全文获取类型
收费全文 | 36673篇 |
免费 | 2547篇 |
国内免费 | 978篇 |
出版年
2023年 | 349篇 |
2022年 | 678篇 |
2021年 | 1226篇 |
2020年 | 842篇 |
2019年 | 1087篇 |
2018年 | 1232篇 |
2017年 | 978篇 |
2016年 | 1354篇 |
2015年 | 1970篇 |
2014年 | 2169篇 |
2013年 | 2669篇 |
2012年 | 3096篇 |
2011年 | 2964篇 |
2010年 | 1696篇 |
2009年 | 1454篇 |
2008年 | 1941篇 |
2007年 | 1845篇 |
2006年 | 1651篇 |
2005年 | 1527篇 |
2004年 | 1374篇 |
2003年 | 1201篇 |
2002年 | 1053篇 |
2001年 | 581篇 |
2000年 | 527篇 |
1999年 | 466篇 |
1998年 | 285篇 |
1997年 | 261篇 |
1996年 | 267篇 |
1995年 | 245篇 |
1994年 | 201篇 |
1993年 | 167篇 |
1992年 | 248篇 |
1991年 | 226篇 |
1990年 | 207篇 |
1989年 | 165篇 |
1988年 | 159篇 |
1987年 | 155篇 |
1986年 | 121篇 |
1985年 | 158篇 |
1984年 | 114篇 |
1983年 | 102篇 |
1982年 | 83篇 |
1981年 | 89篇 |
1980年 | 63篇 |
1979年 | 106篇 |
1978年 | 98篇 |
1976年 | 63篇 |
1975年 | 70篇 |
1974年 | 71篇 |
1971年 | 62篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
81.
Marie-Hélène Chaput Darasinh Sihachakr Georges Ducreux Dominique Marie Nasrine Barghi 《Plant cell reports》1990,9(8):411-414
In order to regenerate somatic hybrids, mesophyll protoplasts from a dihaploid potato, BF15 (H1), were electrofused with those from two other dihaploid clones, Aminca (H6) and Cardinal (H3). Determination of the ploidy level by flow cytometry showed that 10% of plants regenerated from the fusion experiment with BF15 + Aminca were diploids, 14% triploids, 63% tetraploids and very few were mixoploids or had a higher ploidy level. Using morphological markers and vigour in plant growth, we were able to recover a total of 24 somatic hybrid plants, respectively 20 and 4 hybrids (accounting for 12% and 13% of regenerants) from the fusions BF15 + Aminca and BF15 + Cardinal. Most of the somatic hybrids were at the expected tetraploid level (2n=4x=48). The hybrid nature was confirmed by examining isoenzyme patterns for malate dehydrogenase (MDH) and isocitrate dehydrogenase (ICD). 相似文献
82.
M Julkunen R Koistinen A M Suikkari M Sepp?l? O A J?nne 《Molecular endocrinology (Baltimore, Md.)》1990,4(5):700-707
A beta-lactoglobulin homologue (beta LG/PP14) and insulin-like growth factor-binding protein-1 (IGFBP-1) are two major secretory proteins of the human endometrium. In the present study, we have shown that beta LG/PP14 mRNA is expressed in the endometrium in a cyclic manner, being hardly detectable in midcycle and most abundant during the late secretory phase. IGFBP-1 mRNA is also expressed in endometrium, but in amounts smaller than those encoding beta LG/PP14 and with maximal accumulation earlier in the secretory phase. The expression of these two mRNAs occurs in different cell types of the endometrium, as revealed by in situ hybridization techniques using single-stranded RNA probes. The glandular epithelial cells accumulate beta LG/PP14 mRNA during the late secretory phase of the cycle, whereas only the stromal cells of the late secretory endometrium express IGFBP-1 mRNA. In contrast to the endometrium, the two mRNAs are present at very low abundance in the fallopian tubes where they are expressed in the epithelial cells of the mucosa. 相似文献
83.
A new fluorescent acridine orange derivative, 3-amino-6-methoxy-9-(2-hydroxyethylamino) acridine (AMHA), has been applied to Hela cells in order to set up appropriate conditions for the detection of mycoplasma contaminations. Since AMHA staining reveals intensely fluorescent nuclei and slight fluorescent cytoplasm, we can visualize and localize mycoplasma contamination on each cell. In combination with a shortened Chen's staining method (1977), AMHA should allow a better detection of mycoplasma in animal cell cultures than the well established Hoechst dye. 相似文献
84.
The dihydrofolate reductase amplicons in different methotrexate-resistant Chinese hamster cell lines share at least a 273-kilobase core sequence, but the amplicons in some cell lines are much larger and are remarkably uniform in structure. 总被引:10,自引:7,他引:3
下载免费PDF全文
![点击此处可从《Molecular and cellular biology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
J E Looney C Ma T H Leu W F Flintoff W B Troutman J L Hamlin 《Molecular and cellular biology》1988,8(12):5268-5279
We have previously cloned and characterized two different dihydrofolate reductase amplicon types from a methotrexate-resistant Chinese hamster ovary cell line (CHOC 400). The largest of these (the type I amplicon) is 273 kilobases (kb) in length. In the present study, we utilized clones from the type I amplicon as probes to analyze the size and variability of the amplified DNA sequences in five other independently isolated methotrexate-resistant Chinese hamster cell lines. Our data indicated that the predominant amplicon types in all but one of these cell lines are larger than the 273-kb type I sequence. In-gel renaturation experiments as well as hybridization analysis of large SfiI fragments separated by pulse-field gradient gel electrophoresis showed that two highly resistant cell lines (A3 and MK42) have amplified very homogeneous core sequences that are estimated to be at least 583 and 653 kb in length, respectively. Thus, the sizes of the major amplicon types can be different in different drug-resistant Chinese hamster cell lines. However, there appears to be less heterogeneity in size and sequence arrangement within a given methotrexate-resistant Chinese hamster cell line than has been reported for several other examples of DNA sequence amplification in mammalian systems. 相似文献
85.
Hélène Pelletier Nils-Olivier Olsson Catherine Fady Danièle Reisser Patricia Lagadec Jean-François Jeannin 《Cancer immunology, immunotherapy : CII》1988,26(3):263-268
Summary DHD/K12 TRb (PROb) and DHD/K12 TSb (REGb) are two cancer cell variants originating from the same rat colon adenocarcinoma. They differ in their tumorigenicity: when inoculated into syngeneic BDIX rats, PROb cells induce progressive tumors whereas REGb cells induce tumors which always regress. As previously described, there is an inverse relation between their tumorigenicity and their susceptibility to NCMC mediated by syngeneic spleen or peripheral blood lymphocytes: PROb cells are significantly less sensitive to NCMC than REGb cells. This suggests a role for NCMC in the regression of REGb tumors. In this work the BDIX NCMC effector cells active in vitro against REGb cells were identified as NK cells according to four criteria: (1) efficacy in a 4-h 51Cr release assay, (2) sensitivity to anti-asGM1 antibody plus complement, (3) LGL morphology, and (4) ability to bind with the same affinity REGb and YAC-1 cells. In spleen, these NK cells were heterogeneous with respect to their asGM1 surface density and their morphology. PROb cells were not lysed by these NK cells in a short-term cytotoxicity assay, but only in a 16-h assay. It was shown that PROb and REGb cells were bound with the same affinity by NK cells, thus they certainly differ in their ability to resist to NK lytic mechanisms. This difference could play a role in the different tumorigenicity of the two variants.
Abbreviations used: NK, natural killer; NC, natural cytotoxic; NCMC, natural cell-mediated cytotoxicity; asGM1, asialo GM1; LL, large lymphocytes; LGL, large grnular lymphocytes; LAL, large agranular lymphocytes; PBMNC, peripheral blood mononuclear cells; E:T, effector to target cell ratio; C:H, cold to hot cell ratio; FBS, fetal bovine serum 相似文献
86.
A system for the inducible secretion of proteins from Bacillus subtilis during logarithmic growth 总被引:1,自引:0,他引:1
Alex Edelman Gwennaël Joliff ré Klier Georges Rapoport 《FEMS microbiology letters》1988,52(1-2):117-120
Abstract A Bacillus subtilis-Escherichia coli shuttle vector was constructed containing the B. subtilis levansucrase gene promoter and region encoding its signal sequence.
A site for the restriction enzyme Nae I was included to facilitate precise translational fusions to the DNA encoding the levansucrase signal sequence. Fusions of TEM β-lactamase to this construct displayed sucrose-inducible expression and secretion of B. subtilis . 相似文献
A site for the restriction enzyme Nae I was included to facilitate precise translational fusions to the DNA encoding the levansucrase signal sequence. Fusions of TEM β-lactamase to this construct displayed sucrose-inducible expression and secretion of B. subtilis . 相似文献
87.
Antimessenger oligodeoxyribonucleotides: an alternative to antisense RNA for artificial regulation of gene expression--a review 总被引:16,自引:0,他引:16
Synthetic oligodeoxyribonucleotides (oligos) are now widely used as artificial regulators for gene expression both in cell-free media and in cultured cells. We describe the biological consequence of the various chemical modifications that have been introduced into the molecules to improve their resistance against nuclease attack, their affinity for the target mRNA and their uptake by cells. We also describe the rising generation of antimessenger oligos. Covalently linked to reactive groups these molecules direct irreversible modifications of the complementary nucleic acids. We anticipate that these oligos will be targeted to double-stranded nucleic acids to interfere with gene expression at the DNA level. 相似文献
88.
The structure of an HLA-A2.4 functional variant (A2.4c) expressed on donor KLO has been examined by comparative peptide mapping with other HLA-A2 antigens of known structure and radiochemical sequencing. All the peptide differences between A2.4c and A2.1 could be accounted for by five amino acid changes at positions 9, 43, 66, 95, and 156. The nature of residues 9, 43, and 95 in A2.4c was determined by sequencing to be identical to those in A2.2Y. The nature of residue 156 in A2.4c was also assigned as identical to that in A2.2Y on the basis of the identity of the corresponding peptide in its chromatographic comparison with A2.2Y. Position 66 was unique to A2.4c. It was determined to be an Asn residue instead of the Lys present in all other HLA-A2 antigens of known structure. This was the only detected amino acid difference between A2.4c and A2.2Y. The results indicate that, from a structural point of view, A2.4c is most closely related to the A2.2 subtype antigens and not to other A2.4 antigens. The data are compatible with the assumption that A2.4c was derived from A2.2Y by a single point mutation event. 相似文献
89.
Ma I. G. Siso M. A. Murado J. M. Franco J. Miron Ma.P. González 《Biotechnology letters》1988,10(6):431-436
Summary Some possible criteria in selection of amylolytic microorganisms for their mixed culture with non-amylolytic yeasts are discussed, and the growth of several microfungus-yeast mixed cultures on mussel processing wastes are studied. 相似文献
90.
Raúl Castaño Angel Ezquerra Nieves Doménech José A. López de Castro 《Immunogenetics》1988,27(5):345-355
The HLA-A2 antigen expressed by donor OZB can be distinguished from the main HLA-A2.1 subtype by isoelectric focusing - it is one charge unit more acidic — and by some alloreactive T-cell clones but not by cytolytic T lymphocyte lines. The structure of variant OZB has been examined by comparative peptide mapping with A2.1 and radiochemical sequence analysis. The two molecules were found to differ in a single tryptic peptide from the 0 region, spanning residues 220–243. The amino acid sequence of this peptide from variant OZB revealed that there was only one amino acid change of Glu instead of Ala at position 236, a hitherto invariant residue in class I HLA antigens. All previously characterized HLA or H-2 natural variants have structural changes restricted to the 1 and/or 2 domains. Thus, variant OZB is unique in that (1) it has one amino acid change in 3 and (2) it has no changes in l and 2. The only detected substitution of this variant may be accounted for by a single base change at the DNA level, suggesting that it might have resulted from a point mutation in the A2.1 gene. The structural features of variant OZB open a novel way to examine the influence of polymorphism in 3 on cytolytic T-cell recognition of naturally occurring class I antigens.Abbreviations CTL
cytolytic T lymphocytes
- HPLC
high performance liquid chromatography
- IEF
isoelectric focusing
- MHC
major histocompatibility complex 相似文献