全文获取类型
收费全文 | 175504篇 |
免费 | 17619篇 |
国内免费 | 49107篇 |
出版年
2024年 | 916篇 |
2023年 | 3538篇 |
2022年 | 6614篇 |
2021年 | 9484篇 |
2020年 | 7535篇 |
2019年 | 8996篇 |
2018年 | 7442篇 |
2017年 | 6137篇 |
2016年 | 7560篇 |
2015年 | 10841篇 |
2014年 | 13685篇 |
2013年 | 13392篇 |
2012年 | 17087篇 |
2011年 | 16085篇 |
2010年 | 11529篇 |
2009年 | 11196篇 |
2008年 | 12501篇 |
2007年 | 11700篇 |
2006年 | 10580篇 |
2005年 | 8698篇 |
2004年 | 6963篇 |
2003年 | 6119篇 |
2002年 | 5371篇 |
2001年 | 4890篇 |
2000年 | 4457篇 |
1999年 | 3457篇 |
1998年 | 1946篇 |
1997年 | 1572篇 |
1996年 | 1413篇 |
1995年 | 1214篇 |
1994年 | 1121篇 |
1993年 | 883篇 |
1992年 | 1087篇 |
1991年 | 875篇 |
1990年 | 672篇 |
1989年 | 642篇 |
1988年 | 533篇 |
1987年 | 480篇 |
1986年 | 407篇 |
1985年 | 458篇 |
1984年 | 289篇 |
1983年 | 259篇 |
1982年 | 253篇 |
1981年 | 152篇 |
1980年 | 88篇 |
1979年 | 87篇 |
1977年 | 70篇 |
1975年 | 67篇 |
1974年 | 67篇 |
1973年 | 66篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
小麦初生叶接种条锈菌毒性生理小种(CY29)及其弱毒突变菌系(CY29-mut3)后,呈不亲和反应的寄主叶片可溶性蛋白质合成能力在接种后24h显著高于未接种对照,但其后逐渐降低,直至接近对照;而呈亲和性反应的寄主叶片可溶性蛋白质合成在侵染早期与对照相近,但与膜结合蛋白质在96h时大大高于对照。对接种叶中核糖体的密度梯度分析证实:呈不亲和反应寄主叶片游离多聚核糖体及亲和反应的寄主内与膜结合多聚核糖体均有特异性增加。上述结果表明寄主的抗病和感病反应均与蛋白质合成能力的变化有关。 相似文献
972.
经硫酸铵分部沉淀、SephacrylS-300和DEAE-纤维素柱层析纯化了小球藻RubisCO,得率为15%,比活力达1.232μmolCO2ms-1min-1,分子量是500kD,它和菠菜叶片RubisCO在分子量、亚基组成和免疫特性等方面相似,反映RubisCO在高等和低等植物中有较高的同源性。自养小球藻RubisCO占细胞可溶性蛋白质的24%。而异养转变后的小球藻细胞内不含RubisCO。异养小球藻向自养生长转变过程中,20h后细胞内叶绿素含量逐渐增加,24h时细胞内出现RubisCO,24h后大量增加,至41h时含量达最高峰;标志着小球藻细胞光合作用能力的恢复和加强。 相似文献
973.
The current study shows that a clonal derivative of the Jurkat cell line up-regulates both the avidity and density of the α 6/β1 receptor in response to phorbol 12-myristate 13-acetate (PMA). This derivative attaches to fibronectin and, to a lesser degree, laminin constitutively. Adhesion and spreading are dramatically up-regulated following treatment with PMA. The response on fibronectin peaks within 4 hours, is insensitive to cyclohexaminde, can be blocked by monoclonal antibodies (Mabs) to the β1 and α 5 subunits of the β1 family of integrins, and is not associated with increased expression of the α 5 or β1 epitopes at the cell surface. In contrast, the response on laminin is biphasic. The early phase parallels the response on fibronectin. The second phase peaks after 48–72 hours of treatment with PMA, is sensitive to cycloheximide, can be blocked by Mabs to the β1 and α 6 subunits, and is associated with increased expression of the α 6 epitope. Both the density independent and dependent responses to PMA in Jurkat cells are blocked by the protein kinase inhibitor staurosporine. The HSB-2, CEM, Molt-4, and HPB-ALL T-lymphoblastic cell lines also up-regulate attachment to fibronectin and laminin following treatment with PMA. All four lines constitutively attach to fibronectin and show rapid up-regulation of attachment following treatment with PMA. None of the lines attach to laminin prior to PMA treatment; however, specific adhesion developed after 4–120 hours of treatment. The most mature lines (Jurkat and HPB-ALL) up-regulated adhesion on laminin more rapidly than the less phenotypically mature lines (CEM, Molt-4, and HSB-2). In summary, clonal derivatives of the Jurkat cell line up-regulated attachment to laminin through protein kinase dependent increases in α /β1 receptor avidity and density. In addition, the expression of functional receptors for laminin is linked to developmental maturity in a series of T-lymphoblastic cell lines. © 1993 Wiley-Liss, Inc. 相似文献
974.
Wen-Hsiung Li 《Journal of molecular evolution》1993,36(1):96-99
Summary The current convention in estimating the number of substitutions per synonymous site (K
S
) and per nonsynonymous site (K
A
) between two protein-coding genes is to count each twofold degenerate site as one-third synonymous and two-thirds nonsynonymous
because one of the three possible changes at such a site is synonymous and the other two are nonsynonymous. This counting
rule can considerably overestimate theK
S
value because transitional mutations tend to occur more often than transversional mutations and because most transitional
mutations at twofold degenerate sites are synonymous. A new method that gives unbiased estimates is proposed. An application
of the new and the old method to 14 pairs of mouse and rat genes shows that the new method gives aK
S
value very close to the number of substitutions per fourfold degenerate site whereas the old method gives a value 30% higher.
Both methods give aK
A
value close to the number of substitutions per nondegenerate site. 相似文献
975.
Lymphocytes protect against and are not required for reovirus-induced myocarditis. 总被引:11,自引:10,他引:1
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Many studies suggest that host lymphocytes are damaging, rather than protective, in virally induced myocarditis. We have investigated the role of lymphocyte-based immunity in murine myocarditis by using a myocarditic reovirus (reovirus serotype 3 8B), nonmyocarditic reoviruses, adoptive transfer experiments, and mice with severe combined immunodeficiency (SCID mice). Prior to infection, passive transfer of monoclonal antibodies specific for 8B capsid proteins protected neonatal mice against 8B-induced myocarditis, indicating that humoral immunity can protect against myocarditis. Some monoclonal antibodies acted by blocking viral spread to and/or replication in the heart. Passive transfer of reovirus-immune, but not naive, spleen cells prior to infection protected neonatal mice from 8B-induced myocarditis. Depletion of either CD4 or CD8 T cells resulted in increased viral titer in the heart but did not abrogate immune cell-mediated protection against myocardial injury. This shows that both CD4 and CD8 T cells can act independently to protect myocardial tissue from reovirus infection. In addition, reovirus 8B caused extensive myocarditis in SCID mice. This confirms a prior report (B. Sherry, F. J. Schoen, E. Wenske, and B. N. Fields, J. Virol. 63:4840-4849, 1989) that T cells are not required for reovirus-induced myocarditis and demonstrates for the first time that B cells are not required for reovirus-induced myocarditis. We used SCID mice and a panel of reoviruses to assess (i) the relationship between growth in the heart and myocardial damage and (ii) the possibility that nonmyocarditic reoviruses exhibit a myocarditic phenotype in the absence of functional lymphocytes. Growth in the heart was not the sole determinant of myocarditic potential in SCID mice. Although 8B induced myocarditis in SCID mice, no or minimal myocarditis was found in SCID mice infected with four reovirus strains previously shown (B. Sherry and B. N. Fields, J. Virol. 63:4850-4856, 1989) to be nonmyocarditic or poorly myocarditic in normal neonatal mice. We conclude that (i) humoral immunity and cellular immunity are protective against, and not required for, reovirus-induced myocarditis and (ii) the potential to induce cardiac damage is a property of the virus independent of lymphocyte-based immunity. 相似文献
976.
A cellular cofactor facilitates efficient 3CD cleavage of the poliovirus P1 precursor. 总被引:5,自引:4,他引:1
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The production of poliovirus capsid proteins from a capsid protein precursor (P1) is mediated by virus-encoded proteinase 3CD and involves a complicated set of proteinase-substrate interactions. In addition to substrate and enzymatic determinants required for this interaction, we describe a cellular cofactor, which facilitates 3CD recognition of the P1 precursor. Cellular cofactor activity is 3CD dependent and salt dependent. Our analysis shows that proteolytic cleavage of the P1 precursor at the VP0/VP3 cleavage site exhibits a greater dependency on the cellular cofactor than cleavage at the VP3/VP1 site. Such a greater dependency on cellular cofactor activity can be relieved (in part) by the substitution of an Ala residue for the Pro residue at the -4 position of the VP0/VP3 cleavage site. However, mutant viruses containing Pro-to-Ala substitutions at the -4 position of the VP0/VP3 site exhibit defects in viral growth. 相似文献
977.
978.
979.
文章通过贵州盘新塘原下二叠统下部紫松阶珊瑚化石的分析,将该地层划归上石炭统上部。描述上炭统威宁阶,过岩阶和紫松阶的四射珊瑚6属7种,其中5新种,通过对这些珊瑚的形态构造进行了剖析,略述了该地区在晚石炭世的环境特征。 相似文献