全文获取类型
收费全文 | 532409篇 |
免费 | 59630篇 |
国内免费 | 345篇 |
专业分类
592384篇 |
出版年
2018年 | 5218篇 |
2017年 | 5082篇 |
2016年 | 6940篇 |
2015年 | 8755篇 |
2014年 | 10470篇 |
2013年 | 15232篇 |
2012年 | 16934篇 |
2011年 | 17319篇 |
2010年 | 11684篇 |
2009年 | 10726篇 |
2008年 | 15129篇 |
2007年 | 15684篇 |
2006年 | 14651篇 |
2005年 | 14044篇 |
2004年 | 13907篇 |
2003年 | 13265篇 |
2002年 | 12794篇 |
2001年 | 28352篇 |
2000年 | 28209篇 |
1999年 | 21973篇 |
1998年 | 6721篇 |
1997年 | 7300篇 |
1996年 | 6732篇 |
1995年 | 6208篇 |
1994年 | 5980篇 |
1993年 | 5956篇 |
1992年 | 17039篇 |
1991年 | 16287篇 |
1990年 | 15696篇 |
1989年 | 15207篇 |
1988年 | 13919篇 |
1987年 | 12933篇 |
1986年 | 12040篇 |
1985年 | 11812篇 |
1984年 | 9657篇 |
1983年 | 8081篇 |
1982年 | 5992篇 |
1981年 | 5372篇 |
1980年 | 5095篇 |
1979年 | 8940篇 |
1978年 | 6809篇 |
1977年 | 6272篇 |
1976年 | 5645篇 |
1975年 | 6235篇 |
1974年 | 6758篇 |
1973年 | 6540篇 |
1972年 | 5978篇 |
1971年 | 5429篇 |
1970年 | 4679篇 |
1969年 | 4399篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
331.
Oligomycin sensitivity conferral protein (OSCP), factor 6 (F6), and ATPase inhibitor protein are all components of the ATP synthase complex of bovine mitochondria. They are encoded in nuclear DNA. Complementary DNA clones encoding the precursors of these proteins have been isolated from a bovine library by using mixtures of synthetic oligonucleotides as hybridization probes, and their DNA sequences have been determined. The deduced protein sequences show that the OSCP, F6, and inhibitor proteins have N-terminal presequences of 23, 32, and 25 amino acids, respectively. These presequences are not present in the mature proteins. It is assumed that they serve to direct the proteins into the mitochondrial matrix. The cDNA clones have also been employed as hybridization probes to investigate the genetic complexity of the three proteins in cows and humans. These experiments indicate that the bovine and human inhibitor and bovine F6 proteins are encoded by single genes but suggest the possibility of the presence in both species of more than one gene (or pseudogenes) for the OSCP. 相似文献
332.
333.
The recurrence of influenza A epidemics has originally been explained by a “continuous antigenic drift” scenario. Recently, it has been shown that if genetic drift is gradual, the evolution of influenza A main antigen, the haemagglutinin, is punctuated. As a consequence, it has been suggested that influenza A dynamics at the population level should be approximated by a serial model. Here, simple models are used to test whether a serial model requires gradual antigenic drift within groups of strains with the same antigenic properties (antigenic clusters). We compare the effect of status based and history based frameworks and the influence of reduced susceptibility and infectivity assumptions on the transient dynamics of antigenic clusters. Our results reveal that the replacement of a resident antigenic cluster by a mutant cluster, as observed in data, is reproduced only by the status based model integrating the reduced infectivity assumption. This combination of assumptions is useful to overcome the otherwise extremely high model dimensionality of models incorporating many strains, but relies on a biological hypothesis not obviously satisfied. Our findings finally suggest the dynamical importance of gradual antigenic drift even in the presence of punctuated immune escape. A more regular renewal of susceptible pool than the one implemented in a serial model should be part of a minimal theory for influenza at the population level. 相似文献
334.
Studies in biodiversity-ecosystem function and conservation biology have led to the development of diversity indices that take species' functional differences into account. We identify two broad classes of indices: those that monotonically increase with species richness (MSR indices) and those that weight the contribution of each species by abundance or occurrence (weighted indices). We argue that weighted indices are easier to estimate without bias but tend to ignore information provided by rare species. Conversely, MSR indices fully incorporate information provided by rare species but are nearly always underestimated when communities are not exhaustively surveyed. This is because of the well-studied fact that additional sampling of a community may reveal previously undiscovered species. We use the rarefaction technique from species richness studies to address sample-size-induced bias when estimating functional diversity indices. Rarefaction transforms any given MSR index into a family of unbiased weighted indices, each with a different level of sensitivity to rare species. Thus rarefaction simultaneously solves the problem of bias and the problem of sensitivity to rare species. We present formulae and algorithms for conducting a functional rarefaction analysis of the two most widely cited MSR indices: functional attribute diversity (FAD) and Petchey and Gaston's functional diversity (FD). These formulae also demonstrate a relationship between three seemingly unrelated functional diversity indices: FAD, FD and Rao's quadratic entropy. Statistical theory is also provided in order to prove that all desirable statistical properties of species richness rarefaction are preserved for functional rarefaction. 相似文献
335.
C-Type Virus Released from Cultured Human Rhabdomyosarcoma Cells 总被引:44,自引:0,他引:44
R. M. McALLISTER M. NICOLSON M. B. GARDNER R. W. RONGEY S. RASHEED P. S. SARMA R. J. HUEBNER M. HATANAKA S. OROSZLAN R. V. GILDEN A. KABIGTING L. VERNON 《Nature: New biology》1972,235(53):3-6
RD-114 virus, released from human rhabdomyosarcoma cells, has all the characteristics of a mammalian C-type virus. Immunological tests indicate that it differs from all known C-type viruses and is the most likely candidate for a human C-type virus yet described. 相似文献
336.
337.
C57BL/6 nude beige mice (B6 nubq; no T cell, no NK activity) were used as recipients for the adoptive transfer of thymocytes from B6 gld mice (generalized lymphoproliferative disease) which are a model of systemic lupus erythematous. The [gld----nubg] chimeras showed several similarities with gld control mice including the T cell disorders (lymphoproliferation and Con A-response deficiency of splenocytes) and B cell disorders (hyperglobulinemia and elevated anti-single-stranded DNA antibody titers). This suggests that the gld lymphoproliferative disorder has a thymic origin (and does not result from an abnormally extrathymic T cell development) and that the gld T cells have an essential role for the emergence of the disorders of both the T and B cells. 相似文献
338.
Prostaglandin (PG) stimulates female spawning behavior in goldfish and in some other teleosts in which female reproductive behaviors consist of postovulatory oviposition acts. This study examined the effects of PG on female sexual behavior in a teleost fish, Cichlasoma bimaculatum, in which female reproductive behaviors involve both preovulatory courtship and substrate cleaning behaviors, and post-ovulatory oviposition behavior. In females of established pairs, PGF2 alpha injection (5 micrograms, im) at any stage of the spawning cycle, or in the parental phase, rapidly induced substrate cleaning which soon merged into oviposition behavior (without egg release). These results support a role for PG in oviposition behavior of Cichlasoma. However, indomethacin (1 mg, ip), a PG synthesis inhibitor, did not block oviposition in ovulated females which had begun to spawn. Indomethacin may not have lowered PG levels sufficiently. Alternatively, as shown by J.J. Polder (1971, Neth. J. Zool. 21, 265), oviposition behavior may be induced or maintained by other factors associated with the spawning situation. 相似文献
339.
340.