suggested that the ratio of desmosterol to cholesterol gives an indication of the state of maturity of a given region, particularly with respect to myelination. Thus the higher levels of desmosterol detected in the thalamus and hypothalamus may be correlated with earlier myelination in this region compared to the cerebellum which myelinates much later in development. Squalene-2(3)-epoxide-lanosterol cyclase activity in whole brain showed three phases during the period studied; an initial low level up to 15 weeks, a slight increase in activity from 15 to 18 weeks (corresponding to neuroblast multiplication) and a sharp increase after 18 weeks (corresponding to glial multiplication prior to myelination). The enzyme was shown to be microsomal although freezing and thawing of whole brain tended to result in its solubilization. The apparent Km was found to be 2.5 × 10?6m and the effects of the inhibitors iminosqualene and di-iminosqualene were studied. A comparison was made between the brain enzyme and the liver enzyme at the same age and it was seen that at 22 weeks there was a large increase in activity in the brain, prior to myelination, which was not paralleled by an increase in the activity of the liver enzyme. In this study we have measured the levels of cholesterol, desmosterol and squa-lene-2(3)-epoxide-lanosterol cyclase in whole human foctal brain, thalamus and hypothalamus, and cerebellum during early and mid-gestation. We have also examined various properties of the cyclase enzyme and in particular the variations in its activity during early brain development. 相似文献
59.
A structure analysis of the low-angle X-ray diffraction data from nerve myelin is described. The low-angle X-ray data are interpreted in terms of an electron density strip model which has five parameters, these refer to the dimensions of the membrane pair and their component electron densities. Three sets of low-angle X-ray data from peripheral nerve swollen in media of different electron densities are analyzed and membrane pair dimensions and component electron densities on an absolute scale are assigned. Membrane pair dimensions are given for a variety of peripheral nerve myelins and central nervous system myelins. 相似文献
60.
C.R. Worthington T.J. Mcintosh S. Lalitha 《Archives of biochemistry and biophysics》1980,201(2):429-436
Low-angle X-ray diffraction patterns have been recorded from frog sciatic nerve after digestion with trypsin and Pronase. Reproducible X-ray patterns were obtained by swelling the nerves in distilled water before treatment with enzymes. The X-ray patterns of enzyme-treated nerves are distinctly different from the X-ray pattern of normal (live) nerve. It would appear that the normal asymmetric nerve myelin membrane becomes symmetric about its center after treatment with enzymes as a result of proteolytic cleavage and a subsequent redistribution of protein components. 相似文献