全文获取类型
收费全文 | 5223篇 |
免费 | 303篇 |
国内免费 | 1篇 |
出版年
2022年 | 22篇 |
2021年 | 52篇 |
2020年 | 28篇 |
2019年 | 33篇 |
2018年 | 56篇 |
2017年 | 48篇 |
2016年 | 92篇 |
2015年 | 144篇 |
2014年 | 145篇 |
2013年 | 460篇 |
2012年 | 289篇 |
2011年 | 291篇 |
2010年 | 175篇 |
2009年 | 175篇 |
2008年 | 324篇 |
2007年 | 306篇 |
2006年 | 309篇 |
2005年 | 276篇 |
2004年 | 276篇 |
2003年 | 290篇 |
2002年 | 259篇 |
2001年 | 117篇 |
2000年 | 91篇 |
1999年 | 87篇 |
1998年 | 74篇 |
1997年 | 62篇 |
1996年 | 52篇 |
1995年 | 50篇 |
1994年 | 41篇 |
1993年 | 45篇 |
1992年 | 77篇 |
1991年 | 62篇 |
1990年 | 58篇 |
1989年 | 78篇 |
1988年 | 57篇 |
1987年 | 44篇 |
1986年 | 34篇 |
1985年 | 41篇 |
1984年 | 30篇 |
1982年 | 29篇 |
1981年 | 26篇 |
1979年 | 27篇 |
1978年 | 22篇 |
1976年 | 21篇 |
1974年 | 20篇 |
1973年 | 24篇 |
1970年 | 19篇 |
1968年 | 19篇 |
1967年 | 22篇 |
1966年 | 18篇 |
排序方式: 共有5527条查询结果,搜索用时 15 毫秒
41.
The exogenous addition of dolichyl phosphate (Dol-P), an active form of dolichol (Dol) that carries oligosaccharide chains for protein-N-glycosylation, significantly enhanced colony formation of mouse bone marrow hematopoietic progenitors (CFU-e, BFU-e, and CFU-gm) was stimulated by erythropoietin (Epo) and colony-stimulating factor (CSF), but Dol enhanced colony formation of CFU-e only. The effects of Dol or Dol-P on these hematopoietic progenitors were fully dependent on stimulation by Epo or CSF. Other mevalonate-metabolites, such as cholesterol, coenzyme Q10, and isopentenyladenine, had no effect on hematopoietic progenitors. These studies suggest that exogenous Dol-P enhances the frequency of differentiation of hematopoietic progenitors stimulated by Epo or CSF, and there may be a diversity in cellular response of these progenitors to Dol. 相似文献
42.
43.
Koichi Negayama Takako Negayama Kiyomi Kondo 《International journal of primatology》1986,7(4):365-378
Parturitional behavior in 12 caged Macaca fuscatawas analyzed. Wild-caught mothers showed adequate maternal behaviors immediately following the neonate’s expulsion. Parity
differences existed in the behaviors; primiparae were more idiosyncratic than were multiparae. Among multiparae, those with
two or more offspring were uniformly adequate, but those with a single birth experience varied in the adequacy of the maternal
care they provided at parturition. Mothers embraced and licked their neonates and had ventroventral contact with them frequently
immediately after parturition but decreased these behaviors after expulsion of the placenta. In contrast, mothers showed allogrooming
after consuming the placenta. Placentophagy was correlated with the level of orality represented by maternal licking behaviors.
An isolation-reared primipara reacted to her newborn in a basically negative manner, although she showed little actual aggression.
She showed a rapid shift in her negative behavior during the immediate postpartum period. This mother’s newborn sought contact
with her, indicating the neonate’s active role in establishing a stable mother-neonate bond. 相似文献
44.
45.
Summary The avian stomach is composed of two distinct organs, the proventriculus and the gizzard. Pepsinogen expression in the proventricular and gizzard epithelia of chick embryos was investigated immunohistochemically with anti-embryonic chick pepsinogen (anti-ECPg) antiserum. In normal development, the ECPg antigen was expressed only in the glandular epithelial cells of the embryonic proventriculus from the 8th day of incubation onwards. However, both proventricular and gizzard epithelia of 6-day embryos expressed the ECPg antigen when recombined and cultured with the proventricular mesenchyme. Chronological studies revealed that the ECPg antigen was first detected in a few epithelial cells at 3 days of cultivation. The percentage of ECPg-positive cells among the total epithelial cells in each recombinant increased with the length of the culture period and all the glandular epithelial cells were positive at 9 days. During this process, the percentage of ECPg-positive cells in each cultured recombinant was similar in proventricular and gizzard epithelia. Moreover, both epithelia could express the ECPg antigen when recombined and cultured with the oesophageal or small-intestine mesenchyme for 9 days, though the percentage of ECPg-positive cells in each cultured recombinant was much lower than that in the cultured recombinant with the proventricular mesenchyme. These results indicate that the gizzard epithelium of 6-day chick embryos possesses a similar potential for pepsinogen expression as the proventricular epithelium of the same age. 相似文献
46.
Long-chain betulaprenol-type polyprenols from the leaves of Ginkgo biloba. 总被引:6,自引:1,他引:5 下载免费PDF全文
A long-chain betulaprenol-type polyprenol mixture was isolated from the leaves of Ginkgo biloba mainly as acetate. The structure was determined by mass spectroscopy, 1H-n.m.r. spectroscopy and 13C-n.m.r. spectroscopy. The mixture contained polyprenols-14-22, predominantly polyprenols-17, -18 and -19, and consisted of the dimethylallyl terminal unit (omega-terminal), two trans-isoprene residues, a sequence of 11-19 cis-isoprene residues and a terminal hydroxylated isoprene unit (alpha-terminal) aligned in that order. The concentration of these polyprenols in leaves increased from 0.04 to 2.0% of dry wt. with maturing of the leaves, though the content of total lipids was constant. The distribution of chain length in these polyprenols showed little variation throughout the whole life of the leaves. 相似文献
47.
48.
Surface-Bound Nuclease of Staphylococcus aureus: Localization of the Enzyme 总被引:3,自引:2,他引:1 下载免费PDF全文
The cellular localization of staphylococcus nuclease, previously known as an exoenzyme, was investigated, and the following results were obtained. (i) When Staphylococcus aureus cells were converted to protoplasts by cell wall lytic enzyme L-11 (a bacteriolytic enzyme purified from Flavobacterium sp. which specifically hydrolyzes amide and peptide linkages of murein layers), over 80% of the cell-bound nuclease was released into the surrounding sucrose medium. (ii) The cell-bound nuclease was associated with the cell-wall membrane fraction of mechanically disrupted cells. (iii) The nuclease activity of cell-wall membrane fractions from cells during early and late stages of protoplast formation were compared. Less activity was found in the late stage. These results suggest that nuclease may be located at or near the surface of the cells. The distribution of cell-bound nuclease in the cell-wall membrane fraction varied with the growth conditions of S. aureus. The activity of alkaline phosphatase, another surface enzyme, was also investigated. Less of this enzyme than nuclease was released when the cells were converted to protoplasts. 相似文献
49.
Surface-Bound Nuclease of Staphylococcus aureus: Purification and Properties of the Enzyme 下载免费PDF全文
The surface-bound nuclease of Staphylococcus aureus liberated during formation of protoplasts was purified 1,000-fold by chromatography on phosphocellulose. Its properties were compared with those of the known extracellular nuclease, purified 200-fold by the same procedures. The adsorbance of the surface-bound nuclease on phosphocellulose was distinctly different from that of the extracellular nuclease, but other properties of the two enzymes were similar. Both enzymes had a pH optimum of about 10 and required Ca2+ for activity. Both enzymes hydrolyzed deoxyribonucleic acid (DNA) and ribonucleic acid, and denatured DNA was a better substrate than native DNA. Both enzymes were inhibited by the same metal ions. Nuclease-less mutants of S. aureus were isolated from S. aureus 209P by using N-methyl-N′-nitroso-N-nitrosoguanidine. These mutants contained neither surface-bound nor extracellular nuclease activity. These results suggest that the surface-bound and extracellular nucleases are expressed from the same cistron of S. aureus. 相似文献
50.
Stereospecific hydroxylation of 3-deoxy-1,2:5,6-di-O-isopropylidene-3-C-trans-and 3-C-cis-(methoxycarbonylmethylene)-α-D-ribo-hexofuranose (2 and 3, respectively), with potassium permanganate in pyridine afforded 3-C-[S- and R-hydroxy-(methoxycarbonyl)methyl]-1,2:5,6-di-O-isopropylidene-α-D-glucofuranose, (6 and 7, respectively), in a combined yield, after chromatography, of 43%. Selective formation of monomethanesulfonates (9a and 10a) and p-toluenesulfonates (9b and 10b), followed by treatment with sodium azide and reduction of the azide, afforded the methyl 2-D-(and 2-L-)(3-deoxy-1,2:5,6-di-O-isopropylidene-α-D-glucofuranos-3-yl)-glycinates (12a and 13a, respectively). Basic hydrolysis of the latter compounds yielded 2-D- and 2-L-(3-deoxy-1,2:5,6-di-O-isopropylidene-α-D-glucofuranos-3-yl)glycine (12b and 13b, respectively). The structures of the glycosyl amino acids were correlated with that of L-alanine by circular dichroism. 相似文献