全文获取类型
收费全文 | 11939篇 |
免费 | 683篇 |
国内免费 | 35篇 |
出版年
2024年 | 18篇 |
2023年 | 113篇 |
2022年 | 359篇 |
2021年 | 605篇 |
2020年 | 359篇 |
2019年 | 460篇 |
2018年 | 507篇 |
2017年 | 377篇 |
2016年 | 533篇 |
2015年 | 628篇 |
2014年 | 745篇 |
2013年 | 926篇 |
2012年 | 989篇 |
2011年 | 886篇 |
2010年 | 523篇 |
2009年 | 424篇 |
2008年 | 544篇 |
2007年 | 510篇 |
2006年 | 467篇 |
2005年 | 454篇 |
2004年 | 350篇 |
2003年 | 295篇 |
2002年 | 260篇 |
2001年 | 124篇 |
2000年 | 114篇 |
1999年 | 95篇 |
1998年 | 67篇 |
1997年 | 42篇 |
1996年 | 42篇 |
1995年 | 48篇 |
1994年 | 32篇 |
1993年 | 37篇 |
1992年 | 58篇 |
1991年 | 51篇 |
1990年 | 60篇 |
1989年 | 46篇 |
1988年 | 47篇 |
1987年 | 36篇 |
1986年 | 35篇 |
1985年 | 40篇 |
1984年 | 35篇 |
1983年 | 31篇 |
1982年 | 26篇 |
1981年 | 30篇 |
1980年 | 19篇 |
1979年 | 17篇 |
1978年 | 22篇 |
1977年 | 20篇 |
1976年 | 24篇 |
1975年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
Definition of essential sequences and functional equivalence of elements downstream of the adenovirus E2A and the early simian virus 40 polyadenylation sites. 总被引:43,自引:21,他引:22
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)
In addition to the highly conserved AATAAA sequence, there is a requirement for specific sequences downstream of polyadenylic acid [poly(A)] cleavage sites to generate correct mRNA 3' termini. Previous experiments demonstrated that 35 nucleotides downstream of the E2A poly(A) site were sufficient but 20 nucleotides were not. The construction and assay of bidirectional deletion mutants in the adenovirus E2A poly(A) site indicates that there may be redundant multiple sequence elements that affect poly(A) site usage. Sequences between the poly(A) site and 31 nucleotides downstream were not essential for efficient cleavage. Further deletion downstream (3' to +31) abolished efficient cleavage in certain constructions but not all. Between +20 and +38 the sequence T(A/G)TTTTT was duplicated. Function was retained when one copy of the sequence was present, suggesting that this sequence represents an essential element. There may also be additional sequences distal to +43 that can function. To establish common features of poly(A) sites, we also analyzed the early simian virus 40 (SV40) poly(A) site for essential sequences. An SV40 poly(A) site deletion that retained 18 nucleotides downstream of the cleavage site was fully functional while one that retained 5 nucleotides downstream was not, thus defining sequences required for cleavage. Comparison of the SV40 sequences with those from E2A did not reveal significant homologies. Nevertheless, normal cleavage and polyadenylation could be restored at the early SV40 poly(A) site by the addition of downstream sequences from the adenovirus E2A poly(A) site to the SV40 +5 mutant. The same sequences that were required in the E2A site for efficient cleavage also restored activity to the SV40 poly(A) site. 相似文献
52.
A I Mustafa B H Ali A M Satti 《Comp. Biochem. Physiol. C, Comp. Pharmacol. Toxicol.》1985,81(1):167-169
Furazolidone (FZ) was given to Nubian goats at the recommended therapeutic dose of 10 mg/kg body wt, for 5 days. The animals were slaughtered 24 hr after the last dose, and their livers, gluteal muscles, kidneys and hearts analysed for FZ residues by an HPLC method with a detection limit of 0.05 micrograms g-1. The drug was detected in the muscle and liver at concentrations of 0.256 +/- 0.009, 0.101 +/- 0.016 micrograms g-1 tissue, respectively. No detectable concentrations of the drug were found in the kidney and heart. 相似文献
53.
Immunoaffinity purified Sm/RNP antigens from buffalo and goat liver were studied to determine the role of RNA and proteins
towards the antigenicity of Sm and RNP antigens. A more direct approach using enzyme-linked immunosorbent assay on nylon beads
has been utilized to look into the problem. The effect of enzyme treatment and the role of RNA and protein fractions in influencing
antigenicity have been described. RNA seems to be involved in the maintenance of RNP specific polypeptides in suitable conformation
so as to keep them in solution. Removal of RNA leads to insolubilization of RNP specific polypeptides. Antibodies to Sm and
RNP antigens have been shown to cross react with poly A containing heterogeneous nuclear ribonucleoprotein with no cross reactivity
with thymus RNA or DNA. 相似文献
54.
Summary Our ultrastructural study of the retinal pigmented epithelium (RPE) of young, pigmented, and mature silver eels, (Anguilla anguilla), revealed the presence of retinomotor responses in the RPE of both growth stages. Furthermore, while the overall fine structure of the RPE is similar to that of other vertebrates, specific structures were also found. These include various forms of lipid droplets, intercellular membranous whorls, multitubular bodies, macrophages, melanolysosomes, and myeloid bodies (large and numerous in young eels). Structural variations in cellular organelles of the RPE according to different adaptive states (light, dark) and growth stages (young, mature) of the animal are discussed with respect to their significance in its complex life cycle. 相似文献
55.
A heterogeneous particulate fraction of mouse brain homogenates binds NRDC 157 (3-phenoxybenzyl [1,]-3-(2,2-dibromovinyl)-2,2- dimethylcyclopropanecarboxylate), a potent pyrethroid insecticide, stereospecifically and with high affinity. Stereospecific binding is a minor component of total binding (2.8%); the remainder of observed binding is predominantly nonspecific and unsaturable. Stereospecific binding is half-saturated at 4×10?8 and fully saturated at concentrations in excess of 1×10?7. The stereospecific binding capacity of this preparation was 200–250 pmoles of NRDC 157 per gram equivalent of brain tissue (2.3–2.8 pmol/mg protein). This binding site may represent the neural receptor involved in the stereospecific toxic action of pyrethroids. 相似文献
56.
Shahid Ali A. Sitaramayya K. Sree Kumar Padmanabhan S. Krishnan 《The Biochemical journal》1974,137(1):85-92
1. An inhibitor of cytoplasmic guanine deaminase of rat liver was isolated from liver ;heavy mitochondrial' fraction after freezing and thawing and treatment with Triton X-100. 2. Submitochondrial fractionation revealed that the inhibitor was localized in the outer-membrane fraction. 3. The method of purification of inhibitor, involving precipitation with (NH(4))(2)SO(4) and chromatography on DEAE-cellulose, its precipitability by trichloroacetic acid and the pattern of absorption in the u.v. indicated that the inhibitor was a protein. In confirmation, tryptic digestion of the isolated material resulted in destruction of the inhibitor activity. The inhibitor was stable to acid, but labile to heat. 4. The isolated inhibitor required phosphatidylcholine (lecithin) for activity. Phosphatidylcholine also partially protected the inhibitor against heat inactivation. 5. When detergent treatment was omitted, the inhibitor activity of frozen mitochondria was precipitated by (NH(4))(2)SO(4) in a fully active form without supplementation with phosphatidylcholine, indicating that Triton X-100 ruptured the linkage between inhibitor and lipid. 6. A reconstituted sample of inhibitor-phosphatidylcholine complex was precipitated in a fully active form by dialysis against 2-mercaptoethanol, but treatment of the precipitate with NaCl yielded an extract which was inactive unless supplemented with fresh phosphatidylcholine. 7. We interpret the results as evidence that the inhibitor was present in vivo as a lipoprotein and that once the complex was dissociated by the action of detergent and the protein precipitated, there was an absolute need for exogenous phosphatidylcholine for its activity. The manner in which inhibitor associated with the outer membrane of rat liver mitochondria might regulate the activity of the enzyme in the supernatant has been suggested. 相似文献
57.
Ali H. Qizilbash 《CMAJ》1974,110(2):185-186
A case of early bilateral serous papillary adenocarcinoma of the ovaries in an asymptomatic 58-year-old woman was diagnosed by the discovery of psammoma bodies in routine cervicovaginal and endometrial smears. Multiple small foci of carcinoma in-situ involving both fallopian tubes were also present. The significance of psammoma bodies in the cervicovaginal smear is discussed and the literature on the subject briefly reviewed. 相似文献
58.
59.
The apex of a 3-leaf pea plant was chilled in cold chambers maintained at 5–7°C. The lateral shoots 1 through 5 grew, and shoot 5 eventually dominated other lateral shoots. The apex when returned to the ambient temperature did not reimpose apical dominance. The growing lateral shoots competed with the stem apex. The apices of 2- and 3-leaf plants were chilled and P-32 distribution in these plants was studied in the entire plant, at various intervals of time. Phosphorus-32 accumulation followed the growth pattern of the plant. The lateral shoots accumulated P-32 activity and very little activity was accumulated by the apex. The dominating shoots 2 and 5 accumulated the maximum amount of activity in 2- and 3-leaf plants, respectively. Labeled-IAA moved basipetally through the stem when applied to the cut stump simulating the apex. By cold treatment the translocation of IAA was influenced more than its absorption. The plant seems to metabolize this compound in the later periods of application. The plant now becomes “insensitive” to auxin and the lateral shoots grow. 相似文献
60.
The growth OfRhizoctonia solani in different carbohydrates was studied. The rate of growth of the fungus was traced by taking the dry weights of mycelia obtained from the carbohydrate medium at regular intervals and shifts in the pH were recorded. Different carbohydrate sources had different effects on the growth of the organism. The exoenzymes from the organism were capable of cleaving carbohydrates irrespective of whether the fungus grew in them or not. 相似文献