全文获取类型
收费全文 | 85866篇 |
免费 | 7584篇 |
国内免费 | 46篇 |
专业分类
93496篇 |
出版年
2023年 | 281篇 |
2022年 | 707篇 |
2021年 | 1548篇 |
2020年 | 879篇 |
2019年 | 1108篇 |
2018年 | 1439篇 |
2017年 | 1241篇 |
2016年 | 2136篇 |
2015年 | 3631篇 |
2014年 | 4047篇 |
2013年 | 4811篇 |
2012年 | 6536篇 |
2011年 | 6482篇 |
2010年 | 4171篇 |
2009年 | 3799篇 |
2008年 | 5428篇 |
2007年 | 5432篇 |
2006年 | 5233篇 |
2005年 | 5031篇 |
2004年 | 4891篇 |
2003年 | 4667篇 |
2002年 | 4402篇 |
2001年 | 858篇 |
2000年 | 622篇 |
1999年 | 959篇 |
1998年 | 1231篇 |
1997年 | 821篇 |
1996年 | 743篇 |
1995年 | 654篇 |
1994年 | 616篇 |
1993年 | 666篇 |
1992年 | 553篇 |
1991年 | 520篇 |
1990年 | 462篇 |
1989年 | 397篇 |
1988年 | 420篇 |
1987年 | 344篇 |
1986年 | 317篇 |
1985年 | 399篇 |
1984年 | 518篇 |
1983年 | 401篇 |
1982年 | 501篇 |
1981年 | 486篇 |
1980年 | 414篇 |
1979年 | 295篇 |
1978年 | 323篇 |
1977年 | 281篇 |
1976年 | 263篇 |
1975年 | 204篇 |
1974年 | 241篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Dylan J. Weese Swanne P. Gordon Andrew P. Hendry Michael T. Kinnison 《Evolution; international journal of organic evolution》2010,64(6):1802-1815
We conducted 10 mark–recapture experiments in natural populations of Trinidadian guppies to test hypotheses concerning the role of viability selection in geographic patterns of male color variation. Previous work has reported that male guppies are more colorful in low‐predation sites than in high‐predation sites. This pattern of phenotypic variation has been theorized to reflect differences in the balance between natural (viability) selection that disfavors bright male color (owing to predation) and sexual selection that favors bright color (owing to female choice). Our results support the prediction that male color is disfavored by viability selection in both predation regimes. However, it does not support the prediction that viability selection against male color is weaker in low‐predation experiments. Instead, some of the most intense bouts of selection against color occurred in low‐predation experiments. Our results illustrate considerable spatiotemporal variation in selection among experiments, but such variation was not generally correlated with local patterns of color diversity. More complex selective interactions, possibly including the indirect effects of predators on variation in mating behavior, as well as other environmental factors, might be required to more fully explain patterns of secondary sexual trait variation in this system. 相似文献
992.
993.
994.
Gomez-Godinez V Wu T Sherman AJ Lee CS Liaw LH Zhongsheng Y Yokomori K Berns MW 《Nucleic acids research》2010,38(22):e202
In this study the femtosecond near-IR and nanosecond green lasers are used to induce alterations in mitotic chromosomes. The subsequent double-strand break responses are studied. We show that both lasers are capable of creating comparable chromosomal alterations and that a phase paling observed within 1-2 s of laser exposure is associated with an alteration of chromatin as confirmed by serial section electron microscopy, DAPI, γH2AX and phospho-H3 staining. Additionally, the accumulation of dark material observed using phase contrast light microscopy (indicative of a change in refractive index of the chromatin) ~ 34 s post-laser exposure corresponds spatially to the accumulation of Nbs1, Ku and ubiquitin. This study demonstrates that chromosomes selectively altered in mitosis initiate the DNA damage response within 30 s and that the accumulation of proteins are visually represented by phase-dark material at the irradiation site, allowing us to determine the fate of the damage as cells enter G1. These results occur with two widely different laser systems, making this approach to study DNA damage responses in the mitotic phase generally available to many different labs. Additionally, we present a summary of most of the published laser studies on chromosomes in order to provide a general guide of the lasers and operating parameters used by other laboratories. 相似文献
995.
The present state of knowledge of the phytochemistry of small molecules isolated from the roots and leaves of cassava, Manihot esculenta Crantz (Euphorbiaceae), is reviewed. Cassava roots are an important source of dietary and industrial carbohydrates, mainly eaten as a source of starch, forming the staple food to over 500 million; additionally, the roots have value as a raw material for industrial starch production and for animal feed giving the crop high economic value, but it suffers markedly from post-harvest physiological deterioration (PPD). The hydroxycoumarins scopoletin and its glucoside scopolin as well as trace quantities of esculetin and its glucoside esculin are identified from cassava roots during PPD. The biotechnological prospects for cassava are also reviewed including a critical appraisal of transgenic approaches for crop improvement, together with its use for bioethanol production, due to cassava's efficient ability to fix carbon dioxide into carbohydrate. 相似文献
996.
Alexandra S. Solovyova Jonathan A. Pointon Paul R. Race Wendy D. Smith Michael A. Kehoe Mark J. Banfield 《European biophysics journal : EBJ》2010,39(3):469-480
Adhesion of the serotype M1 Streptococcus pyogenes strain SF370 to human tonsil explants and cultured keratinocytes requires extended polymeric surface structures called pili.
In this important human pathogen, pili are assembled from three protein subunits: Spy0125, Spy0128 and Spy0130 through the
action of sortase enzymes. For this study, the structural properties of these pili proteins have been investigated in solution.
Spy0125 and Spy0128 display characteristics of globular, folded proteins. Circular dichroism suggests a largely β-sheet composition
for Spy0128 and Spy0125; Spy0130 appears to contain little secondary structure. Each of the proteins adopts a monodisperse,
monomeric state in solution as assessed by analytical ultracentrifugation. Further, small-angle X-ray scattering curves for
Spy0125, Spy0128 and Spy0130 suggest each protein adopts an elongated shape, likely comprised of two domains, with similar
maximal dimensions. Based on the scattering data, dummy atom models of each of the pili subunits have been reconstructed ab
initio. This study provides the first insights into the structure of Streptococcus pyogenes minor pili subunits, and possible implications for protein function are discussed. 相似文献
997.
Tessa Gordon Michael Hendry Christine A. Lafontaine Holliday Cartar Jennifer J. Zhang Gregory H. Borschel 《PloS one》2015,10(5)
There are currently no available options to promote nerve regeneration through chronically denervated distal nerve stumps. Here we used a rat model of delayed nerve repair asking of prior insertion of side-to-side cross-bridges between a donor tibial (TIB) nerve and a recipient denervated common peroneal (CP) nerve stump ameliorates poor nerve regeneration. First, numbers of retrogradely-labelled TIB neurons that grew axons into the nerve stump within three months, increased with the size of the perineurial windows opened in the TIB and CP nerves. Equal numbers of donor TIB axons regenerated into CP stumps either side of the cross-bridges, not being affected by target neurotrophic effects, or by removing the perineurium to insert 5-9 cross-bridges. Second, CP nerve stumps were coapted three months after inserting 0-9 cross-bridges and the number of 1) CP neurons that regenerated their axons within three months or 2) CP motor nerves that reinnervated the extensor digitorum longus (EDL) muscle within five months was determined by counting and motor unit number estimation (MUNE), respectively. We found that three but not more cross-bridges promoted the regeneration of axons and reinnervation of EDL muscle by all the CP motoneurons as compared to only 33% regenerating their axons when no cross-bridges were inserted. The same 3-fold increase in sensory nerve regeneration was found. In conclusion, side-to-side cross-bridges ameliorate poor regeneration after delayed nerve repair possibly by sustaining the growth-permissive state of denervated nerve stumps. Such autografts may be used in human repair surgery to improve outcomes after unavoidable delays. 相似文献
998.
Jun-Hyuk Choi So-Young Kim Sook-Kyung Kim Michael G. Kemp Aziz Sancar 《The Journal of biological chemistry》2015,290(48):28812-28821
DNA damage by UV and UV-mimetic agents elicits a set of inter-related responses in mammalian cells, including DNA repair, DNA damage checkpoints, and apoptosis. Conventionally, these responses are analyzed separately using different methodologies. Here we describe a unified approach that is capable of quantifying all three responses in parallel using lysates from the same population of cells. We show that a highly sensitive in vivo excision repair assay is capable of detecting nucleotide excision repair of a wide spectrum of DNA lesions (UV damage, chemical carcinogens, and chemotherapeutic drugs) within minutes of damage induction. This method therefore allows for a real-time measure of nucleotide excision repair activity that can be monitored in conjunction with other components of the DNA damage response, including DNA damage checkpoint and apoptotic signaling. This approach therefore provides a convenient and reliable platform for simultaneously examining multiple aspects of the DNA damage response in a single population of cells that can be applied for a diverse array of carcinogenic and chemotherapeutic agents. 相似文献
999.
Huw S. Groucutt Michael D. Petraglia Geoff Bailey Eleanor M. L. Scerri Ash Parton Laine Clark‐Balzan Richard P. Jennings Laura Lewis James Blinkhorn Nick A. Drake Paul S. Breeze Robyn H. Inglis Maud H. Devès Matthew Meredith‐Williams Nicole Boivin Mark G. Thomas Aylwyn Scally 《Evolutionary anthropology》2015,24(4):149-164
Current fossil, genetic, and archeological data indicate that Homo sapiens originated in Africa in the late Middle Pleistocene. By the end of the Late Pleistocene, our species was distributed across every continent except Antarctica, setting the foundations for the subsequent demographic and cultural changes of the Holocene. The intervening processes remain intensely debated and a key theme in hominin evolutionary studies. We review archeological, fossil, environmental, and genetic data to evaluate the current state of knowledge on the dispersal of Homo sapiens out of Africa. The emerging picture of the dispersal process suggests dynamic behavioral variability, complex interactions between populations, and an intricate genetic and cultural legacy. This evolutionary and historical complexity challenges simple narratives and suggests that hybrid models and the testing of explicit hypotheses are required to understand the expansion of Homo sapiens into Eurasia. 相似文献
1000.
Mate Familiarity Affects Pairing Behaviour in a Long‐Term Monogamous Lizard: Evidence from Detailed Bio‐Logging and a 31‐Year Field Study 下载免费PDF全文
Stephan T. Leu Dale Burzacott Martin J. Whiting C. Michael Bull 《Ethology : formerly Zeitschrift fur Tierpsychologie》2015,121(8):760-768
Long‐term monogamy is most prevalent in birds but is also found in lizards. We combined a 31‐year field study of the long‐lived, monogamous Australian sleepy lizard, Tiliqua rugosa, with continuous behavioural observations through GPS data logging, in 1 yr, to investigate the duration of pair bonds, rates of partner change and whether either the reproductive performance hypothesis or the mate familiarity hypothesis could explain this remarkable long‐term monogamy. The reproductive performance hypothesis predicts higher reproductive success in more experienced parents, whereas the mate familiarity hypothesis suggests that effects of partner familiarity select for partner retention and long‐term monogamy. Rates of partner change were below 34% over a 5‐yr period and most sleepy lizards formed long‐term pair bonds: 31 partnerships lasted for more than 15 yr, 110 for more than 10 yr, and the recorded maximum was 27 yr (ongoing). In the year when we conducted detailed observations, familiar pairs mated significantly earlier than unfamiliar pairs. Previous pairing experience (total number of years paired with previous partners) had no significant effect. Early mating often equates to higher reproductive success, and we infer that is the case in sleepy lizards. Early mating of familiar pairs was not due to better body condition. We propose two suggestions about the proximate mechanisms that may allow familiar pair partners to mate earlier than unfamiliar partners. First, they may have improved coordination of their reproductive sexual cycles to reach receptivity earlier and thereby maximise fertilisation success. Second, they may forage more efficiently, benefiting from effective information transfer and/or cooperative predator detection. Those ideas need empirical testing in the future. Regardless of the mechanism, our observations of sleepy lizard pairing behaviour support the mate familiarity hypothesis, but not the reproductive performance hypothesis, as an explanation for its long‐term monogamous mating system. 相似文献