全文获取类型
收费全文 | 165636篇 |
免费 | 5862篇 |
国内免费 | 5658篇 |
专业分类
177156篇 |
出版年
2024年 | 155篇 |
2023年 | 1044篇 |
2022年 | 2424篇 |
2021年 | 4114篇 |
2020年 | 2646篇 |
2019年 | 3271篇 |
2018年 | 14161篇 |
2017年 | 12255篇 |
2016年 | 10244篇 |
2015年 | 5411篇 |
2014年 | 5882篇 |
2013年 | 6313篇 |
2012年 | 11055篇 |
2011年 | 18394篇 |
2010年 | 15214篇 |
2009年 | 11229篇 |
2008年 | 13269篇 |
2007年 | 14419篇 |
2006年 | 3201篇 |
2005年 | 2904篇 |
2004年 | 2940篇 |
2003年 | 2670篇 |
2002年 | 2184篇 |
2001年 | 1491篇 |
2000年 | 1388篇 |
1999年 | 1159篇 |
1998年 | 680篇 |
1997年 | 691篇 |
1996年 | 684篇 |
1995年 | 629篇 |
1994年 | 555篇 |
1993年 | 411篇 |
1992年 | 594篇 |
1991年 | 477篇 |
1990年 | 416篇 |
1989年 | 289篇 |
1988年 | 263篇 |
1987年 | 250篇 |
1986年 | 166篇 |
1985年 | 195篇 |
1984年 | 116篇 |
1983年 | 134篇 |
1982年 | 73篇 |
1981年 | 58篇 |
1980年 | 38篇 |
1979年 | 62篇 |
1974年 | 38篇 |
1973年 | 34篇 |
1972年 | 276篇 |
1971年 | 297篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
961.
Genetic variation and population structure in Portunus trituberculatus along the coast of China were revealed according to 617 bp of mitochondrial DNA control region. 90 polymorphic sites defined
53 distinct haplotypes, showing a moderately high diversity among 72 individuals sampled from eight localities. Neighbor-joining
tree, statistics analyses of gene flow and genetic differentiation index indicated two populations from Beihai and Laizhou
had differentiated. The population from Yingkou, Dandong, Laizhou and Beihai had smaller genetic diversity compared to that
from Ningbo, Lianyungang, Qingdao and Japan according to the genetic distance. And mantel test showed significant positive
correlation between genetic distance and geographic distance for P.
trituberculatus. TCS parsimony network suggested that all the animals sampled were probably the result of recent divergence from a common
ancestral haplotype but for Laizhou population. Moreover, the haplotype distribution appeared to correlate with a recent colonization
followed by localized genetic differentiation. Mismatch distribution results suggested that Ningbo, Yingkou, Qingdao, Lianyungang
and Japan populations, particularly Dandong population had experienced a sudden demographic or spatial expansion. The Pleistocene
glaciations might contribute to this process. 相似文献
962.
Santos ME das C L E Silva F Gomes KB Fernandes AP Freitas FR Faria MC Mota AP Carvalho MG 《Molecular biology reports》2011,38(5):3361-3366
Peripheral arterial disease (PAD) is an atherosclerotic disturbance characterized by a progressive obstruction of lower limb
arteries. Many risk factors associated with PAD development have being reported in the literature. The present study aimed
to investigate whether mutations in the methylenetetrahydrofolate reductase (MTHFR) or in the cystathionine beta synthase
(CBS) genes are associated with higher levels of homocysteine and the risk of PAD in patients from Brazil. This study analyzed
39 patients with PAD and 32 without PAD in whom risk factors and C677T mutations in the MTHFR gene and both 844ins68 and T833C
mutations in the CBS gene were investigated. Although higher levels of homocysteine could be observed in patients with PAD
compared to controls, no association between the increase of homocysteine and the frequency of C677T, 844ins68, and T833C
mutations could be observed. The results suggest that these mutations do not appear to be related to either homocysteine levels
or the development of the disease. However, hyperhomocysteinemia and smoking are important factors in PAD development. 相似文献
963.
The aim of this study was to investigate whether a moderate‐intensity static magnetic field (SMF) can enhance the killing effect of adriamycin (ADM) on K562 cells, and to explore the effects of SMF combined with ADM on K562 cells. We analyzed the metabolic activity of cells, cell cycle distribution, DNA damage, change in cell ultrastructure, and P‐glycoprotein (P‐gp) expression after K562 cells were exposed continuously to a uniform 8.8 mT SMF for 12 h, with or without ADM. Our results showed that the SMF combined with ADM (25 ng/ml) significantly inhibited the metabolic activity of K562 cells (P < 0.05), while neither ADM nor the SMF alone affected the metabolic activity of these cells. Cell ultrastructure was altered in the SMF + ADM group. For example, cell membrane was depressed, some protuberances were observable, and vacuoles in the cytoplasm became larger. Cells were arrested at the G2/M phase and DNA damage increased after cells were treated with the SMF plus ADM. ADM also induced the P‐gp expression. In contrast, in the SMF group and SMF + ADM group, the P‐gp expression was decreased compared with the ADM group. Taken together, our results showed that the 8.8 mT SMF enhanced the cytotoxity potency of ADM on K562 cells, and the decrease in P‐gp expression may be one reason underlying this effect. Bioelectromagnetics 32:191–199, 2011. © 2010 Wiley‐Liss, Inc. 相似文献
964.
利用重组大肠杆菌表达丝氨酸羟甲基转移酶(SHMT)和色氨酸酶(TPase),并利用双酶法合成L-色氨酸。采用PCR从大肠杆菌K12基因组中扩增上述两种酶的基因,利用pET-28a载体,构建单表达重组质粒pET-SHMT、pET-TPase和共表达重组质粒pET-ST。将上述3种重组质粒转入大肠杆菌BL21(DE3)进行表达。SDS-PAGE结果表明,单表达基因工程菌BL21(DE3)/pET-SHMT和BL21(DE3)/pET-TPase分别在47kDa(SHMT)和50kDa(TPase)处有蛋白表达带;共表达基因工程菌BL21(DE3)/pET-ST在上述两处均有蛋白表达带。与宿主菌相比,单表达SHMT基因工程菌产酶活性提高了6.4倍;单表达TPase基因工程菌产酶活性提高了8.4倍;共表达SHMT和TPase基因工程菌产酶活性分别提高了6.1和6.9倍。利用工程菌所产酶进行双菌双酶法和单菌双酶法合成L-色氨酸。两菌双酶合成L-色氨酸的累积量达到41.5g/L,甘氨酸转化率为83.3%,吲哚转化率为92.5%;单菌双酶合成L-色氨酸的累积量达到28.9g/L,甘氨酸转化率为82.7%,吲哚转化率为82.9%。 相似文献
965.
Naoki Kamada Kaori Tano Akiko Oyabu Yoshio Imura Naoko Narita Yasura Tashiro Atsuko Uchida Yoshihiro Komada Masaaki Narita 《International journal of peptide research and therapeutics》2010,16(2):55-61
We recently identified a novel 40-amino acid neuropeptide designated manserin from the rat brain (Yajima in NeuroReport 15:
1755–1759, 2004). Manserin is highly expressed in pituitary and hypothalamic nuclei, which suggests that it plays a role in
the endocrine system. In this study, we employed immunohistochemical methods to investigate the presence of manserin in rat
adrenal glands, as well as its regulation by physical stress. Immunohistochemical analysis using anti-manserin antibody showed
that manserin is present in the rat adrenal medulla but not in the cortex. When the colocalization of manserin and phenylethanolamine
N-methyltransferase (PNMT), an epinephrine-synthesizing enzyme, was examined, virtually all PNMT-positive cells expressed manserin.
Interestingly, the immunoreactivity of manserin was significantly increased when the rats were exposed to water-immersion
restraint stress. These results demonstrate for the first time that adrenal manserin, a novel neuropeptide, may have a potential
physiological role under stress-inducing conditions. 相似文献
966.
Junhui Zhou Dongdong Li Guoming Wang Fuxi Wang Merixia Kunjal Dirk Joldersma Zhongchi Liu 《植物学报(英文版)》2020,62(3):269-286
Fruit crops, including apple, orange, grape,banana, strawberry, watermelon, kiwifruit and tomato, not only provide essential nutrients for human life but also contribute to the major agricultural output and economic growth of many countries and regions in the world. Recent advancements in genome editing provides an unprecedented opportunity for the genetic improvement of these agronomically important fruit crops. Here, we summarize recent reports of applying CRISPR/Cas9 to fruit crops,including efforts to reduce disease susceptibility, change plant architecture or flower morphology, improve fruit quality traits, and increase fruit yield. We discuss challenges facing fruit crops as well as new improvements and platforms that could be used to facilitate genome editing in fruit crops, including d Cas9-base-editing to introduce desirable alleles and heat treatment to increase editing efficiency. In addition, we highlight what we see as potentially revolutionary development ranging from transgene-free genome editing to de novo domestication of wild relatives. Without doubt, we now see only the beginning of what will eventually be possible with the use of the CRISPR/Cas9 toolkit. Efforts to communicate with the public and an emphasis on the manipulation of consumerfriendly traits will be critical to facilitate public acceptance of genetically engineered fruits with this new technology. 相似文献
967.
Self-organization is a key condition to the success of community-based conservation initiatives, including those recognized
by the Equator Initiative of the UNDP. This paper contributes to emerging scholarship that focuses on community-based conservation
in South Asia and in particular examines self-organization strategies within a small-scale community-based conservation initiative
in a cross-cultural setting to further understanding about how such initiatives originate, sustain and grow. This is achieved
through a case study of the Baripada Forest Protection Initiative in India by utilizing in-depth interviews and focus groups.
In addition to certain often-cited strategies for self-organization, the Baripada initiative included unique features of self-organization
such as village community design, implementation and adaptation of rules for local natural resources use and conservation,
little need for financial support, and significant mobilization of human resources. These strategies, along with emerging
social learning opportunities (e.g., a community plant diversity register) inspired by the Baripada initiative, inform and
enrich the criteria for designing and evaluating conservation and development initiatives, irrespective of their scale. 相似文献
968.
969.
970.
The N-glycosylation mutants (mnn1 and mnn1 och1) show different morphological characteristics at the restrictive and nonpermissive temperature. We deleted the MNN1 to eliminate the terminal α1, 3-linked mannose of hypermannosylation and deleted the OCH1 to block the elongation of the main backbone chain. The mnn1 cells exhibited no observable change with respect to the wild-type strain at 28°C and 37°C, but the mnn1 och1 double mutant exhibited defects in cell cytokinesis, showed a slower growth rate, and became temperature-sensitive. Meanwhile,
the mnn1 och1 mutant tended to aggregate, which was probably due to the glycolsylation defect. Loss of mannosyl-phosphate-accepting sites
in this mutant migth result in reduced charge repulsion between cell surfaces. Pyridylaminated glycans were profiled and purified
through an NH2 column by size-fractionation high-performance liquid chromatography. Matrix assisted laser desoption/ionization time of flight
mass spectrometry (MALDI TOF/MS) analysis of the N-glycan structure of the mnn1 och1 mutant revealed that the main component is Man8GlcNAc2. 相似文献