全文获取类型
收费全文 | 11079篇 |
免费 | 1022篇 |
国内免费 | 7篇 |
专业分类
12108篇 |
出版年
2023年 | 61篇 |
2022年 | 166篇 |
2021年 | 329篇 |
2020年 | 160篇 |
2019年 | 225篇 |
2018年 | 262篇 |
2017年 | 229篇 |
2016年 | 327篇 |
2015年 | 624篇 |
2014年 | 618篇 |
2013年 | 701篇 |
2012年 | 939篇 |
2011年 | 920篇 |
2010年 | 564篇 |
2009年 | 514篇 |
2008年 | 674篇 |
2007年 | 630篇 |
2006年 | 533篇 |
2005年 | 499篇 |
2004年 | 496篇 |
2003年 | 512篇 |
2002年 | 446篇 |
2001年 | 84篇 |
2000年 | 50篇 |
1999年 | 84篇 |
1998年 | 115篇 |
1997年 | 59篇 |
1996年 | 65篇 |
1995年 | 66篇 |
1994年 | 58篇 |
1993年 | 56篇 |
1992年 | 54篇 |
1991年 | 62篇 |
1990年 | 44篇 |
1989年 | 41篇 |
1988年 | 35篇 |
1987年 | 32篇 |
1986年 | 38篇 |
1985年 | 63篇 |
1984年 | 47篇 |
1983年 | 54篇 |
1982年 | 45篇 |
1981年 | 44篇 |
1980年 | 36篇 |
1979年 | 34篇 |
1978年 | 38篇 |
1977年 | 37篇 |
1975年 | 29篇 |
1974年 | 36篇 |
1973年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
21.
22.
23.
Chromosome distribution of intracisternal A-particle sequences in the Syrian hamster and mouse 总被引:5,自引:0,他引:5
Edward L. Kuff Joseph E. Fewell Kira K. Lueders Joseph A. DiPaolo Suzanne C. Amsbaugh Nicholas C. Popescu 《Chromosoma》1986,93(3):213-219
Metaphase chromosomes of Syrian hamster and BALB/c mice were hybridized in situ with radiolabeled probes derived from cloned intracisternal A-particle (IAP) genes of the corresponding species. The DNAs of these species are known to contain about 900 and 1,000 copies, respectively, of the retrovirus-like IAP sequence elements per haploid genome. Multiple IAP sequences were found on all chromosomes of both hamster and mouse. In the hamster, more than half of the IAP sequences were located in regions of non-centromeric constitutive heterochromatin, at an average concentration per unit chromosome length 5 times greater than in the euchromatic regions. The other dispersed sequences showed marked local variations in concentration along the chromosome lengths; both discrete foci and large grain clusters were observed as well as regions apparently lacking IAP sequences. Within the resolution of the techniques, IAP sequences appeared to be more evenly distributed over the mouse chromosomes; however, some prominent variations in concentration were seen. The number of potentially active IAP genes in the Syrian hamster, and by extension in the mouse, may be restricted by the preferential location of IAP sequences in genetically inert regions of the genome. 相似文献
24.
Purification and sequencing of the active site tryptic peptide from penicillin-binding protein 1b of Escherichia coli 总被引:8,自引:0,他引:8
This paper reports the sequence of the active site peptide of penicillin-binding protein 1b from Escherichia coli. Purified penicillin-binding protein 1b was labeled with [14C]penicillin G, digested with trypsin, and partially purified by gel filtration. Upon further purification by high-pressure liquid chromatography, two radioactive peaks were observed, and the major peak, representing over 75% of the applied radioactivity, was submitted to amino acid analysis and sequencing. The sequence Ser-Ile-Gly-Ser-Leu-Ala-Lys was obtained. The active site nucleophile was identified by digesting the purified peptide with aminopeptidase M and separating the radioactive products on high-pressure liquid chromatography. Amino acid analysis confirmed that the serine residue in the middle of the sequence was covalently bonded to the [14C]penicilloyl moiety. A comparison of this sequence to active site sequences of other penicillin-binding proteins and beta-lactamases is presented. 相似文献
25.
Uptake Hydrogenase Activity Determined by Plasmid pRL6JI in Rhizobium leguminosarum Does Not Increase Symbiotic Nitrogen Fixation 总被引:2,自引:2,他引:0 下载免费PDF全文
Scott D. Cunningham Yoram Kapulnik Nicholas J. Brewin Donald A. Phillips 《Applied microbiology》1985,50(4):791-794
We examined three groups of wild baboons (Papio cynocephalus) in Amboseli National Park, Kenya, to determine the prevalence of aerobic antibiotic-resistant fecal bacteria in nonhuman primates with and without contact with human refuse. Using standard isolation and replica plating techniques, we found only low numbers of antibiotic-resistant gram-negative enteric bacteria in two groups of baboons leading an undisturbed existence in their natural habitat and having limited or no contact with humans. However, resistance was significantly higher among enteric bacteria from the third group of baboons living in close proximity to a tourist lodge and having daily contact with unprocessed human refuse. Conjugation studies and analysis of the cell DNA by gel electrophoresis showed that in many cases resistance was plasmid-borne and transferable. These data suggest that wild nonhuman primates in frequent contact with human debris have a higher proportion of antibiotic-resistant enteric bacteria than do conspecifics without this contact. The findings further suggest that such groups of wild animals may constitute a heretofore overlooked source of antibiotic resistance in the natural environment. 相似文献
26.
Summary The complete nucleotide sequence of the 5S ribosomal RNA from the cyanobacteriumSynechococcus lividus II has been determined. The sequence is 5-UGCCUAGUGUUUAUGGCGCG-GUGGAACCACGCUGAUCCAUCCCGAACUC-AGAGGUGAAACAUCGCAGCGGUGAAGAU-AGUUGGAGGGUAGCCUCCUGCAAAAAUA-GCUCAAUGCUAGGCAOH-3. This 5S RNA has the cyanobacterial- and chloroplast-specific nucleotide insertion between positions 30 and 31 (using the numbering system of the generalized eubacterial 5S RNA) and the chloroplast-specific nucleotide-deletion signature between positions 34 and 39. The 5S RNA ofS. lividus II has 27 base differences compared with the 5S RNA of the related strainS. lividus III. This large difference may reflect an ancient divergence between these two organisms. The electrophoretic mobilities on nondenaturing polyacrylamide gels of renatured 5S RNAs fromS. lividus II,S. lividus III, and spinach chloroplasts are identical, but differ considerably from that ofEscherichia coli 5S RNA. This most likely reflects differences in higher-order structure between the 5S RNA ofE. coli and these cyanobacterial and chloroplast 5S RNAs. 相似文献
27.
Identification of the active site in penicillin-binding protein 3 of Escherichia coli. 总被引:5,自引:5,他引:0 下载免费PDF全文
We report the sequence of the active site tryptic peptide of penicillin-binding protein 3 from Escherichia coli. Purified penicillin-binding protein 3 was labeled with [14C]penicillin G and digested with trypsin, and the resulting radioactive peptides were isolated by a combination of gel filtration and high-pressure liquid chromatography. The major radioactive peak from high-pressure liquid chromatography was sequenced, and the peptide Thr-Ile-Thr-Asp-Val-Phe-Glu-Pro-Gly-Ser-Thr-Val-Lys, which comprises residues 298 to 310 in the amino acid sequence, was identified. This sequence is compared with the active site sequences from other penicillin-binding proteins and beta-lactamases. 相似文献
28.
R A Nicholas F Ishino W Park M Matsuhashi J L Strominger 《The Journal of biological chemistry》1985,260(10):6394-6397
The localization of the active site of penicillin-binding protein 5 from the dacA mutant of Escherichia coli strain TMRL 1222 has been determined. The protein was purified to homogeneity and labeled with [14C] penicillin G. The labeled protein was digested with trypsin, and the active site tryptic peptide was purified by a combination of gel filtration and high-pressure liquid chromatography. Sequencing of the purified [14C]penicilloyl peptide yielded the sequence Arg-Asp-Pro-Ala-Ser-Leu-Thr-Lys, which corresponds to residues 40-47 of the gene sequence (Broome-Smith, J., Edelman, A., and Spratt, B. G. (1983) in The Target of Penicillin (Hakenbeck, R., Holtje, J.-V., and Labischinski, H., eds) pp. 403-408, Walter de Gruyter, Berlin). The catalytic amino acid residue that forms a covalent bond with penicillin was identified by treating the purified [14C]penicilloyl peptide with a mixture of proteases and then separating the radioactive products using high-pressure liquid chromatography. Analysis of the radioactive peaks by amino acid analysis confirmed that it is the serine residue that reacts with the beta-lactam ring of penicillin. 相似文献
29.
Summary A new species of Rhynchoidomonas Patton was observed in a single adult male winter moth, Operophtera brumata (L.) from England. Intracellular amastigotes, and extracellular epimastigotes and trypomastigotes with an undulating membrane and free flagellum, were present. All stages had a large, reniform kinetoplast. As transmission of the flagellate between generations of winter moths by ingestion of infected faeces is a virtual impossibility, it is suggested that the flagellate's true host may have been a dipteran parasitoid and that an egg, surface-contaminated with the flagellate, was oviposited into or ingested by a winter moth larva. If the parasitoid had died, this flagellate infection could have been carried over to the adult moth. ac]19830601 相似文献
30.
Mammary explants from pregnant rats can be induced in regard to casein synthesis and alpha-lactalbumin activity when cultured in the presence of hydrocortisone, prolactin and levels of insulin approaching physiological concentrations. No detectable induction occurs in the absence of insulin. Although epidermal growth factor and multiplication stimulating activity, in the presence of hydrocortisone, can maintain the initial level of NADH-cytochrome c reductase as well as insulin, neither can substitute effectively for insulin in the induction of the milk proteins. Proinsulin, nerve growth factor, platelet-derived growth factor and fibroblast growth factor are also ineffective substitutes for insulin in this regard. Whereas prolonged tissue exposure to multiplication stimulating activity, hydrocortisone and prolactin does not result in induction of alpha-lactalbumin activity, subsequent addition of insulin leads to prompt response. The results suggest that the ability of insulin to function as a unique, essential factor in the induction of rat milk proteins is independent of its cell-maintenance activity. Thus, in addition to its well established functions in metabolic processes, insulin appears to play a vital role in certain developmental processes. 相似文献