全文获取类型
收费全文 | 4237篇 |
免费 | 451篇 |
国内免费 | 2篇 |
专业分类
4690篇 |
出版年
2022年 | 31篇 |
2021年 | 58篇 |
2020年 | 46篇 |
2019年 | 42篇 |
2018年 | 58篇 |
2017年 | 53篇 |
2016年 | 108篇 |
2015年 | 168篇 |
2014年 | 162篇 |
2013年 | 178篇 |
2012年 | 230篇 |
2011年 | 203篇 |
2010年 | 164篇 |
2009年 | 113篇 |
2008年 | 187篇 |
2007年 | 169篇 |
2006年 | 180篇 |
2005年 | 171篇 |
2004年 | 172篇 |
2003年 | 159篇 |
2002年 | 146篇 |
2001年 | 94篇 |
2000年 | 93篇 |
1999年 | 75篇 |
1998年 | 48篇 |
1997年 | 49篇 |
1996年 | 49篇 |
1995年 | 36篇 |
1994年 | 44篇 |
1993年 | 40篇 |
1992年 | 71篇 |
1991年 | 83篇 |
1990年 | 69篇 |
1989年 | 66篇 |
1988年 | 54篇 |
1987年 | 58篇 |
1986年 | 44篇 |
1985年 | 43篇 |
1984年 | 46篇 |
1983年 | 47篇 |
1982年 | 47篇 |
1981年 | 38篇 |
1979年 | 55篇 |
1978年 | 51篇 |
1977年 | 51篇 |
1975年 | 39篇 |
1974年 | 48篇 |
1973年 | 41篇 |
1972年 | 44篇 |
1968年 | 30篇 |
排序方式: 共有4690条查询结果,搜索用时 0 毫秒
11.
Mouse glandular kallikrein genes. Structure and partial sequence analysis of the kallikrein gene locus 总被引:15,自引:0,他引:15
Mouse glandular kallikreins are encoded by a family of closely linked genes which are located on chromosome 7 at a site corresponding to the genetically defined Tam-1, Prt-4, and Prt-5 loci. We have characterized 24 kallikrein genes by genomic cloning and restriction mapping of 310 kilobase pairs of BALB/c mouse DNA. Most of these genes are highly homologous, have the same exon/intron organization, and are linked in clusters of up to 11 genes. Partial sequence analysis of the kallikrein genes has facilitated identification of those members of the family for which protein sequence data exist and assignment of those which are pseudogenes or encode proteins of unknown function. We find that a maximum of 14 mouse kallikrein genes have the potential to encode functional proteins. 相似文献
12.
13.
14.
15.
A. J. Richards P. N. Ward P. Narcisi A. C. Nicholls J. C. Lloyd F. M. Pope 《Human genetics》1992,89(4):414-418
Summary Ehlers-Danlos syndrome type IV, an inherited connective tissue disease, is usually caused by mutations in the gene for type III collagen. Here, we describe a glycine to glutamic acid substitution in a patient with this syndrome. Previous studies had shown that fibroblasts from the patient, his mother and brother secreted a reduced amount of type III collagen and also produced an overmodified form of the protein that was preferentially retained intracellularly. Peptide mapping experiments indicated that the mutation was located within cyanogen bromide peptide 9. This was supported by chemical cleavage analysis and sequencing of cDNA encoding this region. Allele-specific oligonucleotide hybridisation of genomic DNA confirmed that a G to A mutation converted Gly 847 to Glu. The mutation was present in two other affected family members and also in a third, who was clinically unaffected. Further analysis of this unaffected individual revealed reduced mutant:normal ratios in DNA obtained from both blood and hair samples, showing that she was mosaic for the mutation. 相似文献
16.
Information concerning the cellular localization of dopamine receptor subtypes in the nucleus accumbens (NAcc) was obtained using receptor autoradiographic analysis. Unilateral, stereotaxic injection of the axon-sparing neurotoxin, quinolinic acid, into the NAcc resulted in a prominent loss of dopamine D1 receptors (as labeled by [3H]SCH 23390). Contrarily, no appreciable decrement in D2 receptors (labeled by [3H]raclopride) could be identified within the same region of the NAcc. The findings support the view that accumbens D1 receptors are located postsynaptically on neurons or their processes, while D2 receptors within this nucleus are primarily located on afferent terminals. 相似文献
17.
We analyzed data from Section 404 permits issued in California from January 1971 through November 1987 that involved impacts to wetlands and required compensatory mitigation (wetland creation, restoration, or preservation). The purpose of this study was to determine patterns and trends in permitting activity and to document cumulative effects of associated management decisions on the California wetland resource. The 324 permits examined documented that 387 compensatory wetlands (1255.9 ha) were required as mitigation for impacts to 368 wetlands (1176.3 ha). The utility of the data on wetland area was limited, however, since 38.0% of the impacted wetlands and 41.6% of the compensatory wetlands lacked acreage data. The wetland type most frequently impacted (37.8% of impacted wetlands) and used in compensation (38.2% of compensatory wetlands) was palustrine forested wetlands. Estuarine intertidal emergent wetlands had the most area impacted (52.3%) and compensated (62.5%). The majority of the wetlands were small (less than or equal to 4.0 ha in size). Wildlife habitat was the most frequently listed function of impacted wetlands (90.7% of the permits) and objective of compensatory wetlands (83.3%). Endangered species were listed as affected in 20.4% of impacted and 21.0% of compensatory projects. The number of permits requiring compensatory mitigation and the number of impacted and compensatory wetlands increased from 1971 to 1986.Documentation of the details of Section 404 permit decisions was inadequate for the permits we examined. Area information and specific locations of impacted and compensatory wetlands were lacking or of poor quality. Follow-up information was also inadequate. For example, project completion dates were specified in the permit for only 2.2% of compensatory wetlands. Furthermore, less than one-third (31.5%) of the permits required the compensatory wetland to be monitored by at least one site visit. We recommend improved documentation, regular reporting, and increased monitoring for better evaluation of the Section 404 permitting system. 相似文献
18.
The growth rate, food, reproduction and mortality rates of the red bass, Lutjanus bohar , are described for fish associated with coral reefs in the Tigak Islands of Papua New Guinea. Spawning intensity by month was investigated by examination of gonad maturation stages. The length–at–maturity of female L. bohar was estimated from the relationship between changes in gonad index with increasing size. Female fish matured at about 45–0 cm fork length. The Petersen method was used to obtain age–at–length data from length frequency data of L. bohar captured by hook–and–line fishing. The growth of L. bohar was described by a von Bertalanffy growth curve with the parameters AT=0.27 and L∞ = 81.7 cm. There was a positive correlation between water depth (D) and the length of L. bohar (L) that was described by L = 318–8+ 3.050 D . The mortality rates ( Z ) of L. bohar from shallow (≤30m) and deep (≤80m) water were estimated from length converted catch curves and were 1–181 and 0–64 respectively. The high Z value for the shallow water population of L. bohar was ascribed to the migration of larger specimens into deeper water. The deep water population of L. bohar remains unfished and the estimate of Z was believed to be equivalent to the natural mortality rate ( M ). An empirical method of calculating M gave an estimate of 0.59. Lutjanus bohar is a carnivorous piscivore that feeds throughout the water column. 相似文献
19.
1H-15N heteronuclear NMR studies of Escherichia coli thioredoxin in samples isotopically labeled by residue type 总被引:6,自引:0,他引:6
Ten samples of Escherichia coli thioredoxin were individually isotopically enriched by residue type via growth of an appropriate auxotrophic strain on media supplemented with one 2H, 15N-enriched amino acid. 1H observe-heteronuclear decoupling experiments were conducted on these samples making use of the 95-Hz 1H-15N amide J1 coupling. Subtraction of near-resonance from off-resonance 15N decoupled spectra generated difference patterns corresponding only to protons directly bonded to 15N nuclei. For the ten different enriched residue types observed to date, every labeled position (60) has been observed as a resolved resonance. The spectral dispersion in both the 1H and the 15N dimensions was roughly 1500 Hz (at 500-MHz field strength) with rather little apparent dependence on residue type. With the exception of the glycine-enriched sample, the range of the J1 coupling constants was not much greater than the precision of the measurements (1.5-2.0 Hz). However, for the glycine residues the J1 amide coupling values varied over a range of 10 Hz. 相似文献
20.
Chemically induced vasospasm: the effect of ischemia, vessel occlusion, and adrenergic blockade 总被引:1,自引:0,他引:1
Vasospasm in revascularized tissue can compromise tissue perfusion even though the microsurgical anastomoses remain patent. Circulating catecholamine stimulates peripheral vasoconstriction. Chemical vasospasm was induced by the intraarterial administration of norepinephrine to denervated rat hind limbs. Heel pad blood flow was assessed by laser-Doppler velocimetry. Mean blood flow was 463 +/- 106 in the denervated leg and 337 +/- 50 in the opposite (intact) leg (p less than 0.01). Flow in the denervated leg decreased 78 percent (463 +/- 106 to 100 +/- 23) within 5 minutes of norepinephrine administration and did not return to normal for 30 minutes. Norepinephrine administration in the presence of 1 and 3 hours of ischemia decreased flow at 5 minutes to 6.6 and 6.0 percent of normal, respectively (31 +/- 14, 28 +/- 14, control 100 +/- 23; p less than 0.001). Administration of intraarterial norepinephrine distal to a femoral artery occluded for 1 and 5 minutes reduced flow following clamp release to 11.2 and 7.1 percent of normal 5 minutes after clamp release (52 +/- 9, 33 +/- 7, control 100 +/- 23; p less than 0.001). Comparison of the 1-minute and 5-minute groups to each other showed a significant flow decrease in the 5-minute group (p less than 0.007). This indicates that the observed decrease in flow was related both to the presence of a vessel occlusion and to the length of the occlusion.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献