全文获取类型
收费全文 | 106550篇 |
免费 | 7843篇 |
国内免费 | 211篇 |
专业分类
114604篇 |
出版年
2021年 | 1214篇 |
2020年 | 1091篇 |
2019年 | 1073篇 |
2018年 | 2503篇 |
2017年 | 2359篇 |
2016年 | 3084篇 |
2015年 | 3890篇 |
2014年 | 4041篇 |
2013年 | 5310篇 |
2012年 | 6278篇 |
2011年 | 5805篇 |
2010年 | 3712篇 |
2009年 | 3053篇 |
2008年 | 4682篇 |
2007年 | 4458篇 |
2006年 | 4286篇 |
2005年 | 3659篇 |
2004年 | 3566篇 |
2003年 | 3218篇 |
2002年 | 3002篇 |
2001年 | 4221篇 |
2000年 | 3888篇 |
1999年 | 2888篇 |
1998年 | 1083篇 |
1997年 | 890篇 |
1996年 | 834篇 |
1995年 | 734篇 |
1994年 | 661篇 |
1992年 | 1847篇 |
1991年 | 1649篇 |
1990年 | 1540篇 |
1989年 | 1478篇 |
1988年 | 1351篇 |
1987年 | 1298篇 |
1986年 | 1166篇 |
1985年 | 1167篇 |
1984年 | 946篇 |
1983年 | 776篇 |
1982年 | 608篇 |
1979年 | 874篇 |
1978年 | 676篇 |
1977年 | 601篇 |
1975年 | 731篇 |
1974年 | 835篇 |
1973年 | 824篇 |
1972年 | 798篇 |
1971年 | 723篇 |
1970年 | 726篇 |
1969年 | 741篇 |
1968年 | 625篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
R. Z. German D. W. Hertweck J. E. Sirianni D. R. Swindler 《American journal of physical anthropology》1994,93(3):373-380
Somatic growth is not a simple linear process with a constant rate of growth. The most successful attempts to quantify growth as a function of age or size have employed nonlinear techniques. Sexual dimorphism of primate growth, weight vs. age, was examined using nonlinear models with Sirianni and Swindler's ([1985] Growth and Development of the Pigtailed Macaque, Boca Raton, FL: CRC Press) growth data on the pigtailed macaque (Macaca nemestrina). The best fit of several exponential growth models was the Gompertz curve: Different multiple phase models were also fit, where each phase represents a distinct exponential component. The two-phase models proved to be the best (R2 = .0.84 for females, 0.91 for males), suggesting that there are two growth spurts, one in infancy and one at puberty. The timing of the beginning and end of the first spurt is the same in males and females, but the rate, and value of the asymptote for this phase, is greater in males. The timing of the second spurt is earlier, and the rate of growth for this spurt is smaller in females than males. The sexual dimorphism in these species is not a simple rate change, but a complex interaction of timing and rate over the entire period of growth. It would be impossible to separate these entities with a linear, polynomial, or single-phase model of the data. While these data and results complement much of the existing work on adult dimorphism, they also emphasize the vital role that ontogenetic data have in elucidating the underlying evolutionary mechanisms that generate sexual dimorphism. © 1994 Wiley-Liss, Inc. 相似文献
992.
A. M. Theodosiou K. E. Morrison A. M. Nesbit R. J. Daniels L. Campbell M. J. Francis Z. Christodoulou K. E. Davies 《American journal of human genetics》1994,55(6):1209-1217
Childhood-onset proximal spinal muscular atrophy (SMA) is a heritable neurological disorder, which has been mapped by genetic linkage analysis to chromosome 5q13, in the interval between markers D5S435 and D5S557. Here, we present gene sequences that have been isolated from this interval, several of which show sequence homologies to exons of beta-glucuronidase. These gene sequences are repeated several times across the candidate region and are also present on chromosome 5p. The arrangement of these repetitive gene motifs is polymorphic between individuals. The high degree of variability observed may have some influence on the expression of the genes in the region. Since SMA is not inherited as a classical autosomal recessive disease, novel genomic rearrangements arising from aberrant recombination events between the complex repeats may be associated with the phenotype observed. 相似文献
993.
In comparison with the parental cv. Finale the ‘RisfixC’ supernodulator exhibited higher, continuously increasing nodule number and fresh mass accumulation, but substantially lower individual nodule fresh mass, leghemoglobin concentration, and specific acetylene reduction activity of nodule tissue. There were no substantial differences between Finale and ‘RisfixC’ in total acetylene reduction, nodule leghemoglobin accumulation per nodulated root, total and specific CO2 evolution from nodulated roots and gross CO2 respiratory costs of acetylene reduction. The ‘RisfixC’ also exhibited a substantially lower plant dry mass production (by 30%), but nitrogen concentration in shoots and carotenoid concentration in leaf tissue were significantly higher by 33 and 14%, and the chlorophylla+b content insignficantly higher than in the parental cultivar. In contrast, the nodulation mutant ‘Risnod29’, exhibited a somewhat higher nodule fresh mass accumulation (by 21%) and individual nodule fresh mass (by 23%), total and specific acetylene reduction (by 49 and 19%) and a somewhat more rapid plant dry mass accumulation compared with the cv. Finale. 相似文献
994.
The influence of pH on growth kinetics of yeasts in the presence of benzoate as a sole carbon source
The inhibitory effect and substrate properties of benzoic acid were estimated for 25 yeast strains belonging to generaCondida, Hansenula, Hypopichia, Rhodosporidium, Rhodotorula, Saitoella andTrichosporon. Benzoic acid can serve as a sole carbon source for growth of yeasts belong to generaRhodotorula, Rhodosporidium andSaitoella in synthetic mineral media. Specific growth rate is strongly dependent both on the concentration of benzoate and the pH value
of the cultivation media. Maximum specific growth rate on benzoate is observed in alkaline cultivation media at pH 7.0–7.5
whereas those for growth on glucose in mildly acidic media at pH 5.0. Some of the strains showed weak growth on benzoate even
at pH 8.5. Some carotenoid-containing yeasts of the generaRhodotorula andRhodosporidium lost their ability to synthesize carotenoid pigments during growth in alkaline benzoate media. 相似文献
995.
G. Sekaninová M. Hofer I. Rychlík J. Pillich M. Kolářová V. Zajícová D. Kubíčková 《Folia microbiologica》1994,39(5):381-386
A new bacteriophage typing set, composed of 22 phages, was established as a tool for differentiation ofProteus strains. All the phages were tailed and included 4 morphological types (A1, A2, B1 and C1). They were classified into the
familiesMyoviridae, Siphoviridae andPodoviridae. From the set, 19 phages had double-stranded DNA and 3 were single-stranded DNA phages. 相似文献
996.
The antimicrobial properties of lignin by-products obtained by organic solvent, neutral sulfite semichemical and kraft pulping
were shown in a series of yeasts. Lignin can act as an inhibitor of growth ofSporobolomyces roseus, Candida tropicalis, Trichosporon cutaneum andCandida albicans. Oxidation of all lignin samples tested depresses their inhibitory effects. 相似文献
997.
Properties of a pentapeptide inhibitor of peptidyltransferase that is essential for cat gene regulation by translation attenuation. 总被引:11,自引:5,他引:6 下载免费PDF全文
Inducible chloramphenicol resistance genes cat and cmlA are regulated by translation attenuation. For both genes, the leader codons that must be translated to deliver a ribosome to the induction site specify a peptide that inhibits peptidyltransferase in vitro. The antipeptidyltransferase activity of the peptides is thought to select the site of ribosome stalling that is essential for induction. Using variations of the cat-86 leader-encoded 5-mer peptide MVKTD, we demonstrate a correlation between the in vitro antipeptidyltransferase activity and the ability of the same peptide to support induction by chloramphenicol in vivo. MVKTD footprints to nucleotides 2058, 2059, and 2060 in 23S rRNA. In vivo methylation of nucleotide 2058 by the ermC methylase interferes neither with cat-86 induction nor with peptide inhibition of peptidyltransferase. The methylation eliminates the competition that normally occurs in vitro between erythromycin and MVKTD. MVKTD inhibits the peptidyltransferase of several eubacteria, a representative Archaea species, and the eukaryote Saccharomyces cerevisiae. Bacillus stearothermophilus supports the in vivo induction of cat-86, and the RNA that is phenol extracted from the 50S ribosomes of this gram-positive thermophile is catalytically active in the peptidyltransferase assay and sensitive to peptide inhibition. Our results indicate that peptidyltransferase inhibition by a cat leader peptide is essential to induction, and this activity can be altered by minor changes in the amino acid sequence of the peptide. The broad range of organisms shown to possess peptide-inhibitable peptidyltransferase suggests that the target is a highly conserved component of the ribosome and includes 23S rRNA. 相似文献
998.
A new species of Boeckella from limnetic samples of Laguna del Diamante, a high lake in the Andes (34°10 S) is described and illustrated. The species is defined by the characters of the male fifth leg: the right two segmented endopod bears four peculiar, short, claw-like spines, the left endopod is a simple finger-like projection. This species is related to B. gibbosa, also a species from the Andes and B. vallentini from Malvinas (Falkland Islands) and other subantarctic islands. It is distinguished from them by diagnostic features of the fifth legs of the male and abdominal structure and fifth legs of the female. Some current views on the features used in the taxonomy of the genus Boeckella are discussed. 相似文献
999.
Tesniere C.; Romieu C.; Dugelay I.; Nicol M.Z.; Flanzy C.; Robin J.P. 《Journal of experimental botany》1994,45(1):145-151
The metabolic changes of mature grape berries during periodsof anoxia and upon return to air were examined using two cultivars.Glutamate declined during anoxia, together with a correspondingaccumulation of -aminobutyrate (GABA). The reverse occurredduring a 24 h period of air. Total adenine nucleotide (AdN),ATP level, and adenylate energy charge (AEC) all declined duringperiods of anoxia but showed a reversible pattern upon subsequentaeration. However, aerobic recovery of metabolite levels wasnot observed when the duration of anoxia at 30°C exceeded6 d. Accumulated ethanol during anoxia (up to 0.22 M) triggered afurther increase in the rate of ethanol synthesis when stressedberries were returned to air. Ethanol may be the principal determinantgoverning the ability of grapes to withstand and recover fromanoxic stress. We propose that the imbalance between aerobicand fermentative pathways may be due to the ability of ethanolto impair mitochondrial membrane function and uncouple oxidativephosphorylation, the rate of anaerobic respiration being insufficientto meet energy requirements. Key words: Grape, energy metabolism, anaerobic stress, aeration, ethanol production 相似文献
1000.