全文获取类型
收费全文 | 16178篇 |
免费 | 1950篇 |
国内免费 | 5篇 |
出版年
2021年 | 215篇 |
2019年 | 126篇 |
2018年 | 171篇 |
2017年 | 172篇 |
2016年 | 270篇 |
2015年 | 403篇 |
2014年 | 498篇 |
2013年 | 624篇 |
2012年 | 739篇 |
2011年 | 765篇 |
2010年 | 491篇 |
2009年 | 434篇 |
2008年 | 615篇 |
2007年 | 667篇 |
2006年 | 600篇 |
2005年 | 574篇 |
2004年 | 551篇 |
2003年 | 519篇 |
2002年 | 489篇 |
2001年 | 480篇 |
2000年 | 505篇 |
1999年 | 434篇 |
1998年 | 249篇 |
1997年 | 209篇 |
1996年 | 193篇 |
1995年 | 180篇 |
1994年 | 165篇 |
1993年 | 183篇 |
1992年 | 373篇 |
1991年 | 289篇 |
1990年 | 326篇 |
1989年 | 287篇 |
1988年 | 305篇 |
1987年 | 319篇 |
1986年 | 252篇 |
1985年 | 280篇 |
1984年 | 223篇 |
1983年 | 233篇 |
1982年 | 203篇 |
1981年 | 180篇 |
1980年 | 168篇 |
1979年 | 239篇 |
1978年 | 209篇 |
1977年 | 183篇 |
1976年 | 184篇 |
1975年 | 170篇 |
1974年 | 171篇 |
1973年 | 182篇 |
1972年 | 162篇 |
1971年 | 134篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
142.
D Waller M Agass D Mant A Coulter A Fuller L Jones 《BMJ (Clinical research ed.)》1990,300(6732):1115-1118
OBJECTIVE--To assess attendance at and the characteristics of patients attending health checks for cardiovascular disease offered in a general practice over a period of five years (1984-9). DESIGN--Medical record audit and postal questionnaire survey. SETTING--One general practice in Oxfordshire with a socially diverse population. PARTICIPANTS--1101 Men and 1110 women aged 35-64 registered with the practice. MAIN OUTCOME MEASURES--Age, sex, marital state, social class, smoking habits, alcohol consumption, and diet. RESULTS--Of the 2211 men and women in the target age group (35-64) in 1989, 1458 (65.9%) had been offered screening and 963 (43.6%) had attended for a health check. Attenders were more likely to be women, aged greater than or equal to 45, married, non-smokers, and of higher social class than patients who did not respond to the invitation. The relative likelihood of non-attendance was 1.24 for smokers, 1.20 for the overweight, 1.16 for heavy drinkers, and 1.28 for those with a less healthy diet, even after adjustment for age, sex, marital state, and social class. CONCLUSIONS--After five years of offering health checks, opportunistically (to men) and in the context of cervical smear tests (to women), less than half of the eligible patients had attended. The likelihood of acceptance of an invitation to attend was inversely related to the patient''s cardiovascular risk for all factors measured except age. A coherent strategy to reduce cardiovascular disease depends on more careful targeting of scarce health service resources and more emphasis on public health measures (such as dietary regulation and tobacco taxation). Doctors should be careful not to absolve the government of its public health obligations by substituting unproved preventive interventions aimed at the individual patient. 相似文献
143.
S. Schmitz M. Loeffler J. B. Jones R. D. Lange† H. E. Wichmann‡ 《Cell proliferation》1990,23(5):425-442
Abstract. Cyclic haemopoiesis in Grey Collie dogs is characterized by stable oscillations in all haemopoietic lineages. It is proposed that in these animals, in contrast to normal animals, the maturation process of haemopoietic (in particular granuloid) cells from the primitive progenitors to the functional cells is characterized by an abnormally strong synchrony. It is conjectured that the marrow maturation time has a very small variance compared with non-cyclic normal dogs. With a mathematical model of haemopoiesis it is shown that small fluctuations are amplified via regular feedback processes such that stable granuloid oscillations are established. Erythroid oscillations are induced indirectly by granuloid feedback to the stem cell pool. The model calculations further show that the synchrony hypothesis of bone marrow maturation can quantitatively explain the following experimental results: (1) the maintenance of stable cycles of granuloid and erythroid bone marrow and blood cells with a period of approximately 14 d; (2) the disappearance of granuloid and erythroid cycles during the administration of the colony stimulating factor rhG-CSF; (3) the reappearance of oscillations when the administration of CSF is discontinued; (4) the cessation of cycles during endotoxin application; and (5) the persistence of cycles during erythroid manipulations (bleeding anaemia, hypoxia, hypertransfusion). We therefore conclude that cyclic haemopoiesis is not caused by a defect in the regulatory control system but by an unusual maturation process. 相似文献
144.
Methylation and expression of the Myo D1 determination gene 总被引:5,自引:0,他引:5
P A Jones M J Wolkowicz M A Harrington F Gonzales 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》1990,326(1235):277-284
Mouse embryo cells induced to differentiate with the demethylating agent 5-azacytidine represent an excellent model system to investigate the molecular control of development. Clonal derivatives of 10T1/2 cells that have become determined to the myogenic or adipogenic lineages can be isolated from the multipotential parental line after drug treatment. These determined derivatives can be cultured indefinitely and will differentiate into end-stage phenotypes on appropriate stimulation. A gene called Myo D1, recently isolated from such a myoblast line, will confer myogenesis when expressed in 10T1/2 or other cell types (Davis et al. 1987). The cDNA for Myo D1 contains a large number of CpG sequences and the gene is relatively methylated in 10T1/2 cells and an adipocyte derivative, but is demethylated in myogenic derivatives. Myo D1 may therefore be subject to methylation control in vitro. On the other hand, preliminary observations suggest that Myo D1 is not methylated at CCGG sites in vivo so that a de novo methylation event may have occurred in vitro. These observations may have significance in the establishment of immortal cell lines and tumours. 相似文献
145.
Rapid turnover of adenovirus E1A is determined through a co-translational mechanism that requires an aminoterminal domain. 总被引:5,自引:1,他引:4 下载免费PDF全文
The product of the adenovirus E1A 13S mRNA can both stimulate and repress the expression of certain viral and cellular genes. As with several other regulatory proteins, E1A has a short half-life, approximately 40 min. Although this short half-life is observed in cells expressing the E1A gene, it is not the case with cells injected with E1A protein, where its half-life is very long, generally greater than 15 h. We have sought to reconcile these apparent differences in E1A stability. Using Xenopus oocytes, we find that E1A exhibits its characteristic short half-life when it is synthesized from injected mRNA while it has a very long half-life when it is injected as a protein synthesized originally in Escherichia coli or reticulocyte lysates. In order to delineate the amino acids responsible for rapid E1A turnover, several deletion mRNAs were constructed, injected into oocytes, and E1A half-life determined. Carboxyl-terminal deletions and an internal deletion of residues 38-86 failed to increase the half-life of E1A. In contrast, amino-terminal deletions of 70 and 14 residues resulted in very stable E1A proteins (t1/2 greater than 20 h). Furthermore, deletion of the second amino acid, an arginine, resulted in a stable E1A protein. The amino-terminal region of E1A was able to induce the rapid turnover of a normally stable protein, beta-globin, in oocytes injected with an E1A-globin chimeric mRNA. This E1A-induced instability of globin was abolished, however, when the protein was first synthesized in reticulocyte lysates and then injected into oocytes. The amino-terminal region of E1A is also important in governing halflife in adenovirus-infected HeLa cells. These results demonstrate that the half-life of E1A is established cotranslationally through a mechanism involving sequences within the amino-terminal 37 residues. 相似文献
146.
The role of beta-hydroxyaspartate and adjacent carboxylate residues in the first EGF domain of human factor IX. 总被引:21,自引:7,他引:14 下载免费PDF全文
D J Rees I M Jones P A Handford S J Walter M P Esnouf K J Smith G G Brownlee 《The EMBO journal》1988,7(7):2053-2061
beta-Hydroxyaspartic acid is a post-translationally modified amino acid found in a number of plasma proteins in a domain homologous to epidermal growth factor. Its presence can be correlated with a high affinity Ca2+ binding site, with a dissociation constant of 10-100 microM. We describe a system for the expression of human coagulation factor IX in dog kidney cells in tissue culture, in which the post-translational modifications and the biochemical activity are indistinguishable from factor IX synthesized in vivo. This system has been used to express eight different point mutations of human factor IX in the first epidermal growth factor domain in order to study the role of beta-hydroxyaspartate at residue 64, and the adjacent carboxylate residues at positions 47, 49 and 78. We conclude that this domain is essential for factor IX function and suggest that Ca2+ binds to carboxylate ions in this domain and stabilizes a conformation necessary for the interaction of factor IXa with factor X, factor VIII and phospholipid in the next step of the clotting cascade. 相似文献
147.
Gender difference in the relationship of performance in the handgrip and standing long jump tests to lean limb volume in young adults 总被引:1,自引:0,他引:1
B. N. Davies Elizabeth J. Greenwood S. R. Jones 《European journal of applied physiology and occupational physiology》1988,58(3):315-320
Groups of young, adult males and females performed the handgrip and standing long jump tests. Their total forearm and leg volumes were calculated from a series of circumference and length measurements, and the lean volumes (bone + muscle) calculated by taking the skinfold thickness into consideration. In the handgrip, the mean female performance was 298 N compared with 496 N for the males. In the standing long jump, mean performance expressed as distance x body mass was 87.3 kg.m for females compared with 137.7 kg.m for males. These superior performances of males could simply reflect their greater muscle mass, as the mean lean volumes of female and male limbs respectively were 0.54 l and 0.89 l for forearms, and 11.82 l and 14.82 l for the two legs. However, when the performances of males and females were grouped by lean limb volume, it was found that while in both tests there were linear relationships, males and females did not share a common line. In both tests the male relationship was at a higher level than the female; therefore, for a given lean volume, the male performance was significantly superior to that of the female. The gender difference found in this study has not been seen in other studies in which the performance of skeletal muscle has been related to the cross-sectional area of the active muscles and the possible reasons for the differences are considered. 相似文献
148.
149.
150.