全文获取类型
收费全文 | 51744篇 |
免费 | 4300篇 |
国内免费 | 1726篇 |
出版年
2023年 | 767篇 |
2022年 | 1120篇 |
2021年 | 1413篇 |
2020年 | 1766篇 |
2019年 | 2298篇 |
2018年 | 2089篇 |
2017年 | 1496篇 |
2016年 | 1440篇 |
2015年 | 1616篇 |
2014年 | 3100篇 |
2013年 | 3663篇 |
2012年 | 2349篇 |
2011年 | 3029篇 |
2010年 | 2279篇 |
2009年 | 2524篇 |
2008年 | 2712篇 |
2007年 | 2647篇 |
2006年 | 2232篇 |
2005年 | 2033篇 |
2004年 | 1823篇 |
2003年 | 1565篇 |
2002年 | 1339篇 |
2001年 | 937篇 |
2000年 | 738篇 |
1999年 | 791篇 |
1998年 | 704篇 |
1997年 | 643篇 |
1996年 | 586篇 |
1995年 | 569篇 |
1994年 | 561篇 |
1993年 | 463篇 |
1992年 | 436篇 |
1991年 | 400篇 |
1990年 | 307篇 |
1989年 | 280篇 |
1988年 | 249篇 |
1987年 | 250篇 |
1986年 | 193篇 |
1985年 | 370篇 |
1984年 | 526篇 |
1983年 | 450篇 |
1982年 | 466篇 |
1981年 | 385篇 |
1980年 | 396篇 |
1979年 | 315篇 |
1978年 | 239篇 |
1977年 | 234篇 |
1976年 | 230篇 |
1975年 | 198篇 |
1974年 | 178篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
981.
982.
Using artificial electron donors and acceptors, it is shown here that the major HCO3? effect in the Hill reaction is after the “primary” electron acceptor (Q) of Photosystem II and before the site of action of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (at the plastoquinone pool). Chloroplasts in the presence of both 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea, which blocks electron flow from the reduced primary acceptor Q? to the plastoquinone pool, and silicomolybdate, which accepts electrons from Q?, show no significant bicarbonate stimulation of electron flow. However, a 6–7-fold stimulation is clearly observed when oxidized diaminodurene, as an electron acceptor, and dibromothymoquinone, as an inhibitor of electron flow beyond the plastoquinone pool, are used. In the same chloroplast preparation no measurable effect of bicarbonate is observed in a Photosystem I reaction as monitored by electron flow from reduced diaminodurene to methyl viologen in the presence of 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea. The insensitivity of the bicarbonate effect to uncouplers of photophosphorylation and the dependence of this effect on the presence of a weak acid anion and on external pH are also reported. 相似文献
983.
Freeze cleaving electron microscopy has shown that fusion of isolated secretory vesicles from bovine neurohypophyses was induced by Ca2+ in micromolar concentrations. Mg2+ and Sr2+ were ineffective. Mg2+ inhibited Ca2+-induced fusion.In suspensions containing secretory vesicles as well as sheets of cell membrane, release of vasopressin parallel to intervesicular fusion of secretory vesicles with sheets of cell membrane was observed after exposure to Ca2+. Mg2+ and Sr2+ were ineffective in replacing Ca2+ as trigger for fusion or vasopressin release.Intervesicular fusion and exocytotic profiles were observed when isolated neurohypophyses or neurosecretosome were exposed to cold. 相似文献
984.
The influence of prednisolone (corticosteroid, C.S.) treatment upon established cell-mediated immunity (CMI) induced by complete Freund's adjuvant (CFA) has been studied in rats by using in vitro migration inhibitory factor (MIF) assays, type IV skin reactions, and regional lymph node and spleen histology. Additionally, changes in the mononuclear-polymorphonuclear ratio of peripheral blood and T-cell accumulation in bone marrow in response to C.S. treatment have been determined. These results have been evaluated by comparison with equivalent experiments upon animals treated with anti-rat lymphocyte serum (ALS), oxisuran, or 2-[(methylsulfinyl)-acetyl]pyridine, which selectively suppresses CMI. The results suggest the existence of a population of “educated” T-cells in the thymic cortex of sensitized rats, and they suggest that prolonged C.S. administration does not suppress T-effector cells involved in established CMI but, rather, affects lymphocyte and monocyte migration patterns, including the migration of educated T-cells from the thymic cortex into other tissue compartments. 相似文献
985.
Mouse lymphoid cells and tumor cell lines were employed as target cells for the investigation of the mechanism of heterocytotoxicity in human serum samples. It was shown that the heterocytotoxic effects were due to two differing mechanisms. Cytotoxicity was mediated in part, by activation of the alternative complement pathway on target cell membrane, a process which was antibody-independent. A second mechanism of cytotoxicity was induced by natural antibodies to a target cell, which probably mediated activation of the classical complement pathway. These data may shed light on the frequently observed cytotoxicity in mammalian sera for various target cells. 相似文献
986.
B J Smith 《Chemico-biological interactions》1977,16(3):275-280
The reaction of cytosine and 5-hydroxymethyl-cytosine (OHMeCyt) with a variety of monofunctional alkylating agents has been investigated to evaluate further the possible role of cytosine alkylation in mutagenesis and the possibility that the immunity of T-even phages to mutation by methyl methanesulphonate (MMS) was due to the unreactivity of OHMeCyt towards this agent. Both cytosine and OHMeCyt reacted equally well with the methylating agents MMS and N-methyl-N-nitrosourea (MNU) affording 6% and less than 1% respectively of the 3-substituted derivative. No product was isolated following subjection of the bases to reaction with ethyl methane-sulphonate (EMS), N-ethyl-N-nitrosourea (ENU) or iso-propyl methane-sulphonate (iPMS). 相似文献
987.
Ram S. Verma Carole Rubenstein Harvey Dosik 《In vitro cellular & developmental biology. Plant》1977,13(11):806-807
Summary There was no significant difference in the mitotic indices of the cultures maintained at different CO2 concentrations, i.e. 0%, 5% and 10%. However, considerable variation was recorded among different individuals.
Supported by National Cancer Institute Contract No. 1 CP 43251. 相似文献
988.
Use of lambda phasmids for deletion mapping of non-selectable markers cloned in plasmids 总被引:5,自引:0,他引:5
A nonselectable gene carried on a poorly selectable recombinant plasmid has been physically mapped by deletion analysis. Our method involved cloning the plasmid into a coliphage lambda vector and treating the recombinant phage with a chelator. Virtually all particles surviving this treatment carried large deletions within the plasmid insert. Further deletion analysis was done by inserting a selectable lambda sequence into one such deletion derivative and repeating the chelator selection. Chelator selection was also used to isolate deletions constructed in vitro. The deleted phage are readily characterized by restriction mapping, and the gene in question scored after infection of a mutant host strain. These techniques have enabled us to physically assign the cyclopropane fatty acid synthase gene of Escherichia coli to 0.8 kb of a 16-kb segment after characterizing only a small number of isolates. This approach should be generally useful in the mapping of plasmids for which no convenient method exists for selecting or scoring the gene in question. 相似文献
989.
Aphid transmission and a polypeptide are specified by a defined region of the cauliflower mosaic virus genome 总被引:15,自引:0,他引:15
Infection of young turnip leaves with an aphid-transmissible isolate, Cabb B-JI, of cauliflower mosaic virus (CaMV) causes synthesis of an Mr 18 000 polypeptide (p18) which co-purifies with virus inclusion bodies. This polypeptide is not detectable in leaves infected with either of two aphid non-transmissible isolates. Campbell and CM4-184. Construction in vitro, of hybrid genomes between Cabb B-JI and Campbell isolates demonstrates that aphid transmissibility and presence of p18 is dependent on the small genome fragment from the BstEII site to the XhoI site. A deletion made in this fragment within open reading frame (ORF) II causes loss of aphid transmissibility and also terminates production of p18. We conclude that aphid transmissibility and the presence of p18 are related to the expression of ORF II of the CaMV genome. 相似文献
990.
Molecular cloning of bovine leukemia virus DNA integrated into the bovine tumor cell genome 总被引:3,自引:0,他引:3
The bovine leukemia virus (BLV) DNA harbored in the bovine tumor cell genome was cloned in lambda Charon 4A phage. Using either representative or 3' half-enriched BLV cDNA as a blot hybridization probe, clone lambda BLV-1 was shown to carry 9 kb of the BLV genome, flanked by cellular sequences at both ends. Restriction mapping with twelve endonucleases and hybridization of the DNA fragments to BLV cDNA representing a 3'-end portion of the viral genome revealed the presence and precise location of two long terminal repeats (LTRs) and virus-cell junctions. Thus, lambda BLV-1 appears to contain the complete BLV genome and flanking tumor cellular sequences. The restriction map of the cloned BLV proviral DNA closely resembles that previously reported for unintegrated linear proviral DNA, but differs significantly from that of the integrated provirus of another BLV isolate, the difference occurring preferentially in the putative gag and pol genes. 相似文献