全文获取类型
收费全文 | 5006篇 |
免费 | 490篇 |
国内免费 | 1篇 |
出版年
2022年 | 66篇 |
2021年 | 134篇 |
2020年 | 76篇 |
2019年 | 87篇 |
2018年 | 112篇 |
2017年 | 75篇 |
2016年 | 161篇 |
2015年 | 236篇 |
2014年 | 286篇 |
2013年 | 295篇 |
2012年 | 389篇 |
2011年 | 373篇 |
2010年 | 227篇 |
2009年 | 186篇 |
2008年 | 246篇 |
2007年 | 253篇 |
2006年 | 202篇 |
2005年 | 216篇 |
2004年 | 203篇 |
2003年 | 174篇 |
2002年 | 146篇 |
2001年 | 74篇 |
2000年 | 73篇 |
1999年 | 52篇 |
1998年 | 60篇 |
1997年 | 48篇 |
1996年 | 37篇 |
1995年 | 42篇 |
1994年 | 42篇 |
1993年 | 37篇 |
1992年 | 64篇 |
1991年 | 36篇 |
1990年 | 40篇 |
1989年 | 43篇 |
1988年 | 35篇 |
1987年 | 34篇 |
1986年 | 31篇 |
1985年 | 38篇 |
1984年 | 32篇 |
1983年 | 27篇 |
1982年 | 24篇 |
1981年 | 30篇 |
1980年 | 32篇 |
1979年 | 26篇 |
1978年 | 34篇 |
1977年 | 26篇 |
1976年 | 27篇 |
1975年 | 30篇 |
1974年 | 24篇 |
1969年 | 23篇 |
排序方式: 共有5497条查询结果,搜索用时 15 毫秒
81.
Axial filament formation in Bacillus subtilis: induction of nucleoids of increasing length after addition of chloramphenicol to exponential-phase cultures approaching stationary phase. 总被引:6,自引:1,他引:5 下载免费PDF全文
When chloramphenicol was added to a culture of Bacillus subtilis in early exponential growth, microscopic observation of cells stained by 4',6-diamidino-2-phenylindole showed nucleoids that had changed in appearance from irregular spheres and dumbbells to large, brightly stained spheres and ovals. In contrast, the addition of chloramphenicol to cultures in mid- and late exponential growth showed cells with elongated nucleoids whose frequency and length increased as the culture approached stationary phase. The kinetics of nucleoid elongation after the addition of chloramphenicol to exponential-phase cultures was complex. Immediately after treatment, the rate of nucleoid elongation was very rapid. The nucleoid then elongated steadily for about 4 min, after which the rate of elongation decreased considerably. Nucleoids of cells treated with 6-(p-hydroxyphenylazo)-uracil (an inhibitor of DNA synthesis) exhibited the immediate rapid elongation upon chloramphenicol treatment but not the subsequent changes. These observations suggest that axial filament formation during stationary phase (stage I of sporulation) in the absence of chloramphenicol results from changes in nucleoid structure that are initiated earlier, during exponential growth. 相似文献
82.
Single anion channels reconstituted from cardiac mitoplasts 总被引:4,自引:0,他引:4
Ion channels from sheep cardiac mitoplast (inverted inner mitochondrial membrane vesicle) preparations were incorporated into
voltage-clamped planar lipid bilayers. The appearance of anion rather than cation channels could be promoted by exposing the
bilayers to osmotic gradients formed by Cl− salts of large, relatively imperment, cations at a pH of 8.8. Two distinct activities were identified. These comprised a
multisubstate anion channel of intermediate conductance (∼60 pS in 300vs. 50mm choline Cl, ∼100 pS in symmetric 150mm KCl), and a lower-conductance anion channel (∼25 or ∼50 pS in similar conditions), which only displayed two well-defined
substates, at ∼25 and ∼50% of the fully open state. The larger channels were not simple multiples of the lower-conductance
channels, but both discriminated poorly, and to a similar extent, between anions and cations (PCl
−/Pcholine
+ ∼12, PCl
−/PK
+∼8). The lower-conductance channel was only minimally selective between different anions (PNO
3
−
(1.0)=PCl
−>PBr
−>PI
−>PSCN
−(0.8)), and its conductance failed to saturate even in high (>1.0 M) activities of KCl. The channels were not obviously voltage
dependent, and they were unaffected by 0.5 mM SITS, H2O2, propranolol, quinine or amitriptyline, or by 2 mM ATP, or by variations in pH (5.5–8.8). Ca2+ and Mg2+ did not alter single channel activity, but did modify single current amplitudes in the lower-conductance channel. This effect,
together with voltage-dependent substate behavior, is described in the following paper. 相似文献
83.
An index marker map of chromosome 9 provides strong evidence for positive interference. 总被引:8,自引:7,他引:1 下载免费PDF全文
D J Kwiatkowski C Dib S A Slaugenhaupt S Povey J F Gusella J L Haines 《American journal of human genetics》1993,53(6):1279-1288
An index marker map of chromosome 9 has been constructed using the Centre d'Etude du Polymorphisme Humain reference pedigrees. The map comprises 26 markers, with a maximum intermarker interval of 13.1 cM and only two intervals > 10 cM. Placement of all but one marker into the map was achieved with > 10,000:1 odds. The sex-equal length is 151 cM, with male length of 121 cM and female length of 185 cM. The map extends to within 2%-3% of physical length at the telomeres, and its coverage therefore is expected to be within 20-30 cM of full map length. The markers are all of the GT/CA repeat type and have average heterozygosity .77, with a range of .60-.89. The map shows both marked contraction of genetic distance relative to physical distance in the pericentromeric region and expansion in the telomeric regions. Genotypic data were carefully examined for errors by using the crossover routine of the program DATAMAN. Five new mutations were observed among 17,316 meiotic events examined. There were two double-crossover events occurring within an interval of 0-10 cM, and another eight were observed within an interval of 10-20 cM. Many of these could be due to additional mutational events in which one parental allele converted to the other by either gene conversion or random strand slippage. When there was no correction for these possible mutational events, the number of crossovers displayed by the maternal and paternal chromosomes was significantly different (P < .001) from that predicted by the Poisson distribution, which would be expected in the absence of interference. In addition, the observed crossover distribution for paternally derived chromosomes was similar to that predicted from cytogenetic chiasma frequency observations. In all, the data strongly support the occurrence of strong positive interference on human chromosome 9 and suggest that flanking markers at an interval of < or = 20 cM are generally sufficient for disease gene inheritance predictions in presymptomatic genetic counseling by linkage analysis. 相似文献
84.
Localization of juvenile, but not late-infantile, neuronal ceroid lipofuscinosis on chromosome 16. 总被引:1,自引:1,他引:0
W Yan R M Boustany C Konradi L Ozelius T Lerner J A Trofatter C Julier X O Breakefield J F Gusella J L Haines 《American journal of human genetics》1993,52(1):89-95
The neuronal ceroid lipofuscinoses (NCL) are a group of progressive neurodegenerative disorders characterized by the deposition of autofluorescent proteinaceous fingerprint or curvilinear bodies. We have found that CLN3, the gene underlying the juvenile form of NCL, is very tightly linked to the dinucleotide repeat marker D16S285 on chromosome 16. Integration of D16S285 into the genetic map of chromosome 16 by using the Centre d'Etude du Polymorphisme Humain panel of reference pedigrees yielded a favored marker order in the CLN3 region of qtel-D16S150-.08-D16S285-.04-D16S148-.02-D16S 67-ptel. The most likely location of the disease gene, near D16S285 in the D16S150-D16S148 interval, was favored by odds of greater than 10(4):1 over the adjacent D16S148-D16S67 interval, which was recently reported as the minimum candidate region. Analysis of D16S285 in pedigrees with late-infantile NCL virtually excluded the CLN3 region, suggesting that these two forms of NCL are genetically distinct. 相似文献
85.
Extra- and Intracellular Laccases of the Chestnut Blight Fungus, Cryphonectria parasitica 总被引:1,自引:0,他引:1 下载免费PDF全文
A double-stranded RNA virus of the chestnut blight pathogen, Cryphonectria parasitica, has been shown previously to reduce accumulation of mRNAs of extracellular laccase (laccase A) produced by this fungus. Both extra- and intracellular laccases have been detected after growth of the fungus in liquid culture. In addition to cellular localization, the two laccases are distinguishable by time of appearance during growth and electrophoretic mobility. Laccase A was purified from the culture filtrate by standard protein purification procedures. The enzyme was characterized as a glycoprotein with a molecular mass of approximately 77 kDa. Both laccase A and laccase B activities were significantly reduced in the hypovirulent (double-stranded RNA-infected) strain UEP1 compared with the isogenic virulent (double-stranded RNA-free) strain EP155/2. 相似文献
86.
Evaluation of indoxyl-beta-D-glucuronide as a chromogen in media specific for Escherichia coli. 总被引:2,自引:1,他引:1 下载免费PDF全文
Indoxyl-beta-D-glucuronide (indoxyl) was evaluated as a specific chromogen for detection of Escherichia coli by the membrane filter method. In all, 413 colonies were tested from the indoxyl-supplemented media, yielding 93.3% confirmation, as E. coli. Compared with the indoxyl medium, other media gave either much lower recovery with high verification or equal recovery with poor verification. 相似文献
87.
G. K. Haines G. D. Ghadge S. Becker M. Kies H. Pelzer B. Thimmappaya J. A. Radosevich 《Virchows Archiv. B, Cell pathology including molecular pathology》1993,63(1):289-295
p68 is an inducible protein kinase which is believed to be an important factor in the regulation of both viral and cellular
protein synthesis. We have produced a monoclonal antibody (TJ4C4) which specifically detects p68, and which can be used to
detect this antigen in formalin-fixed, paraffin-embedded tissues. Because p68 plays an important role in cellular protein
synthesis, we hypothesized that it may correlate with normal and neoplastic cellular differentiation. One hundred and seventy-seven
head and neck squamous cell carcinoma specimens, representing 82 patients, were studied. The relative amount, frequency, and
distribution of p68 expression were determined by microscopic evaluation of ABC immunoperoxidase-stained specimens. A spectrum
of immunoreactivity was detected in 156 of 177 tumors, as well as within the normal squamous epithelium. Normal, actively
proliferating cells, such as the basal layer of squamous epithelium, expressed comparatively little p68. Increased p68 expression
was noted to parallel the morphologic features of cellular differentiation. In neoplastic tissue, p68 expression also increased
with the degree of cellular differentiation. These data demonstrate that the expression of p68 parallels the degree of cellular
differentiation in squamous cell carcinoma of the head and neck region, as well as within normal squamous mucosa. Therefore,
p68 may provide an objective biologic measure of cellular differentiation which does not depend on morphologic features. 相似文献
88.
Jennifer D. Watts Mary Farina John S. Kimball Luke D. Schiferl Zhihua Liu Kyle A. Arndt Donatella Zona Ashley Ballantyne Eugénie S. Euskirchen Frans-Jan W. Parmentier Manuel Helbig Oliver Sonnentag Torbern Tagesson Janne Rinne Hiroki Ikawa Masahito Ueyama Hideki Kobayashi Torsten Sachs Daniel F. Nadeau John Kochendorfer Marcin Jackowicz-Korczynski Anna Virkkala Mika Aurela Roisin Commane Brendan Byrne Leah Birch Matthew S. Johnson Nima Madani Brendan Rogers Jinyang Du Arthur Endsley Kathleen Savage Ben Poulter Zhen Zhang Lori M. Bruhwiler Charles E. Miller Scott Goetz Walter C. Oechel 《Global Change Biology》2023,29(7):1870-1889
Arctic-boreal landscapes are experiencing profound warming, along with changes in ecosystem moisture status and disturbance from fire. This region is of global importance in terms of carbon feedbacks to climate, yet the sign (sink or source) and magnitude of the Arctic-boreal carbon budget within recent years remains highly uncertain. Here, we provide new estimates of recent (2003–2015) vegetation gross primary productivity (GPP), ecosystem respiration (Reco), net ecosystem CO2 exchange (NEE; Reco − GPP), and terrestrial methane (CH4) emissions for the Arctic-boreal zone using a satellite data-driven process-model for northern ecosystems (TCFM-Arctic), calibrated and evaluated using measurements from >60 tower eddy covariance (EC) sites. We used TCFM-Arctic to obtain daily 1-km2 flux estimates and annual carbon budgets for the pan-Arctic-boreal region. Across the domain, the model indicated an overall average NEE sink of −850 Tg CO2-C year−1. Eurasian boreal zones, especially those in Siberia, contributed to a majority of the net sink. In contrast, the tundra biome was relatively carbon neutral (ranging from small sink to source). Regional CH4 emissions from tundra and boreal wetlands (not accounting for aquatic CH4) were estimated at 35 Tg CH4-C year−1. Accounting for additional emissions from open water aquatic bodies and from fire, using available estimates from the literature, reduced the total regional NEE sink by 21% and shifted many far northern tundra landscapes, and some boreal forests, to a net carbon source. This assessment, based on in situ observations and models, improves our understanding of the high-latitude carbon status and also indicates a continued need for integrated site-to-regional assessments to monitor the vulnerability of these ecosystems to climate change. 相似文献
89.
Jeffries et al. (2018) investigated testing for a treatment difference in the setting of a randomized clinical trial with a single outcome measured longitudinally over a series of common follow-up times while adjusting for covariates. That paper examined the null hypothesis of no difference at any follow-up time versus the alternative of a difference for at least one follow-up time. We extend those results here by considering multivariate outcome measurements, where each individual outcome is examined at common follow-up times. We consider the case where there is interest in first testing for a treatment difference in a global function of the outcomes (e.g., weighted average or sum) with subsequent interest in examining the individual outcomes, should the global function show a treatment difference. Testing is conducted for each follow-up time and may be performed in the setting of a group sequential trial. Testing procedures are developed to determine follow-up times for which a global treatment difference exists and which individual combinations of outcome and follow-up time show evidence of a difference while controlling for multiplicity in outcomes, follow-up, and interim analyses. These approaches are examined in a study evaluating the effects of tissue plasminogen activator on longitudinally obtained stroke severity measurements. 相似文献
90.
mtDNA diversity in rhesus monkeys reveals overestimates of divergence time and paraphyly with neighboring species 总被引:4,自引:0,他引:4
Reconstructions of the human-African great ape phylogeny by using
mitochondrial DNA (mtDNA) have been subject to considerable debate. One
confounding factor may be the lack of data on intraspecific variation. To
test this hypothesis, we examined the effect of intraspecific mtDNA
diversity on the phylogenetic reconstruction of another Plio- Pleistocene
radiation of higher primates, the fascicularis group of macaque (Macaca)
monkey species. Fifteen endonucleases were used to identify 10 haplotypes
of 40-47 restriction sites in M. mulatta, which were compared with similar
data for the other members of this species group. Interpopulational,
intraspecific mtDNA diversity was large (0.5%- 4.5%), and estimates of
divergence time and branching order incorporating this variation were
substantially different from those based on single representatives of each
species. We conclude that intraspecific mtDNA diversity is substantial in
at least some primate species. Consequently, without prior information on
the extent of genetic diversity within a particular species, intraspecific
variation must be assessed and accounted for when reconstructing primate
phylogenies. Further, we question the reliability of hominoid mtDNA
phylogenies, based as they are on one or a few representatives of each
species, in an already depauperate superfamily of primates.
相似文献