全文获取类型
收费全文 | 12041篇 |
免费 | 1502篇 |
国内免费 | 3篇 |
专业分类
13546篇 |
出版年
2021年 | 178篇 |
2019年 | 123篇 |
2018年 | 138篇 |
2017年 | 131篇 |
2016年 | 227篇 |
2015年 | 355篇 |
2014年 | 356篇 |
2013年 | 468篇 |
2012年 | 584篇 |
2011年 | 542篇 |
2010年 | 363篇 |
2009年 | 331篇 |
2008年 | 446篇 |
2007年 | 426篇 |
2006年 | 426篇 |
2005年 | 414篇 |
2004年 | 388篇 |
2003年 | 393篇 |
2002年 | 374篇 |
2001年 | 362篇 |
2000年 | 380篇 |
1999年 | 328篇 |
1998年 | 165篇 |
1997年 | 179篇 |
1996年 | 178篇 |
1995年 | 126篇 |
1994年 | 139篇 |
1993年 | 139篇 |
1992年 | 265篇 |
1991年 | 241篇 |
1990年 | 247篇 |
1989年 | 231篇 |
1988年 | 220篇 |
1987年 | 219篇 |
1986年 | 176篇 |
1985年 | 224篇 |
1984年 | 175篇 |
1983年 | 149篇 |
1982年 | 154篇 |
1981年 | 140篇 |
1980年 | 117篇 |
1979年 | 171篇 |
1978年 | 137篇 |
1977年 | 128篇 |
1976年 | 116篇 |
1975年 | 108篇 |
1974年 | 134篇 |
1973年 | 135篇 |
1972年 | 111篇 |
1971年 | 119篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
971.
972.
Taylor RW Wardell TM Connolly BA Turnbull DM Lightowlers RN 《Nucleic acids research》2001,29(16):3404-3412
Mutations in mitochondrial DNA (mtDNA) cause a spectrum of human pathologies, which predominantly affect skeletal muscle and the central nervous system. In patients, mutated and wild-type mtDNAs often co-exist in the same cell (mtDNA heteroplasmy). In the absence of pharmacological therapy, a genetic strategy for treatment has been proposed whereby replication of mutated mtDNA is inhibited by selective hybridisation of a nucleic acid derivative to the single-stranded replication intermediate, allowing propagation of the wild-type genome and correction of the associated respiratory chain defect. Previous studies have shown the efficacy of this anti-genomic approach in vitro, targeting pathogenic mtDNA templates with only a single point mutation. Pathogenic molecules harbouring deletions, however, present a more difficult problem. Deletions often occur at the site of two short repeat sequences (4–13 residues), only one of which is retained in the deleted molecule. With the more common larger repeats it is therefore difficult to design an anti-genomic molecule that will bind selectively across the breakpoint of the deleted mtDNA. To address this problem, we have used linker-substituted oligodeoxynucleotides to bridge the repeated residues. We show that molecules can be designed to bind more tightly to the deleted as compared to the wild-type mtDNA template, consistent with the nucleotide sequence on either side of the linker co-operating to increase binding affinity. Furthermore, these bridging molecules are capable of sequence-dependent partial inhibition of replication in vitro. 相似文献
973.
Hoffmann W Petit G Schulz-Key H Taylor D Bain O Le Goff L 《Parasitology today (Personal ed.)》2000,16(9):387-389
Onchocerciasis and lymphatic filariasis (LF) are major causes of severe morbidity and considerable socio-economic problems throughout the tropics. Vector control and mass chemotherapy have helped to control these infections in some regions, but the temporary success of such measures argues strongly for the development of vaccines. Success in such a venture will require detailed knowledge of protective immune responses in conjunction with the identification of target antigens. By comparison with other important parasitic infections, such as schistosomiasis and leishmaniasis, work on the development of vaccines for onchocerciasis and LF has been constrained because of the difficulties of producing cyclical and patent filarial infection in laboratory mice. Wolfgang Hoffmann and colleagues here outline the opportunities presented by the rodent filaria Litomosoides sigmodontis for filarial research. 相似文献
974.
A cell cycle-specific requirement for the XRCC1 BRCT II domain during mammalian DNA strand break repair 总被引:1,自引:0,他引:1 下载免费PDF全文
Taylor RM Moore DJ Whitehouse J Johnson P Caldecott KW 《Molecular and cellular biology》2000,20(2):735-740
XRCC1 protein is essential for viability in mammals and is required for efficient DNA single-strand break repair and genetic stability following DNA base damage. We report here that XRCC1-dependent strand break repair in G(1) phase of the cell cycle is abolished by mutations created within the XRCC1 BRCT domain that interact with DNA ligase III. In contrast, XRCC1-dependent DNA strand break repair in S phase is largely unaffected by these mutations. These data describe a cell cycle-specific role for a BRCT domain, and we conclude that the XRCC1-DNA ligase III complex is required for DNA strand break repair in G(1) phase of the cell cycle but is dispensable for this process in S phase. The S-phase DNA repair process can remove both strand breaks induced in S phase and those that persist from G(1) and can in part compensate for lack of repair in G(1). This process correlates with the appearance of XRCC1 nuclear foci that colocalize with Rad51 and may thus function in concert with homologous recombination. 相似文献
975.
Phosphorylation of tyrosine residues in the kinase domain and juxtamembrane region regulates the biological and catalytic activities of Eph receptors 总被引:5,自引:0,他引:5 下载免费PDF全文
Binns KL Taylor PP Sicheri F Pawson T Holland SJ 《Molecular and cellular biology》2000,20(13):4791-4805
Members of the Eph family of receptor tyrosine kinases exhibit a striking degree of amino acid homology, particularly notable in the kinase and membrane-proximal regions. A mutagenesis approach was taken to address the functions of specific conserved tyrosine residues within these catalytic and juxtamembrane domains. Ligand stimulation of wild-type EphB2 in neuronal NG108-15 cells resulted in an upregulation of catalytic activity and an increase in cellular tyrosine phosphorylation, accompanied by a retraction of neuritic processes. Tyrosine-to-phenylalanine substitutions within the conserved juxtamembrane motif abolished these responses. The mechanistic basis for these observations was examined using the highly related EphA4 receptor in a continuous coupled kinase assay. Tandem mass spectrometry experiments confirmed autophosphorylation of the two juxtamembrane tyrosine residues and also identified a tyrosine within the kinase domain activation segment as a phosphorylation site. Kinetic analysis revealed a decreased affinity for peptide substrate upon substitution of activation segment or juxtamembrane tyrosines. Together, our data suggest that the catalytic and therefore biological activities of Eph receptors are controlled by a two-component inhibitory mechanism, which is released by phosphorylation of the juxtamembrane and activation segment tyrosine residues. 相似文献
976.
The toxin-co-regulated pilus (TCP), a type 4 pilus that is expressed by epidemic strains of Vibrio cholerae O1 and O139, is required for colonization of the human intestine. The TCP structure is assembled as a polymer of repeating subunits of TcpA pilin that form long fibres, which laterally associate into bundles. Previous passive immunization studies have suggested that the C-terminal region of TcpA is exposed on the surface of the pilus fibre and has a critical role in mediating the colonization functions of TCP. In the present study, we have used site-directed mutagenesis to delineate two domains within the C-terminal region that contribute to TCP structure and function. Alterations in the first domain, termed the structural domain, result in altered pilus stability or morphology. Alterations in the second domain, termed the interaction domain, affect colonization and/or infection by CTX-bacteriophage without affecting pilus morphology. In vitro and in vivo analyses of the tcpA mutants revealed that a major function of TCP is to mediate bacterial interaction through direct pilus-pilus contact required for microcolony formation and productive intestinal colonization. The importance of this function is supported by the finding that intragenic suppressor mutations that restore colonization ability to colonization-deficient mutants simultaneously restore pilus-mediated bacterial interactions. The alterations resulting from the suppressor mutations also provide insight into the molecular interactions between pilin subunits within and between pilus fibres. 相似文献
977.
Escherichia coli TehB requires S-adenosylmethionine as a cofactor to mediate tellurite resistance 总被引:1,自引:0,他引:1 下载免费PDF全文
Liu M Turner RJ Winstone TL Saetre A Dyllick-Brenzinger M Jickling G Tari LW Weiner JH Taylor DE 《Journal of bacteriology》2000,182(22):6509-6513
The Escherichia coli chromosomal determinant for tellurite resistance consists of two genes (tehA and tehB) which, when expressed on a multicopy plasmid, confer resistance to K(2)TeO(3) at 128 microg/ml, compared to the MIC of 2 microg/ml for the wild type. TehB is a cytoplasmic protein which possesses three conserved motifs (I, II, and III) found in S-adenosyl-L-methionine (SAM)-dependent non-nucleic acid methyltransferases. Replacement of the conserved aspartate residue in motif I by asparagine or alanine, or of the conserved phenylalanine in motif II by tyrosine or alanine, decreased resistance to background levels. Our results are consistent with motifs I and II in TehB being involved in SAM binding. Additionally, conformational changes in TehB are observed upon binding of both tellurite and SAM. The hydrodynamic radius of TehB measured by dynamic light scattering showed a approximately 20% decrease upon binding of both tellurite and SAM. These data suggest that TehB utilizes a methyltransferase activity in the detoxification of tellurite. 相似文献
978.
979.
980.
Allison Gray C Bartol FF Taylor KM Wiley AA Ramsey WS Ott TL Bazer FW Spencer TE 《Biology of reproduction》2000,62(2):448-456
Ovine endometrial gland development is a postnatal event that can be inhibited epigenetically by chronic exposure of ewe lambs to a synthetic progestin from birth to puberty. As adults, these neonatally progestin-treated ewes lack endometrial glands and display a uterine gland knockout (UGKO) phenotype that is useful as a model for study of endometrial function. Here, objectives were to determine: 1) length of progestin exposure necessary from birth to produce the UGKO phenotype in ewes; 2) if UGKO ewes display normal estrous cycles; and 3) if UGKO ewes could establish and/or maintain pregnancy. Ewe lambs (n = 22) received a Norgestomet (Nor) implant at birth and every two weeks thereafter for 8 (Group I), 16 (Group II), or 32 (Groups III and IV) weeks. Control ewe lambs (n = 13) received no Nor treatment (Groups V and VI). Ewes in Groups I, II, III, and VI were hemihysterectomized (Hhx) at 16 weeks of age. After puberty, the remaining uterine horn in Hhx ewes was removed on either Day 9 or 15 of the estrous cycle (Day 0 = estrus). Histological analyses of uteri indicated that progestin exposure for 8, 16, or 32 weeks prevented endometrial adenogenesis and produced the UGKO phenotype in adult ewes. Three endometrial phenotypes were consistently observed in Nor-treated ewes: 1) no glands, 2) slight glandular invaginations into the stroma, and 3) limited numbers of cyst- or gland-like structures in the stroma. Overall patterns of uterine progesterone, estrogen, and oxytocin receptor expression were not different in uteri from adult cyclic control and UGKO ewes. However, receptor expression was variegated in the ruffled luminal epithelium of uteri from UGKO ewes. Intact UGKO ewes displayed altered estrous cycles with interestrous intervals of 17 to 43 days, and they responded to exogenous prostaglandin F(2 approximately ) (PGF) with luteolysis and behavioral estrus. During the estrous cycle, plasma concentrations of progesterone in intact control and UGKO ewes were not different during metestrus and diestrus, but levels did not decline in many UGKO ewes during late diestrus. Peak peripheral plasma concentrations of PGF metabolite, in response to an oxytocin challenge on Day 15, were threefold lower in UGKO compared to control ewes. Intact UGKO ewes bred repeatedly to intact rams did not display evidence of pregnancy based on results of ultrasound. Collectively, results indicate that 1) transient, progestin-induced disruption of ovine uterine development from birth alters both structural and functional integrity of the adult endometrium; 2) normal adult endometrial integrity, including uterine glands, is required to insure a luteolytic pattern of PGF production; and 3) the UGKO phenotype, characterized by the absence of endometrial glands and a compact, disorganized endometrial stroma, limits or inhibits the capacity of uterine tissues to support the establishment and/or maintenance of pregnancy. 相似文献