全文获取类型
收费全文 | 9678篇 |
免费 | 818篇 |
国内免费 | 2篇 |
专业分类
10498篇 |
出版年
2022年 | 35篇 |
2021年 | 82篇 |
2020年 | 58篇 |
2019年 | 70篇 |
2018年 | 152篇 |
2017年 | 186篇 |
2016年 | 279篇 |
2015年 | 421篇 |
2014年 | 458篇 |
2013年 | 557篇 |
2012年 | 747篇 |
2011年 | 750篇 |
2010年 | 447篇 |
2009年 | 384篇 |
2008年 | 568篇 |
2007年 | 570篇 |
2006年 | 532篇 |
2005年 | 539篇 |
2004年 | 465篇 |
2003年 | 431篇 |
2002年 | 406篇 |
2001年 | 175篇 |
2000年 | 185篇 |
1999年 | 155篇 |
1998年 | 67篇 |
1997年 | 55篇 |
1996年 | 53篇 |
1995年 | 47篇 |
1994年 | 39篇 |
1993年 | 39篇 |
1992年 | 73篇 |
1991年 | 82篇 |
1990年 | 50篇 |
1989年 | 84篇 |
1988年 | 69篇 |
1987年 | 62篇 |
1986年 | 54篇 |
1985年 | 60篇 |
1984年 | 52篇 |
1983年 | 57篇 |
1982年 | 47篇 |
1981年 | 45篇 |
1979年 | 42篇 |
1978年 | 37篇 |
1977年 | 37篇 |
1976年 | 46篇 |
1975年 | 41篇 |
1974年 | 57篇 |
1973年 | 41篇 |
1971年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
51.
The murine homologue of the T lymphocyte antigen CD28. Molecular cloning and cell surface expression 总被引:15,自引:0,他引:15
J A Gross T St John J P Allison 《Journal of immunology (Baltimore, Md. : 1950)》1990,144(8):3201-3210
The human T lymphocyte Ag CD28 (Tp44) is a homodimeric glycoprotein expressed on the surface of a majority of human peripheral T cells and thymocytes. Although exposure of T cells to anti-CD28 mAb does not activate T cells, stimulation of CD28 can synergize with signals transmitted through the TCR or other stimuli to augment proliferation and lymphokine production. We have used a portion of the human CD28 cDNA to isolate a homologous murine cDNA from an EL4 T lymphoma library. The murine clone has 61% nucleotide identity with the human cDNA. Both human and murine sequences exhibit homology with members of the Ig supergene family and CTLA-4, a T cell specific murine gene. Many characteristics of the human CD28 molecule are conserved within the putative murine CD28 polypeptide. The murine cDNA sequence encodes a polypeptide of 218 amino acids that has 68% identity with the human sequence. Both the murine and human molecules are integral membrane glycoproteins with hydrophobic signal peptide sequences and transmembrane region. All five potential N-linked glycosylation sites are conserved and six of the seven cysteine residues of the mouse protein are found in the human CD28 polypeptide. The murine cDNA is encoded by a single copy nonrearranging gene whose expression at the mRNA level is restricted to T cells. A rabbit antiserum was raised against a synthetic peptide corresponding to a hydrophilic portion of the translated murine cDNA sequence. This antiserum identifies an 80-kDa homodimer consisting of disulfide-bonded subunits of 40 kDa that is expressed on splenic T cells, thymocytes, and several T cell tumors, but not on B cells. deglycosylation studies indicate that four of the five N-linked glycosylation sites are used and that the mature core protein has a molecular mass of 25 kDa, close to that predicted by the cDNA sequence. Transfection of the murine cDNA into Chinese hamster ovary cells resulted in the expression of an 80-kDa dimeric molecule that was immunoprecipitated by the antipeptide antiserum. Taken together, these data provide strong support that we have identified the murine homologue of CD28. 相似文献
52.
Stéphan G. Reebs N. Mrosovsky 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1989,165(6):819-825
Summary Bouts of induced wheel-running, 3 h long, accelerate the rate of re-entrainment of hamsters' activity rhythms to light-dark (LD) cycles that have been phase-advanced by 8 h (Mrosovsky and Salmon 1987). The bouts of running are given early in the first night of the new LD cycle, and by the second night the phase advance in activity onset already averages 7 h. Such large shifts contrast with the mean phase advance of <1 h at the peak of the phase response curve when hamsters in constant darkness (DD) experience 2-h pulses of induced activity (Reebs and Mrosovsky 1989). The present paper investigates pulse duration and light as possible causes for the discrepancy in shift amplitude between these two studies. In a first experiment, pulses of induced wheel-running 1 h, 3 h, or 5 h long were given at circadian times (CT) 6 and 22-2 to hamsters free-running in DD. Pulses given at CT 6 caused phase-advances of up to 2.8 h, whereas pulses at CT 22-2 resulted in delays of up to 1.0 h. Shifts after 3-h and 5-h pulses did not differ, but were larger than after 1-h pulses, and larger than after the 2-h pulses given in DD by Reebs and Mrosovsky (1989). Thus 3 h appears to be the minimum pulse duration necessary to obtain maximum phase-shifting effects. In a second experiment, the re-entrainment design of Mrosovsky and Salmon (1987) was repeated with the light portion of the shifted LD cycle eliminated. Hamsters exercised for 3 h phase-advanced 2.9 h on average (excluding 2 animals who ran poorly). When the same hamsters were exposed 7 days later to a 14-h light pulse starting 5 h after their activity onset, they advanced by an average of 3.3 h. Adding the average values for activity-induced shifts and light-induced shifts gives a total of about 6 h. Possible synergism between the effects of induced activity and those of light may account for the remaining small difference between this total and the 7-h advances previously reported.Abbreviations
CT
circadian time
-
DD
constant darkness
-
LD
light-dark
-
PRC
phase response curve
-
free-running period of rhythm 相似文献
53.
The acute dose-dependent effects of epinephrine and cocaine on heart rate and coronary flow rate (CFR) were examined in isolated, perfused (Langendorff) rat hearts from animals: i) pretreated with daily cocaine injections (20 mg/kg/day) for 8 weeks; ii) after 2-day withdrawal from 8-week cocaine pretreatment; iii) vehicle-treated controls. Chronic cocaine (CC) hearts were significantly less sensitive to the chronotropic effects of epinephrine than control (C) or withdrawal (CW) hearts. CW hearts exhibited significantly higher heart rates in response to epinephrine than C and CC hearts. Epinephrine alone (2.5 x 10(-7) M) decreased CFR 11% (C), 9%(CC), 14%(CW) from respective baseline levels. Cocaine alone had no significant effect on CFR in C hearts but produced slight dose-dependent decrements in CFR in CC and particularly CW hearts at higher doses. Cocaine plus epinephrine markedly decreased CFR in all groups, particularly in CW hearts. The results indicate that chronic daily cocaine administration produces a functional tolerance of the heart to the chronotropic actions of epinephrine but a 2-day withdrawal from chronic cocaine results in a rebound supersensitivity to adrenergic stimulation and cocaine's sympathomimetic effects. In addition, cocaine produces only minor decrements in coronary flow in the rat heart, while cocaine acts synergisticallly with epinephrine to produce a marked decrease in CFR. 相似文献
54.
Two nuclear oncogenic proteins, P135gag-myb-ets and p61/63myc, cooperate to induce transformation of chicken neuroretina cells 总被引:7,自引:4,他引:3 下载免费PDF全文
P Amouyel V Laudet P Martin R P Li B Quatannens D Stéhelin S Saule 《Journal of virology》1989,63(8):3382-3388
Several studies have shown that full transformation of primary rodent fibroblasts can be achieved in vitro through the cooperation of two oncogenes (usually one nuclear and one cytoplasmic) classified on the basis of different complementation groups. We have shown previously that cooperation between v-mil (cytoplasmic, serine-threonine kinase product), and v-myc (nuclear, DNA-binding product) is required to transform 7-day-old chicken neuroretina cells, which in usual culture medium do not rapidly proliferate. v-mil induces sustained growth of chicken neuroretina cells without transformation; v-myc fails to stimulate the proliferation of chicken neuroretina cells but is required to achieve transformation of the proliferating cells. Here, we present results indicating that the P135gag-myb-ets nuclear protein of avian erythroblastosis virus E26 is able to induce proliferation but not transformation of chicken neuroretina cells. v-myc is required in addition to P135gag-myb-ets to achieve chicken neuroretina cell transformation. In contrast, we found that the P135gag-myb-ets and P100gag-mil proteins are not able to cooperate in this system. 相似文献
55.
56.
Grant St. Julian Rodney J. Bothast Larry H. Krull 《Journal of industrial microbiology & biotechnology》1990,5(6):391-394
Summary By succesive recycling of the thin stillage in mashing and fermenting fresh corn, the glycerol content in each fermentation increased by about 0.4% and accumulated to a high of 2.1% in the beer of the fifth recycle. Glycerol concentration declined after the fifth recycle. The original fermentation contained 0.8% glycerol.Presented in part at the Society for Industrial Microbiology Annual Meeting, August 7–12, 1988, Chicago, IL.The mention of firm names or trade products does not imply that they are endorsed or recommended by the U.S. Department of Agriculture over other firms or similar products not mentioned. 相似文献
57.
Neurogenesis, control, and functional significance of gasping 总被引:6,自引:0,他引:6
W M St John 《Journal of applied physiology》1990,68(4):1305-1315
Gasps are frequently the first and last breaths of life. Gasping, which is generated by intrinsic medullary mechanisms, differs fundamentally from other automatic ventilatory patterns. A region of the lateral tegmental field of the medulla is critical for the neurogenesis of the gasp but has no role in eupnea. Neuronal mechanisms in separate brain stem regions may be responsible for the neurogenesis of different ventilatory patterns. This hypothesis is supported by the recording of independent respiratory rhythms simultaneously from isolated brain stem segments. Data from fetal and neonatal animals also support gasping and eupnea being generated by separate mechanisms. Gasping may represent the output of a simple but rugged pattern generator that functions as a backup system until the control system for eupnea is developed. Pacemaker elements are hypothesized as underlying the onset of inspiratory activity in gasping. Similar elements, in a different brain stem region, may be responsible for the onset of the eupneic inspiration with neural circuits involving the pons, the medulla, and the spinal cord serving to shape efferent respiratory-modulated neural discharges. 相似文献
58.
S. K. St. Martin K. S. Lewers R. G. Palmer B. R. Hedges 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1996,92(1):78-82
Methods for identifying germplasm carrying alleles with the potential to improve a particular single-cross hybrid have been proposed and discussed in recent years. There is a need for similar methods to be used in breeding crops for which pure-line cultivars, rather than hybrids, are the goal. The objective of this research was to develop a method to identify germplasm lines with the potential to contribute favorable alleles not present in a specified pure line or set of pure lines. Given a set of adapted pure lines (A
1, A
2 ..., A
m) to be improved and a set of germplasm lines (P
1
P
2 ..., P
f), the procedure consists of producing all f x m possible hybrids and evaluating them along with the parents. The testcross statistic T
ij is defined by T
ij=(F
ij–A
j)+(1–) (F
ij–P
i), where A
j, P
i, and F
ij represent the performance of thej
th adapted line, the i
th germplasm line, and their hybrid, respectively. The statistic
is the mean value of T
ij over all adapted parents A
j. If =(1/2)(1+d), where d = the mean degree of dominance, then T
ij measures the potential for alleles from P
i to improve A
j and
measures the potential for alleles from P
i to improve the set A
1, A
2 ..., A
m. Use of data on soybean and peanut hybrids published by other researchers suggests that the value assumed for d has little effect on the P
i chosen. The ability of the T
ij and
statistics to identify germplasm strains carrying rare favorable alleles should be assessed in empirical studies.Joint contribution: OARDC (Journal Articale No. 161-94), USDAARS, Iowa Agriculture and Home Economics Expriment Station (Journal Paper No. J-16109; Project 2985), and Agreculture and Agri-Food Canada. Salaries and research support for S. K. St. Martin Provided by state and federal funds appropriated to the Ohio Agricultural Research and Development Center, Ohio State University 相似文献
59.
T. Cassol D. A. St. Clair 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1994,88(5):581-588
Inheritance of resistance to blackmold, a disease of ripe tomato fruit caused byAlternaria alternata, was studied in two interspecific crosses. The parents, F1 and F2 generations of a cross between the susceptibleLycopersicon esculentum Mill. cultivar Hunt 100 and the resistantL. Cheesmanii f.typicum Riley accession LA 422, and the parents, F1, F2, F3, and BC1 P2 generations of a cross between the susceptibleL. Esculentum cv. VF 145B-7879 and LA 422 were evaluated. The following disease evaluation traits were used: symptom rating (a symptom severity rating based on visual evaluation of lesions), diseased fruit (the number of diseased fruits divided by the total number of fruit scored), and lesion size (a function derived from the actual lesion diameter). Generation means analysis was used to determine gene action. The data of the Hunt 100 × LA 422 cross fit an additive-dominance model for all three traits. The VF 145B-7879 × LA 422 cross data best fit a model that included the additive × additive and additive × dominance interaction components for the trait diseased fruit, whereas higher-order epistatic models would have to be invoked to fit the data for the traits symptom rating and lesion size. A minimum of one gene segregated for all three traits. Broad-sense heritability estimates ranged from 0.09 to 0.16 for all three traits, indicating that selection for improved resistance to blackmold will require selection on a family performance basis. 相似文献
60.
In a comprehensive study batch and continuous production of citric acid has been investigated. Fermentations in the reciprocation-jet-bioreactor (RJBR) have been carried out with the fungi Aspergillus niger.In the present paper only the results of continuous fermentations are presented. The paper discusses the influence of medium composition in the influent, input of biomass and frequency of reciprocating motion on citric acid production. 相似文献