全文获取类型
收费全文 | 5987篇 |
免费 | 389篇 |
出版年
2023年 | 24篇 |
2022年 | 33篇 |
2021年 | 117篇 |
2020年 | 69篇 |
2019年 | 89篇 |
2018年 | 108篇 |
2017年 | 101篇 |
2016年 | 156篇 |
2015年 | 234篇 |
2014年 | 234篇 |
2013年 | 405篇 |
2012年 | 340篇 |
2011年 | 360篇 |
2010年 | 202篇 |
2009年 | 197篇 |
2008年 | 345篇 |
2007年 | 317篇 |
2006年 | 278篇 |
2005年 | 263篇 |
2004年 | 276篇 |
2003年 | 225篇 |
2002年 | 258篇 |
2001年 | 161篇 |
2000年 | 156篇 |
1999年 | 120篇 |
1998年 | 59篇 |
1997年 | 60篇 |
1996年 | 57篇 |
1995年 | 50篇 |
1994年 | 57篇 |
1993年 | 39篇 |
1992年 | 104篇 |
1991年 | 76篇 |
1990年 | 65篇 |
1989年 | 65篇 |
1988年 | 62篇 |
1987年 | 60篇 |
1986年 | 68篇 |
1985年 | 58篇 |
1984年 | 55篇 |
1983年 | 45篇 |
1982年 | 21篇 |
1981年 | 26篇 |
1980年 | 26篇 |
1979年 | 24篇 |
1978年 | 27篇 |
1977年 | 23篇 |
1976年 | 20篇 |
1975年 | 23篇 |
1971年 | 19篇 |
排序方式: 共有6376条查询结果,搜索用时 281 毫秒
121.
Integration is essential for efficient gene expression of human immunodeficiency virus type 1. 总被引:24,自引:18,他引:6 下载免费PDF全文
H Sakai M Kawamura J Sakuragi S Sakuragi R Shibata A Ishimoto N Ono S Ueda A Adachi 《Journal of virology》1993,67(3):1169-1174
A mutant of human immunodeficiency virus type 1 which carries a frameshift insertion in the integrase/endonuclease region of pol gene was constructed in vitro. Upon transfection into cells, although this mutant exhibited a normal phenotype with respect to expression of gag, pol, and env genes and to generation of progeny virions, no replication-competent virus in CD4-positive cells emerged. An assay for the single-step replication of a defective viral genome dependent on trans complementation by rev protein was established and used to monitor the early phase of viral infection process. Viral clones with a mutation in the vif, vpr, or vpu gene displayed no abnormality in the early phase. In contrast, the integrase mutant did not direct a marker gene expression after infection. Together with an observation that the mutant lacked the ability to integrate, these results indicated that the integration was required for efficient viral gene expression and productive infection of human immunodeficiency virus type 1. 相似文献
122.
Individual identification of ancient human remains is one of the most fundamental requisites for studies of paleo-population genetics, including kinship among ancient people, intra- and interpopulation structures in ancient times, and the origin of human populations. However, knowledge of these subjects has been based mainly on circumstantial archaeological evidence for kinship and intrapopulation structure and on genetic studies of modern human populations. Here we describe individual identification of ancient humans by using short-nucleotide tandem repeats and mtDNAs as genetic markers. The application of this approach to kinship analysis shows clearly the presence or absence of kinship among the ancient remains examined. 相似文献
123.
124.
125.
126.
Pancreatic cancer is a highly aggressive form of cancer with a poor prognosis, partly due to ‘austerity’, a phenomenon of tolerance to nutrient deprivation and survival in its hypovascular tumor microenvironment. Anti-austerity agents which preferentially diminish the survival of cancer cells under nutrition starvation is regarded as new generation anti-cancer agents. This study investigated the potential of Piper longum constituents as anti-austerity agents. The ethanolic extract of Piper longum was found to have preferential cytotoxicity towards PANC-1 human pancreatic cancer cells in a nutrient-deprived medium (NDM). Further investigation led to the identification of pipernonaline ( 3 ) as the lead compound with the strongest anti-austerity activity, inducing cell death and inhibiting migration in a normal nutrient medium, as well as strongly inhibiting the Akt/mTOR/autophagy pathway. Therefore, pipernonaline ( 3 ) holds promise as a novel antiausterity agent for the treatment of pancreatic cancer. 相似文献
127.
The gonyaulacoid dinofiagellate Alexandrium satoanum Yuki et Fukuyo sp. nov. is described from Matoya Bay, Pacific coast of central Japan. The species is distinctive in its conical epitheca with almost straight sides and dorsal concavity of the hypotheca. The plate formula is Po, pc, 4′, 6″, 6c, 10s, 5″″, and 2″″, including two accessory plates inside the sulcus. The apical pore plate is triangular and possesses an anterior attachment pore at the right margin. The first apical plate does not make contact with the apical pore plate and lacks a ventral pore. A posterior attachment pore lies at the center of the posterior sulcal plate. In Matoya Bay, vegetative cells occur as solitary cells or sometimes in pairs during late spring and early summer in low concentrations. In connection with this study, the following new combination is proposed: Alexandrium pseudogonyaulax (Biecheler) Horiguchi ex Yuki et Fukuyo comb. nov. 相似文献
128.
129.
Cryopreservation of wild mouse spermatozoa 总被引:3,自引:0,他引:3
Nakagata N Ueda S Yamanouchi K Okamoto M Matsuda Y Tsuchiya K Nishimura M Oda S Koyasu K Azuma S Toyoda Y 《Theriogenology》1995,43(3):635-643
Spermatozoa of wild mice from China, Czechoslovakia, Denmark, India, Japan and Switzerland were frozen and stored at -196 degrees C. After thawing, intact oocytes were inseminated in vitro with relatively high motility frozen-thawed mouse spermatozoa from Czechoslovakia, Denmark and India, while oocytes with a partially dissected zona were inseminated with low motility frozen-thawed spermatozoa from China, Japan and Switzerland. Embryos developing to the 2-cell stage from oocytes fertilized with frozen-thawed spermatozoa were transferred to the oviducts of female recipients on the first day of pseudopregnancy (day when a vaginal plug was confirmed). Successful embryo development to the 2-cell stage was 46 to 67%. Offspring resulted from 17 to 51% of these transferred 2-cell embryos. 相似文献
130.
Chloroplast division in Nannochloris bacillaris Naumann (Chlorophyta) was examined by electron microscopy after preparation of samples by freeze-substitution. A pair of belts appeared on the surface of the outer and inner envelope membranes at the middle of the chloroplast. These belts seemed to be constructed of thin fibrils that run parallel to the longitudinal direction of the belts. The outer fibrillar belt increased in width as the constriction of the chloroplast advanced. It appears that the fibrillar belt is the division apparatus of the chloroplast. It encircles the chloroplast and finally divides the chloroplast in two as the diameter of the belt decreases. 相似文献