全文获取类型
收费全文 | 1495篇 |
免费 | 185篇 |
出版年
2021年 | 16篇 |
2019年 | 12篇 |
2018年 | 17篇 |
2017年 | 14篇 |
2016年 | 22篇 |
2015年 | 42篇 |
2014年 | 43篇 |
2013年 | 66篇 |
2012年 | 89篇 |
2011年 | 72篇 |
2010年 | 55篇 |
2009年 | 52篇 |
2008年 | 53篇 |
2007年 | 55篇 |
2006年 | 54篇 |
2005年 | 52篇 |
2004年 | 52篇 |
2003年 | 60篇 |
2002年 | 49篇 |
2001年 | 37篇 |
2000年 | 37篇 |
1999年 | 32篇 |
1998年 | 27篇 |
1997年 | 21篇 |
1996年 | 24篇 |
1995年 | 24篇 |
1994年 | 21篇 |
1993年 | 22篇 |
1992年 | 32篇 |
1991年 | 37篇 |
1990年 | 40篇 |
1989年 | 33篇 |
1988年 | 25篇 |
1987年 | 21篇 |
1986年 | 30篇 |
1985年 | 33篇 |
1984年 | 23篇 |
1982年 | 19篇 |
1981年 | 20篇 |
1980年 | 16篇 |
1979年 | 22篇 |
1978年 | 14篇 |
1977年 | 14篇 |
1976年 | 13篇 |
1975年 | 16篇 |
1974年 | 14篇 |
1973年 | 11篇 |
1972年 | 12篇 |
1969年 | 11篇 |
1968年 | 14篇 |
排序方式: 共有1680条查询结果,搜索用时 375 毫秒
51.
Nefiracetam is a novel pyrrolidone derivative which attenuates scopolamine-induced learning and post-training consolidation
deficits. Given that apomorphine inhibits passive avoidance retention when given during training or in a defined 10–12h post-training
period, we evaluated the ability of nefiracetam to attenuate amnesia induced by dopaminergic agonism. A step-down passive
avoidance paradigm was employed and nefiracetam (3 mg/kg) and apomorphine (0.5 mg/kg) were given alone or in combination during
training and at the 10–12h post-training period of consolidation. Co-administration of nefiracetam and apomorphine during
training or 10h thereafter produced no significant anti-amnesic effect. However, administration of nefiracetam during training
completely reversed the amnesia induced by apomorphine at the 10h post-training time and the converse was also true. These
effects were not mediated by a dopaminergic mechanism as nefiracetam, at millimolar concentrations, failed to displace either
[3H]SCH 23390 or [3H]spiperone binding from D1 or D2 dopamine receptor subtypes, respectively. It is suggested that nefiracetam augments molecular processes in the early stages
of events which ultimately lead to consolidation of memory. 相似文献
52.
Anonymous nuclear DNA markers in the American oyster and their implications for the heterozygote deficiency phenomenon in marine bivalves 总被引:4,自引:0,他引:4
A puzzling population-genetic phenomenon widely reported in allozyme
surveys of marine bivalves is the occurrence of heterozygote deficits
relative to Hardy-Weinberg expectations. Possible explanations for this
pattern are categorized with respect to whether the effects should be
confined to protein-level assays or are genomically pervasive and expected
to be registered in both protein- and DNA-level assays. Anonymous nuclear
DNA markers from the American oyster were employed to reexamine the
phenomenon. In assays based on the polymerase chain reaction (PCR), two
DNA-level processes were encountered that can lead to artifactual genotypic
scorings: (a) differential amplification of alleles at a target locus and
(b) amplification from multiple paralogous loci. We describe symptoms of
these complications and prescribe methods that should generally help to
ameliorate them. When artifactual scorings at two anonymous DNA loci in the
American oyster were corrected, Hardy-Weinberg deviations registered in
preliminary population assays decreased to nonsignificant values.
Implications of these findings for the heterozygote-deficit phenomenon in
marine bivalves, and for the general development and use of PCR-based
assays, are discussed.
相似文献
53.
Control of flower development and phyllotaxy by meristem identity genes in antirrhinum. 总被引:9,自引:0,他引:9
下载免费PDF全文
![点击此处可从《The Plant cell》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The flower meristem identity genes floricaula (flo) and squamosa (squa) promote a change in phyllotaxy from spiral to whorled in Antirrhinum. To determine how this might be achieved, we have performed a combination of morphological, genetic, and expression analyses. Comparison of the phenotypes and RNA expression patterns of single and double mutants with the wild type showed that flo and squa act together to promote flower development but that flo is epistatic to squa with respect to early effects on phyllotaxy. We propose that a common process underlies the phyllotaxy of wildtype, flo, and squa meristem development but that the relative timing of primordium initiation or growth is altered. This process depends on two separable events: setting aside zones for potential primordium initiation and partitioning these zones into discrete primordia. Failure of the second event can lead to the formation of continuous double spirals, which are occasionally seen in flo mutants. 相似文献
54.
We have used the lacZ reversion assay to study the mutation spectra induced by the Escherichia coli chromosomal umuDC operon and of its two plasmid-borne analogues impCAB and mucAB following exposure of cells to UV light and methyl methane-sulfonate (MMS). We have shown that the impCAB, mucAB and umuDC operons all produce a similar response to UV light which results almost exclusively in AT → GC transitions. However, we found that the three operons produced different responses to alkylating agents. We found that with MMS the chromosomal umuDC operon produced almost exclusively AT → GC transitions, whilst both mucAB and impCAB produced predominantly transversions. In the case of the impCAB operon the mutation spectrum contained more AT → TA than GC → TA transversions; this balance was reversed with mucAB. The effect of the copy number of the error-prone DNA repair operons upon the mutagenic spectra was also studied. The results obtained suggest that the copy number of the imp operon does not greatly affect the specificity of base substitutions observed. However, an increase in the copy number of the umuDC operon greatly affected the specificity of base substitution, such that virtually no transitions were produced and the spectrum was dominated by GC/AT → TA transversions. It appears that the three error-prone DNA repair operons impCAB, mucAB and umuDC, despite showing strong structural and functional homologies, can display major differences in the spectrum of base changes induced during mutagenesis. We propose that the type of misincorporation/chain extension which DNA polymerase III is allowed to synthesize on a damaged DNA template is extremely sensitive to both the amount and type of error-prone repair proteins present. The modulation of these events by the different proteins can result in widely different mutagenic changes in the repaired DNA. 相似文献
55.
Nitrogen fixation by periphyton and plankton on the Amazon floodplain at Lake Calado 总被引:2,自引:1,他引:1
Nitrogen fixation by periphyton and plankton was measured on the Amazon flood-plain using the acetylene reduction method calibrated with15N-N2. The average ratio (± SD) of moles C2H4 reduced per mole N2-N fixed was 3.4 ± 0.7, similar to other studies. Periphyton and plankton had high rates of light-dependent nitrogen fixation, with dark nitrogen fixation averaging 26% of the average rates in the light. The average daily (24 h) rates for periphyton nitrogen fixation in 1989 and 1990 were 1.79 and 0.51 mmol N2-N·m–2·d–1 respectively, which are comparable to summer rates in many temperate cyanobacterial assemblages. Nitrogen fixation was depressed at N03
– concentrations as low as 0.5 M, and was below detection limits at concentrations of 4 M, which occurred during periods of river flooding. Planktonic nitrogen fixation rates were high (0.5–0.8 mmol N2-N·m–2·d–1) during the high-water and drainage phases of the annual hydrograph when the floodplain waters were draining towards the river (low NO3
–), but rates were undetectable (< 0.05 mmol N2-N·m–2·d–1) when there was river flooding (high NO3
–). Nitrogen fixation by periphyton and plankton in 1989–1990 accounted for approximately 8% of previously reported total annual nitrogen inputs to the floodplain at Lake Calado. 相似文献
56.
57.
Marta Clariano Vanda Marques João Vaz Salma Awam Marta B. Afonso Maria Jesus Perry Cecília MP Rodrigues 《化学与生物多样性》2023,20(3):e202300222
Curcumin has a plethora of biological properties, making this compound potentially effective in the treatment of several diseases, including cancer. However, curcumin clinical use is compromised by its poor pharmacokinetics, being crucial to find novel analogs with better pharmacokinetic and pharmacological properties. Here, we aimed to evaluate the stability, bioavailability and pharmacokinetic profiles of monocarbonyl analogs of curcumin. A small library of monocarbonyl analogs of curcumin 1a–q was synthesized. Lipophilicity and stability in physiological conditions were both assessed by HPLC-UV, while two different methods assessed the electrophilic character of each compound monitored by NMR and by UV-spectroscopy. The potential therapeutic effect of the analogs 1a–q was evaluated in human colon carcinoma cells and toxicity in immortalized hepatocytes. Our results showed that the curcumin analog 1e is a promising agent against colorectal cancer, with improved stability and efficacy/safety profile. 相似文献
58.
59.
The glucan-binding lectin (GBL) ofStreptococcus sobrinus is cell associated, enabling the bacteria to be aggregated by -1,6 glucans. Glucans, such as amylose, pullulan, laminarin and nigeran, have no affinity for the lectin. High molecular weight -1,6 glucans (dextrans) readily aggregate the bacteria, whereas low molecular weight glucans inhibit the aggregation brought about by the high molecular weight species. Methylated glucan T-2000 (an -1,6 glucan with an average molecular weight of 2 × 106 Da) aggregated the bacteria very poorly when the extent of methylation (DS, or degree of substitution) was high, and less poorly when the DS was low. Similarly, methylated low molecular weight -1,6 glucan was a poor inhibitor of aggregation induced by the high molecular weight glucan T-2000. Because the methylation occurred primarily on the hydroxyl of C-2, it is suggested that the hydroxyl is needed for formation of the lectin-glucan complex. It appears that the GBL is not only stereospecific in interaction with glucans, but also regiospecific, interacting only with the underivatized -1,6-glucan. 相似文献
60.