全文获取类型
收费全文 | 20390篇 |
免费 | 1891篇 |
国内免费 | 13篇 |
专业分类
22294篇 |
出版年
2023年 | 121篇 |
2022年 | 300篇 |
2021年 | 563篇 |
2020年 | 266篇 |
2019年 | 388篇 |
2018年 | 435篇 |
2017年 | 340篇 |
2016年 | 586篇 |
2015年 | 985篇 |
2014年 | 1020篇 |
2013年 | 1399篇 |
2012年 | 1598篇 |
2011年 | 1566篇 |
2010年 | 967篇 |
2009年 | 844篇 |
2008年 | 1212篇 |
2007年 | 1192篇 |
2006年 | 1073篇 |
2005年 | 1015篇 |
2004年 | 914篇 |
2003年 | 852篇 |
2002年 | 825篇 |
2001年 | 198篇 |
2000年 | 153篇 |
1999年 | 178篇 |
1998年 | 168篇 |
1997年 | 140篇 |
1996年 | 116篇 |
1995年 | 110篇 |
1994年 | 117篇 |
1993年 | 112篇 |
1992年 | 119篇 |
1991年 | 107篇 |
1990年 | 97篇 |
1989年 | 85篇 |
1988年 | 92篇 |
1987年 | 72篇 |
1986年 | 66篇 |
1985年 | 89篇 |
1984年 | 96篇 |
1983年 | 62篇 |
1982年 | 81篇 |
1981年 | 72篇 |
1980年 | 67篇 |
1979年 | 77篇 |
1978年 | 54篇 |
1977年 | 60篇 |
1976年 | 65篇 |
1975年 | 70篇 |
1973年 | 55篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
961.
Genetic manipulation of floral pigmentation genes 总被引:4,自引:0,他引:4
Joseph N. M. Mol Antoine R. Stuitje Alexander van der Krol 《Plant molecular biology》1989,13(3):287-294
962.
Alexander Kieneke Wilko H. Ahlrichs Pedro Martínez Arbizu Thomas Bartolomaeus 《Zoomorphology》2008,127(1):1-20
In an attempt to obtain detailed information on the entire protonephridial system in Gastrotricha, we have studied the protonephridial
ultrastructure of two paucitubulatan species, Xenotrichula carolinensis syltensis and Chaetonotus maximus by means of complete sets of ultrathin sections. In spite of some differences in detail, the morphology of protonephridia
in both examined species shows a common pattern: Both species have one pair of protonephridia that consist of a bicellular
terminal organ, a voluminous, aciliar canal cell and an adjacent, aciliar nephridiopore cell. The terminal organ consists
of two monociliar terminal cells each with a distal cytoplasmic lobe. These lobes interdigitate and surround cilia and microvilli
of the terminal cells. Where both lobes interdigitate, a meandering cleft is formed that is covered by the filtration barrier.
We here term the entire structure composite filter. The elongated, in some regions convoluted protonephridial lumen opens
distally to the outside via a permanent nephridiopore. A comparison with the protonephridia of other species of the Gastrotricha
allows hypothesising the following autapomorphies of the Paucitubulata: The bicellular terminal organ with a composite filter,
the convoluted distal canal cell lumen and the absence of cilia, ciliary basal structures and microvilli within the canal
cell. Moreover, this comparative survey could confirm important characteristics of the protonephridial system assumed for
the ground pattern of Gastrotricha like, for example, the single terminal cell with one cilium surrounded by eight microvilli. 相似文献
963.
During the RNA World, organisms experienced high rates of genetic errors, which implies that there was strong evolutionary
pressure to reduce the errors’ phenotypical impact by suitably structuring the still-evolving genetic code. Therefore, the
relative rates of the various types of genetic errors should have left characteristic imprints in the structure of the genetic
code. Here, we show that, therefore, it is possible to some extent to reconstruct those error rates, as well as the nucleotide
frequencies, for the time when the code was fixed. We find evidence indicating that the frequencies of G and C in the genome
were not elevated. Since, for thermodynamic reasons, RNA in thermophiles tends to possess elevated G+C content, this result
indicates that the fixation of the genetic code occurred in organisms which were either not thermophiles or that the code’s
fixation occurred after the rise of DNA.
Supplementary Materials Original data and programs are available at the author’s web site: . 相似文献
964.
Arturo Zenone Alexander Kovalev Fabio Badalamenti Stanislav N. Gorb 《Restoration Ecology》2024,32(3):e14084
Seagrasses provide various ecosystem functions in coastal areas of the world. In the Mediterranean Sea, Posidonia oceanica is an endemic species threatened by several activities despite being protected by national and international laws. Currently, several transplanting initiatives have been carried out using different methods, among which those including seeds and seedlings are considered the most ecological and low-cost ones. Beach-cast fruits and seeds can be found in spring and their appearance can easily be reported, through a citizen science approach, by the community. One of the obstacles in using these methods is identifying the best substrate in which to place P. oceanica seeds to facilitate root adhesion of the seedlings prior to their transplantation into the sea. In the present study, we analyzed, using a 3D surface optical microscope, the roughness of natural rocks to identify the availability of specific roughness ranges suitable for adhesion and root anchoring of P. oceanica seedlings. Conventional roughness parameters and roughness power spectral density were calculated for the inner and outer surfaces of 9 different rock samples. Among the rock samples examined, the calcarenitic ones and in particular marsala calcarenite, due to the presence of the “ideal roughness for seedlings” can be considered one of the best consolidated substrates to be used for the construction of ad hoc devices on which plantlet of P. oceanica can grow for the purpose of restoration. 相似文献
965.
The trans-Golgi network (TGN) is one of the main, if not the main, sorting stations in the process of intracellular protein trafficking.
It is therefore of central importance to understand how the key players in the TGN-based sorting and delivery process, the
post-Golgi carriers (PGCs), form and function. Over the last few years, modern morphological approaches have generated new
insights into the questions of PGC biogenesis, structure and dynamics. Here, we present a view by which the “lifecycle” of
a PGC consists of several distinct stages: the formation of TGN tubular export domains (where different cargoes are segregated
from each other and from the Golgi enzymes); the docking of these tubular domains onto molecular motors and their extrusion
towards the cell periphery along microtubules; the fission of the forming PGC from the donor membrane; and the delivery of
the newly formed PGC to its specific acceptor organelle. It is now important to add the many molecular machineries that have
been described as operating at the TGN to this “morphofunctional map” of the TGN export process. 相似文献
966.
Effects of chronic food restriction and treatments with leptin or ghrelin on different reproductive parameters of male rats 总被引:1,自引:0,他引:1
Sirotkin AV Chrenková M Nitrayová S Patras P Darlak K Valenzuela F Pinilla L Tena-Sempere M 《Peptides》2008,29(8):1362-1368
The existence of a close relationship between energy status and reproductive function is well-documented, especially in females, but its underlying mechanisms remain to be fully unfolded. This study aimed to examine the effects of restriction of daily calorie intake, as well as chronic treatments with the metabolic hormones leptin and ghrelin, on the secretion of different reproductive hormones, namely pituitary gonadotropins and prolactin, as well as testosterone, in male rats. Restriction (50%) in daily food intake for 20 days significantly reduced body weight as well as plasma PRL and T levels, without affecting basal LH and FSH concentrations and testicular weight. Chronic administration of leptin to rats fed ad libitum increased plasma PRL levels and decreased circulating T, while it did not alter other hormonal parameters under analysis. In contrast, in rats subjected to 50% calorie restriction, leptin administration increased plasma T levels and reduced testis weight. Conversely, ghrelin failed to induce major hormonal changes but tended to increase testicular weight in fed animals, while repeated ghrelin injections in food-restricted males dramatically decreased plasma LH and T concentrations and reduced testis weight. In sum, we document herein the isolated and combined effects of metabolic stress (50% food restriction) and leptin or ghrelin treatments on several reproductive hormones in adult male rats. Overall, our results further stress the impact and complex way of action of different metabolic cues, such as energy status and key hormones, in reproductive function also in the male. 相似文献
967.
Timothy A. McCaffrey Constantine Tziros Jannet Lewis Richard Katz Robert Siegel William Weglicki Jay Kramer I. Tong Mak Ian Toma Liang Chen Elizabeth Benas Alexander Lowitt Shruti Rao Linda Witkin Yi Lian Yinglei Lai Zhaoqing Yang Sidney W. Fu 《International journal of biological sciences》2013,9(4):350-360
968.
Vladimir Sentchilo Antonia P Mayer Lionel Guy Ryo Miyazaki Susannah Green Tringe Kerrie Barry Stephanie Malfatti Alexander Goessmann Marc Robinson-Rechavi Jan R van der Meer 《The ISME journal》2013,7(6):1173-1186
Plasmids have long been recognized as an important driver of DNA exchange and genetic innovation in prokaryotes. The success of plasmids has been attributed to their independent replication from the host''s chromosome and their frequent self-transfer. It is thought that plasmids accumulate, rearrange and distribute nonessential genes, which may provide an advantage for host proliferation under selective conditions. In order to test this hypothesis independently of biases from culture selection, we study the plasmid metagenome from microbial communities in two activated sludge systems, one of which receives mostly household and the other chemical industry wastewater. We find that plasmids from activated sludge microbial communities carry among the largest proportion of unknown gene pools so far detected in metagenomic DNA, confirming their presumed role of DNA innovators. At a system level both plasmid metagenomes were dominated by functions associated with replication and transposition, and contained a wide variety of antibiotic and heavy metal resistances. Plasmid families were very different in the two metagenomes and grouped in deep-branching new families compared with known plasmid replicons. A number of abundant plasmid replicons could be completely assembled directly from the metagenome, providing insight in plasmid composition without culturing bias. Functionally, the two metagenomes strongly differed in several ways, including a greater abundance of genes for carbohydrate metabolism in the industrial and of general defense factors in the household activated sludge plasmid metagenome. This suggests that plasmids not only contribute to the adaptation of single individual prokaryotic species, but of the prokaryotic community as a whole under local selective conditions. 相似文献
969.
Yuri S. Fantin Alexey D. Neverov Alexander V. Favorov Maria V. Alvarez-Figueroa Svetlana I. Braslavskaya Maria A. Gordukova Inga V. Karandashova Konstantin V. Kuleshov Anna I. Myznikova Maya S. Polishchuk Denis A. Reshetov Yana A. Voiciehovskaya Andrei A. Mironov Vladimir P. Chulanov 《PloS one》2013,8(1)
Sanger sequencing is a common method of reading DNA sequences. It is less expensive than high-throughput methods, and it is appropriate for numerous applications including molecular diagnostics. However, sequencing mixtures of similar DNA of pathogens with this method is challenging. This is important because most clinical samples contain such mixtures, rather than pure single strains. The traditional solution is to sequence selected clones of PCR products, a complicated, time-consuming, and expensive procedure. Here, we propose the base-calling with vocabulary (BCV) method that computationally deciphers Sanger chromatograms obtained from mixed DNA samples. The inputs to the BCV algorithm are a chromatogram and a dictionary of sequences that are similar to those we expect to obtain. We apply the base-calling function on a test dataset of chromatograms without ambiguous positions, as well as one with 3–14% sequence degeneracy. Furthermore, we use BCV to assemble a consensus sequence for an HIV genome fragment in a sample containing a mixture of viral DNA variants and to determine the positions of the indels. Finally, we detect drug-resistant Mycobacterium tuberculosis strains carrying frameshift mutations mixed with wild-type bacteria in the pncA gene, and roughly characterize bacterial communities in clinical samples by direct 16S rRNA sequencing. 相似文献
970.
Joshua Ladau Thomas J Sharpton Mariel M Finucane Guillaume Jospin Steven W Kembel James O'Dwyer Alexander F Koeppel Jessica L Green Katherine S Pollard 《The ISME journal》2013,7(9):1669-1677
Genomic approaches to characterizing bacterial communities are revealing significant differences in diversity and composition between environments. But bacterial distributions have not been mapped at a global scale. Although current community surveys are way too sparse to map global diversity patterns directly, there is now sufficient data to fit accurate models of how bacterial distributions vary across different environments and to make global scale maps from these models. We apply this approach to map the global distributions of bacteria in marine surface waters. Our spatially and temporally explicit predictions suggest that bacterial diversity peaks in temperate latitudes across the world''s oceans. These global peaks are seasonal, occurring 6 months apart in the two hemispheres, in the boreal and austral winters. This pattern is quite different from the tropical, seasonally consistent diversity patterns observed for most macroorganisms. However, like other marine organisms, surface water bacteria are particularly diverse in regions of high human environmental impacts on the oceans. Our maps provide the first picture of bacterial distributions at a global scale and suggest important differences between the diversity patterns of bacteria compared with other organisms. 相似文献