全文获取类型
收费全文 | 3565篇 |
免费 | 218篇 |
国内免费 | 4篇 |
专业分类
3787篇 |
出版年
2023年 | 14篇 |
2022年 | 16篇 |
2021年 | 46篇 |
2020年 | 29篇 |
2019年 | 38篇 |
2018年 | 42篇 |
2017年 | 42篇 |
2016年 | 76篇 |
2015年 | 101篇 |
2014年 | 147篇 |
2013年 | 184篇 |
2012年 | 215篇 |
2011年 | 224篇 |
2010年 | 248篇 |
2009年 | 228篇 |
2008年 | 229篇 |
2007年 | 246篇 |
2006年 | 215篇 |
2005年 | 204篇 |
2004年 | 213篇 |
2003年 | 219篇 |
2002年 | 199篇 |
2001年 | 45篇 |
2000年 | 39篇 |
1999年 | 43篇 |
1998年 | 48篇 |
1997年 | 39篇 |
1996年 | 35篇 |
1995年 | 29篇 |
1994年 | 26篇 |
1993年 | 28篇 |
1992年 | 24篇 |
1991年 | 25篇 |
1990年 | 11篇 |
1989年 | 22篇 |
1988年 | 25篇 |
1987年 | 15篇 |
1986年 | 14篇 |
1985年 | 16篇 |
1984年 | 18篇 |
1983年 | 10篇 |
1982年 | 10篇 |
1981年 | 10篇 |
1980年 | 13篇 |
1979年 | 7篇 |
1978年 | 10篇 |
1977年 | 5篇 |
1976年 | 12篇 |
1975年 | 6篇 |
1974年 | 7篇 |
排序方式: 共有3787条查询结果,搜索用时 0 毫秒
181.
We have investigated conformational transitions in the Klenow fragment polymerase reaction by stopped-flow fluorescence using DNA substrates containing the fluorescent reporter 2-aminopurine (2-AP) on the template strand, either at the templating position opposite the incoming nucleotide (designated the 0 position) or 5' to the templating base (the +1 position). By using both deoxy- and dideoxy-terminated primers, we were able to distinguish steps that accompany ternary complex formation from those that occur during nucleotide incorporation. The fluorescence changes revealed two extremely rapid steps that occur early in the pathway for correct nucleotide incorporation. The first, detectable with the 2-AP reporter at the 0 position, occurs within the first few milliseconds and is associated with dNTP binding. This is followed by a rapid step involving relative movement of the +1 base, detectable when the 2-AP reporter is at the +1 position. Finally, when the primer had a 3'-OH, a fluorescence decrease with a rate equal to the rate of nucleotide incorporation was observed with both 0 and +1 position reporters. When the primer was dideoxy-terminated, the only change observed at the rate expected for nucleotide incorporation had a very small amplitude, suggesting that the rate-limiting conformational change does not produce a large fluorescence change, and is therefore unlikely to involve a significant change in the environment of the fluorophore. Fluorescence changes observed during misincorporation were substantially different from those observed during correct nucleotide incorporation, implying that the conformations adopted during correct and incorrect nucleotide incorporation are distinct. 相似文献
182.
The use of in-situ near infrared spectroscopy (NIRS) as a tool for monitoring four key analytes in a CHO-K1 animal cell culture was investigated. Previous work using on-line NIRS to monitor bioprocesses has involved its application ex-situ where the analyzer is physically outside the fermentor, or to microbial bioprocesses. This novel application of NIRS to monitor analytes within an animal cell culture using a steam sterilizable in-situ fiber optic probe is very important for furthering the use of NIRS within the bioprocessing industry. The method of calibration used to develop the models involved the use of large data sets so that all likely variation in stoichiometry was incorporated within the models. Successful models for glucose, lactate, glutamine, and ammonia were built with Standard Error of Predictions (SEP's) of 0.072 (g/L), 0.0144 (g/L), 0.308 (mM), and 0.036 (mM), respectively of the total concentration range. 相似文献
183.
Colitis induced by proteinase-activated receptor-2 agonists is mediated by a neurogenic mechanism 总被引:6,自引:0,他引:6
Nguyen C Coelho AM Grady E Compton SJ Wallace JL Hollenberg MD Cenac N Garcia-Villar R Bueno L Steinhoff M Bunnett NW Vergnolle N 《Canadian journal of physiology and pharmacology》2003,81(9):920-927
Proteinase-activated receptor-2 (PAR2) activation induces colonic inflammation by an unknown mechanism. We hypothesized that PAR2 agonists administered intracolonically in mice induce inflammation via a neurogenic mechanism. Pretreatment of mice with neurokinin-1 and calcitonin-gene-related peptide (CGRP) receptor antagonists or with capsaicin showed attenuated PAR2-agonist-induced colitis. Immunohistochemistry demonstrated a differential expression of a marker for the type-1 CGRP receptor during the time course of PAR2-agonist-induced colitis, further suggesting a role for CGRP. We conclude that PAR2-agonist-induced intestinal inflammation involves the release of neuropeptides, which by acting on their receptors cause inflammation. These results implicate PAR2 as an important mediator of intestinal neurogenic inflammation. 相似文献
184.
185.
186.
The ability of membrane ultra- and diafiltration and two chromatography media, Matrex Cellufine Sulfate (Millipore) and Macro-Prep ceramic hydroxyapatite (Bio-Rad), to adsorb, elute, and purify gene therapy vectors based on Moloney murine leukaemia virus (MoMuLV) carrying the 4070A amphotropic envelope protein was studied. Membrane ultra- and diafiltration provided virus concentration up to 160-fold with an average recovery of infectious viruses of 77 +/- 14%. In batch experiments, Macro-Prep ceramic hydroxyapatite (type 2, particle size 40 microm) proved superior to Matrex Cellufine Sulfate for MoMuLV vector particle adsorption. Furthermore, functional vector particles could be eluted using phosphate buffer pH 6.8 (highest titres from >or=300 mM phosphate) from the Macro-Prep adsorbent, with higher specific titres (cfu/mg protein) than the starting material. Similar results were obtained when this ceramic hydroxyapatite was packed into a column and used in a liquid chromatography system. Recovery of transduction-competent virus was between 18 and 31% for column experiments and 32 and 46% for batch experiments. 相似文献
187.
The role of competition in task switching during colony emigration in the ant Leptothorax albipennis
The decision-making process that determines when an animal should switch between tasks is a fundamental issue in the study of animal behaviour. We investigated, for the first time, such task switching in terms of the dynamics of worker populations in ant colonies. During colony emigration in the ant Leptothorax albipennis, the colony has to carry out the following three tasks: (1) transport of brood and nestmates to the new nest; (2) sorting of the brood into its characteristic pattern; and (3) building the nest wall. At the beginning of the emigration, the stimuli for all three tasks increase simultaneously but the tasks are performed sequentially by populations of workers in the colony. The issue here is how decision making at the colony level is based on the behaviour of individual workers. We used a mathematical model to explore the hypothesis that such colony level task switching is based on tasks competing for workers. The essential feature of this model is that the sequence of tasks performed by an individual worker need not match the sequence of tasks on which the colony concentrates. We base the parameterization of our model on our detailed experimental study of eight emigrations, one for each of eight L. albipennis colonies. We compared our results with earlier work that emphasizes the role of response thresholds in task-related decisions. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved. 相似文献
188.
189.
190.