全文获取类型
收费全文 | 54998篇 |
免费 | 4625篇 |
国内免费 | 36篇 |
专业分类
59659篇 |
出版年
2023年 | 268篇 |
2022年 | 579篇 |
2021年 | 1053篇 |
2020年 | 771篇 |
2019年 | 835篇 |
2018年 | 1203篇 |
2017年 | 1005篇 |
2016年 | 1641篇 |
2015年 | 2350篇 |
2014年 | 2311篇 |
2013年 | 2881篇 |
2012年 | 3435篇 |
2011年 | 3164篇 |
2010年 | 1933篇 |
2009年 | 1873篇 |
2008年 | 2292篇 |
2007年 | 2269篇 |
2006年 | 2064篇 |
2005年 | 2375篇 |
2004年 | 2351篇 |
2003年 | 1961篇 |
2002年 | 1582篇 |
2001年 | 1465篇 |
2000年 | 1395篇 |
1999年 | 1249篇 |
1998年 | 573篇 |
1997年 | 542篇 |
1996年 | 598篇 |
1995年 | 462篇 |
1994年 | 467篇 |
1993年 | 441篇 |
1992年 | 935篇 |
1991年 | 838篇 |
1990年 | 775篇 |
1989年 | 765篇 |
1988年 | 765篇 |
1987年 | 689篇 |
1986年 | 648篇 |
1985年 | 626篇 |
1984年 | 584篇 |
1983年 | 443篇 |
1982年 | 339篇 |
1981年 | 336篇 |
1980年 | 317篇 |
1979年 | 438篇 |
1978年 | 363篇 |
1977年 | 299篇 |
1975年 | 299篇 |
1974年 | 302篇 |
1973年 | 300篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Mössbauer spectroscopic studies on a heme peptide (HP) derived from cytochrome c and on the HP recombined non-covalently with the remaining cleaved section are reported. The results suggest that the environment of the heme site in the known crystal structure of cytochrome c may differ in detail from the environment of the heme in the working protein. 相似文献
992.
Christina Brahe Isabella Velonà Gerrit van der Steege Stefania Zappata Anneke Y. van de Veen Jan Osinga Carli M. J. Tops Riccardo Fodde P. Meera Khan Charles H. C. M. Buys Giovanni Neri 《Human genetics》1994,93(5):494-501
The locus responsible for the childhood-onset proximal spinal muscular atrophies (SMA) has recently been mapped to an area of 2–3 Mb in the region q12–13.3 of chromosome 5. We have used a series of radiation hybrids (RHs) containing distinct parts of the SMA region as defined by reference markers. A cosmid library was constructed from one RH. Thirteen clones were isolated and five of these were mapped within the SMA region. Both RH mapping and fluorescence in situ hybridization analysis showed that two clones map in the region between loci D5S125 and D5S351. One of the cosmids contains expressed sequences. Polymorphic dinucleotide repeats were identified in both clones and used for segregation analysis of key recombinant SMA families. One recombination between the SMA locus and the new marker 9Ic (D5S685) indicates that 9Ic is probably the closest distal marker. The absence of recombination between the SMA locus and marker Fc (D5S684) suggests that Fc is located close to the disease gene. These new loci should refine linkage analysis in SMA family studies and may facilitate the isolation of the disease gene. 相似文献
993.
Frida E. Kleiman Raquel Dodelson de Kremer Ana Oller de Ramirez Roy A. Gravel Carlos E. Argaraña 《Human genetics》1994,94(3):279-282
The level of -hexosaminidase activity in plasma and leukocytes and the frequency of three known HEXB mutations were studied in an Argentinean deme with high incidence of infantile Sandhoff disease. Two mutations were previously identified in one of two Sandhoff patients from the region, a splice mutation, IVS-2+1 GA, and a 4-bp deletion, CTTT782–785. These mutations, and a 16kb deletion from the 5' end of the HEXB gene common in non-Argentineans, were screened in 9 Sandhoff patients (all unrelated), 24 obligate heterozygotes, 33 additional individuals belonging to families with affected members, and 64 randomly ascertained individuals from the high risk region. Of 31 independent alleles examined, including those of the two patients previously reported, 30 had the IVS-2 splice mutation and only the originally reported patient had the CTTT deletion. The 16-kb deletion was not observed. Further, among the 57 unaffected members of families with a previous history of Sandhoff disease, and absolute correlation was found between carrier diagnosis by enzyme assay of leukocytes and the DNA-based tests for mutation. One of the 64 controls was classified as a carrier by enzyme assay but did not have one of the three mutations screened. We conclude that a single mutation predominates in this Argentinean population and that the DNA-based test can be an effective supplement or alternative to enzyme-based testing. 相似文献
994.
Christopher M. de Fiebre Ping Wu Dean Notabartolo William J. Millard Edwin M. Meyer 《Neurochemical research》1994,19(6):643-648
The ability of Sendai virosomes or LipofectinTM to introduce an AAV vector into primary rat brain astroglial cultures was characterized. The pJDT95npy vector was constructed by inserting rat NPY cDNA downstream from the indigenous AAV p5, p19 and p40 promoters in pJDT95. LipofectinTM-mediated transfection with pJDT95npy (10 g) resulted in pronounced expression of several NPY mRNA species: p5-driven (3.3 kb), p19-driven (2.7 kb) and p40-driven (0.6, 0.8, 1.1, and 1.8 kb). Exposure to virosomally encapsulated pJDT95npy (50 or 100 ng) resulted in transient expression of some p40-driven mRNA species (0.8 and 1.8 kb). Neither method produced astroglia cells which synthesized mature NPY immunoreactivity. This demonstrates that an AAV-derived vector can drive gene expression in astroglia, that Sendai virosomes can infuse vectors into astroglia, but that the amount of DNA infused in this manner may limit long term expression. 相似文献
995.
Maria Ieda Siqueira Linhares Yoshito Eizuru Gildete Patriota de Andrade Ivailda Barbosa Fonseca Luiz Bezerra Carvalho Ivanildo Tenorio Moreira Yoichi Minamishima 《Microbiology and immunology》1994,38(6):475-478
The seroprevalence of human T cell leukemia virus type 1 (HTLV-1) infection was investigated in Brazilians (570): native inhabitants (298) and descendants from Japanese (272) living in Recife and its neighborhoods—North-east of Brazil. Furthermore, polytransfused renal transplanted patients (54) were also examined for the serological status to this virus. The seropositivity to HTLV-1, screened by enzyme-linked immunosorbent assay (ELISA), was low: 1.34% for the local population and 0.73% for the descendants from Japanese. However, the seropositivity for the renal transplanted patients was found to be 11.1%. This higher value suggests that this retrovirus infection seems to be of importance in this clinical condition. 相似文献
996.
水分胁迫对小麦光系统Ⅱ的影响 总被引:10,自引:0,他引:10
水分胁迫可降低小麦叶绿体的室温荧光产量和Mg2+ 在两个光系统间的调节能力、叶片的可变荧光产量、可变荧光猝灭速率以及荧光上升互补面积,表明光系统Ⅱ受到了伤害。光系统Ⅱ氧化侧的人工电子供体DPC能部分恢复受到抑制的叶绿体可变荧光,说明水分胁迫对光系统Ⅱ的损伤部位不仅位于氧化侧,也可能在反应中心上 相似文献
997.
Maryna van de Venter Derek Litthauer Willem Oelofsen 《Journal of cellular biochemistry》1994,54(1):1-10
Adipocyte precursors from the stromal vascular fraction of human adipose tissue were allowed to differentiate in serum-free defined medium, whereafter their catecholamine stimulated lipolytic response was compared to that of mature isolated human adipocytes. Seventy-five to ninety percent of the fibroblast-like cells accumulated lipid droplets and glycerol-3-phosphate dehydrogenase activities of 1,000–2,800 mU/mg protein were measured in cell homogenates of differentiated cells. Lipolysis could be stimulated by both isoproterenol and norepinephrine in both differentiated preadipocytes as well as mature adipocytes. The results obtained with β-adrenergic agents suggested the presence of a higher affinity receptor in differentiated preadipocytes as compared to mature adipocytes. Mature adipocytes responded well to β-adrenergic agents, but no antilipolytic α2-adrenergic response was observed in the differentiated preadipocytes. The presence of Gi proteins in the differentiated preadipocytes was suggested by the antilipolytic effect of adenosine as well as the lipolytic activity generated by pertussis toxin. In conclusion, our medium supported the differentiation of a very high percentage of human preadipocytes which developed a sensitive β-adrenergic lipolytic response but which lacked an α2-adrenergic antilipolytic response. 相似文献
998.
A. R. Carlini M. E. I. Márquez G. Soave D. F. Vergani P. A. Ronayne de Ferrer 《Polar Biology》1994,14(1):37-42
An analysis of milk constituents during various stages of lactation in the southern elephant seal Mirounga leonina was carried out. Forty-six milk samples were taken from 30 females throughout lactation during 1985, 1987, 1990 and 1991 on Stranger Point, King George Island, South Shetland Islands. Total nitrogen (TN), non-protein nitrogen (NPN), sugar, fat, ash and water were measured, and from some of these data true protein and energy content were calculated. The results showed a high degree of variation in water and fat concentrations among samples at different stages of lactation. During the first 20 days the fat content of milk increased from about 12 to approximately 52%, while water content fell from 70 to 33%. The composition of milk changes rapidly during the first days post-partum. Protein, minerals and sugar appear to remain stable after the fourth day of lactation. Milk samples contain significant levels of sugars; thin layer chromatography indicates the presence of lactose and glucose together with other unidentified components. There is evidence of a striking change in composition of the milk in the later part of lactation; the progressive increase in the fat:water ratio is abruptly reversed just prior to weaning. 相似文献
999.
M. Joëls W. Hesen H. Karst E.R. de Kloet 《The Journal of steroid biochemistry and molecular biology》1994,49(4-6):391-398
Corticosteroid hormones can enter the brain and bind to two receptor subtypes: the high affinity mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR) with approximately 10-fold lower affinity. Under physiological conditions the degree of receptor occupation will range from a predominant MR occupation (at the beginning of the inactive period, under rest) to concurrent activation of MRs and GRs (at the circadian peak and after stress). With in vitro electrophysiological recording techniques we observed that neuronal excitability in the CA1 hippocampal field is under a long-term control of MR- and GR-mediated events. The predominant occupation of MRs is associated with a stable amino acid-carried synaptic transmission; calcium- and potassium-currents are small, as are the responses to biogenic amines. Occupation of GRs in addition to MRs results in a gradual failure of CA1 neurons to respond to repeated stimulation of amino acid-mediated input; ionic conductances and responses to biogenic amines are large. In general, electrical properties recorded when both MRs and GRs are unoccupied (i.e. after adrenalectomy) resemble the responses observed when both receptor types are activated. The corticosterone dependency of electrical properties is thus U-shaped. We conclude that MR occupation may be responsible for the maintenance of information processing in the CA1 field and the stability of the circuit. Additional activation of GRs will initially suppress synaptic activity, but may eventually result in an increased instability and even vulnerability of the neuronal networks. 相似文献
1000.
The radiosensitivity of spermatogonial stem cells of C3H/HeH × 101/H F1 hybrid mice was determined by counting undifferentiated spermatogonia at 10 days after X-irradiation. During the spermatogenic cycle, differences in radiosensitivity were found, which were correlated with the proliferative activity of the spermatogonial stem cells. In stage VIIIirr, during quiescence, the spermatogonial stem cells were most radiosensitive with a D0 of 1.4 Gy. In stages XIirr−Virr, when the cells were proliferatively active, the D0 was about 2.6 Gy. Based on the D0 values for sensitive and resistant spermatogonia and on the D0 for the total population, a ratio of 45:55% of sensitive to resistant spermatogonial stem cells was estimated for cell killing.
When the present data were compared with data on translocation induction obtained in mice of the same genotype, a close fit was obtained when the translocation yield (Y; in % abnormal cells) after a radiation dose D was described by Y = eτD, with τ = 1 for the sensitive and τ = 0.1 for the resistant spermatogonial stem cells, with a maximal eτD of 100. 相似文献