全文获取类型
收费全文 | 29394篇 |
免费 | 2546篇 |
国内免费 | 1582篇 |
出版年
2024年 | 33篇 |
2023年 | 359篇 |
2022年 | 754篇 |
2021年 | 1248篇 |
2020年 | 827篇 |
2019年 | 1065篇 |
2018年 | 1110篇 |
2017年 | 892篇 |
2016年 | 1244篇 |
2015年 | 1828篇 |
2014年 | 2006篇 |
2013年 | 2321篇 |
2012年 | 2764篇 |
2011年 | 2513篇 |
2010年 | 1571篇 |
2009年 | 1439篇 |
2008年 | 1621篇 |
2007年 | 1472篇 |
2006年 | 1380篇 |
2005年 | 1151篇 |
2004年 | 1038篇 |
2003年 | 860篇 |
2002年 | 785篇 |
2001年 | 385篇 |
2000年 | 277篇 |
1999年 | 316篇 |
1998年 | 271篇 |
1997年 | 262篇 |
1996年 | 204篇 |
1995年 | 178篇 |
1994年 | 169篇 |
1993年 | 139篇 |
1992年 | 140篇 |
1991年 | 119篇 |
1990年 | 100篇 |
1989年 | 86篇 |
1988年 | 74篇 |
1987年 | 61篇 |
1986年 | 45篇 |
1985年 | 76篇 |
1984年 | 57篇 |
1983年 | 35篇 |
1982年 | 30篇 |
1981年 | 35篇 |
1980年 | 29篇 |
1979年 | 20篇 |
1978年 | 17篇 |
1977年 | 20篇 |
1976年 | 18篇 |
1973年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
测定了3T3细胞、人和大鼠一些组织中DNA拓扑异构酶Ⅰ的活性;估计了核酸内切酶对拓扑酶Ⅰ松弛活性测定的干扰程度;发现增殖组织全细胞抽提液中酶比活高于正常分化组织,而且在异常增殖组织中酶比活的增高更为显著。 相似文献
62.
63.
Enzymatic activity of poliovirus RNA polymerase mutants with single amino acid changes in the conserved YGDD amino acid motif. 总被引:8,自引:4,他引:4
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
RNA-dependent RNA polymerases contain a highly conserved region of amino acids with a core segment composed of the amino acids YGDD which have been hypothesized to be at or near the catalytic active site of the molecule. Six mutations in this conserved YGDD region of the poliovirus RNA-dependent RNA polymerase were made by using oligonucleotide site-directed DNA mutagenesis of the poliovirus cDNA to substitute A, C, M, P, S, or V for the amino acid G. The mutant polymerase genes were expressed in Escherichia coli, and the purified RNA polymerases were tested for in vitro enzyme activity. Two of the mutant RNA polymerases (those in which the glycine residue was replaced with alanine or serine) exhibited in vitro enzymatic activity ranging from 5 to 20% of wild-type activity, while the remaining mutant RNA polymerases were inactive. Alterations in the in vitro reaction conditions by modification of temperature, metal ion concentration, or pH resulted in no significant differences in the activities of the mutant RNA polymerases relative to that of the wild-type enzyme. An antipeptide antibody directed against the wild-type core amino acid segment containing the YGDD region of the poliovirus polymerase reacted with the wild-type recombinant RNA polymerase and to a limited extent with the two enzymatically active mutant polymerases; the antipeptide antibody did not react with the mutant RNA polymerases which did not have in vitro enzyme activity. These results are discussed in the context of secondary-structure predictions for the core segment containing the conserved YGDD amino acids in the poliovirus RNA polymerase. 相似文献
64.
Developing pods of pea ( Pisum sativum L. cv. Alaska no 7) were used to study the enzymes of sucrose metabolism. Acid and neutral invertase (EC 3.2.1.26). sucrose synthase (SS, EC 2.4.1.13) and sucrose phosphate synthase (SPS, EC 2.4.1.14) have been localized in the soluble fraction. Acid invertase activity was also present in the insoluble fraction and in pea ovary apoplast. In pea pods, sucrose breakdown was dominated by the invertase pathway. SS specific activity only increased at late stages of parthenocarpic pod development, while SPS did so in pods obtained by pollination. Changes in time course of invertase activities have been correlated with the growth rate of fruits induced to develop either by fertilization or by exogenous application of giberellic acid (GA), 2,4-dichloro-phenoxy acetic acid (2,4-D) or 6-benzylaminopurine (6-BAP). The soluble neutral activities might be associated with pod elongation, while the acid ones were rather related to assimilate import by the induced fruits. Application of gibberellic acid to non-pollinated ovaries significantly enhanced the soluble neutral invertase activity before any ovary outgrowth was detected (up to 2 h after treatment). Within the same period of time. GA-treated ovaries showed a decrease in the acid invertase activity of the soluble fraction and an increase of the acid invertase activity in the apopiast. preceding in time the increment of the acid invertase activity associated with the insoluble fraction. Our results suggest that the early GA response may be mediated through a promotion of processes of protein secretion. 相似文献
65.
Ramon Latorre Juan Bacigalupo Ricardo Delgado Pedro Labarca 《Journal of bioenergetics and biomembranes》1991,23(4):577-597
Four different nucleotide-gated ion channels are discussed in terms of their biophysical properties and their importance in cell physiology. Channels activated directly by cGMP are present in vertebrate and invertebrate photoreceptors. In both cases cGMP increases the fraction of time the channel remains in the open state. At least three cGMP molecules are involved in channel opening in vertebrate photoreceptors and the concentration of the cyclic nucleotide to obtain the half maximal effect is about 15 µM. The light-dependent channel of both vertebrates and invertebrates is poorly cation selective. The vertebrate channel allows divalent cations to pass through 10–15-fold more easily than monovalent ions. In agreement with their preference for divalent cations, this channel is blocked byl-cis Dialtazem, a molecule that blocks certain types of calcium channels. In olfactory neurons a channel activated by both cAMP and cGMP is found and, as in the light-dependent channel, several molecules of the nucleotide are needed to open the channel with a half maximal effect obtained in the range of 1–40 µM. The channel is poorly cationic selective. A K+ channel directly and specifically activated by cAMP is found inDrosophila larval muscle. At least three cAMP molecules are involved in the opening reaction. Half-maximal effect is obtained at about 50 µM. This channel is blocked by micromolar amount of tetraethylammonium applied internally. Interestingly, this channel has a probability of opening 10–20-fold larger in the mutantdunce, a mutant that possesses abnormally elevated intracellular cAMP level, than in the wild type. 相似文献
66.
Nuclear genome characterization of the carrageenophyteAgardhiella subulata (Rhodophyta) 总被引:2,自引:0,他引:2
Donald F. Kapraun Julie A. Dutcher Juan Lopez-Bautista 《Journal of applied phycology》1992,4(2):129-137
DNA reassociation kinetics were used to determine nuclear genome organization and complexity inAgardhiella subulata (Gigartinales, Rhodophyta). Results indicate the presence of three second-order components corresponding to fast (22%), intermediate
(68%) and slow (10%) fractions. Thus, the genome consists of 90% repetitive sequences. Microspectrophotoometry with the DNA-localizing
fluorochrome DAPI was used to confirm ploidy level differences in the gametophytic and tetrasporophytic phases. Results indicate
that meiosis occurs during tetrasporogenesis. Comparison of mean nuclear DNA (If) values to chicken erythrocytes (RBC) resulted in an estimate of 0.9 pg/2C genome forAgardhiella. Karyological studies using aceto-orcein revealed a chromosome complement of 2N = 44 in carposporangia and the presence of 22 bivalents during diakinesis of tetraspore mother cells. 相似文献
67.
68.
69.
大斑啄木鸟取食行为的研究 总被引:4,自引:0,他引:4
大斑啄木鸟(Dendrocopos major)是左家地区的常见鸟类,对它的研究,国内有过一些报道。1990年4—5月,笔者对它的取食行为做了较为系统的观察,现整理报道如下。 相似文献
70.