全文获取类型
收费全文 | 3215篇 |
免费 | 223篇 |
国内免费 | 406篇 |
出版年
2023年 | 37篇 |
2022年 | 46篇 |
2021年 | 67篇 |
2020年 | 71篇 |
2019年 | 108篇 |
2018年 | 96篇 |
2017年 | 79篇 |
2016年 | 84篇 |
2015年 | 100篇 |
2014年 | 135篇 |
2013年 | 169篇 |
2012年 | 107篇 |
2011年 | 109篇 |
2010年 | 128篇 |
2009年 | 138篇 |
2008年 | 180篇 |
2007年 | 146篇 |
2006年 | 162篇 |
2005年 | 184篇 |
2004年 | 154篇 |
2003年 | 142篇 |
2002年 | 151篇 |
2001年 | 104篇 |
2000年 | 116篇 |
1999年 | 86篇 |
1998年 | 74篇 |
1997年 | 61篇 |
1996年 | 55篇 |
1995年 | 76篇 |
1994年 | 54篇 |
1993年 | 53篇 |
1992年 | 55篇 |
1991年 | 44篇 |
1990年 | 35篇 |
1989年 | 55篇 |
1988年 | 54篇 |
1987年 | 51篇 |
1986年 | 37篇 |
1985年 | 46篇 |
1984年 | 44篇 |
1983年 | 33篇 |
1982年 | 28篇 |
1981年 | 24篇 |
1980年 | 16篇 |
1979年 | 17篇 |
1978年 | 9篇 |
1977年 | 11篇 |
1976年 | 8篇 |
1975年 | 3篇 |
1974年 | 1篇 |
排序方式: 共有3844条查询结果,搜索用时 31 毫秒
21.
Leo S. Melchers Dave V. Thompson Ken B. Idler Saskia T. C. Neuteboom Ruud A. de Maagd Rob A. Schilperoort Paul J. J. Hooykaas 《Plant molecular biology》1988,11(2):227-237
The virulence loci play an essential role in tumor formation by Agrobacterium tumefaciens. Induction of vir gene expression by plant signal molecules is solely dependent on the virulence loci virA and virG. This study focused on the virA locus of the octopine type Ti plasmid pTi15955. The nucleic acid sequence of a 5.7-kilobase fragment encompassing virA was determined. Genetic analysis of this region revealed that virA contains one open reading frame coding for a protein of 91 639 daltons. Immunodetection with antibodies raised against a 35-kDa VirA fusion protein produced in E. coli identified the VirA product in wild-type Agrobacterium cells. Moreover, it is shown that the VirA protein is located in the cytoplasmic membrane fraction of Agrobacterium. These data confirm the proposed regulatory function of VirA whereby VirA acts as a membrane sensor protein to identify plant signal molecules in the environment. The proposed sensory function of VirA strikingly resembles the function of the chemotaxis receptor proteins of E. coli. 相似文献
22.
Roger J. Kemble Tina L. Barsby Stephen A. Yarrow 《Molecular & general genetics : MGG》1988,213(2-3):202-205
Summary
Brassica napus cybrid plants which contain novel nucleus-mitochondria-chloroplast combinations have been constructed, via protoplast fusion. Such fusions resulted in mitochondrial DNA plasmids being lost (at a frequency of 12.5%) or, more surprisingly, being transferred from mitochondria of one protoplast population to mitochondria of the other population (at a frequency of 6.1%). Mitochondria containing their new DNA complement became the dominant organelle population in regenerated plants and were faithfully maternally inherited through successive sexnal generations. No concomitant alterations in mitochondrial chromosome organization or nuclear chromosome number occurred. Protoplast fusion can, therefore, cure plant mitochondria of extrachromosomal DNA and, more importantly, be used to transform plant mitochondria with naturally occurring mitochondrial plasmids. The potential for mitochondrial transformation with recombinant vectors is discussed. 相似文献
23.
B. Rosén C. Halldén W. K. Heneen 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1988,76(2):197-203
Summary Five somatic hybrids between Brassica campestris and B. oleracea were obtained. Molecular, morphological and cytological information all suggest that the resynthesized B. napus plants were hybrids. All five plants were diploid (2n=38) and had mainly bivalents at meiosis. Seedset was low after selfing but normal after crossing with B. napus. Molecular proof of the hybrid nature of these plants was obtained by hybridization of a rDNA repeat to total DNA. Analysis of chloroplast DNA restriction patterns revealed that all hybrids had chloroplasts identical to the B. oleracea parent. The analysis of mitochondrial DNA indicated that three hybrids had restriction patterns identical to those of B. campestris, and the other two had restriction patterns similar to those of B. oleracea. The 11.3 kb plasmid present in mitochondria of the B. campestris parent was also found in mitochondria of all five hybrids. This suggests that the plasmid from a B. campestris type of mitochondria was transferred into mitochondria of a B. oleracea type. 相似文献
24.
Summary Somatic hybrid plants were recovered following fusion of leaf mesophyll protoplasts isolated from tomato (Lycopersicon esculentum) cultivar UC82 with protoplasts isolated from suspension cultured cells of L. chilense, LA 1959. Iodoacetate was used to select against the growth of unfused tomato protoplasts. Two somatic hybrids were recovered in a population of 16 regenerants. No tomato regenerants were recovered; all of the non-hybrid regenerants were L. chilense. The L. chilense protoplast regenerants were tetraploid. The hybrid nature of the plants was verified using species-specific restriction fragment length polymorphisms for the nuclear, chloroplast and mitochondrial genomes. The somatic hybrids had inherited the chloroplast DNA of the tomato parent, and portions of the mitochondrial DNA of the L. chilense parent. The somatic hybrids formed flowers and developed seedless fruit. 相似文献
25.
Ellen M. Johnson Linda S. Schnabelrauch Barbara B. Sears 《Molecular & general genetics : MGG》1991,225(1):106-112
Summary A bovine tRNA gene cluster has been characterized and the sequences of four tDNAs determined. Two of the tDNAs could encode tRNASer
IGA, one tDNASer
UGA, and the fourth tRNAGln
CUG. The three serine tDNAs representing the UCN codon isoacceptor family are almost identical. However, the sequence of the tDNASer
TGA differs from a previously sequenced bovine tDNASer
TGA at 12 positions (ca. 14%). This finding suggests that in the bovine genome, two subfamilies of genes might encode tRNASer
UGA. It also raises the possibility that new genes for a specific UCN isoacceptor might arise from the genes of a different isoacceptor, and could explain previously observed differences between species in the anticodons of coevolving pairs of tRNAsSer
UCN. The gene cluster also contains complete and partial copies, and fragments, of the BCS (bovine consensus sequence) SINE (short interspersed nuclear element) family, six examples of which were sequenced. Some of these elements occur in close proximity to two of the serine tDNAs. 相似文献
26.
Pedro Labarca Daniel Wolff Ubaldo Soto Cecilia Necochea Federico Leighton 《The Journal of membrane biology》1986,94(3):285-291
Summary Fusion of a highly purified fraction of rat liver peroxisomal membranes to planar lipid bilayers incorporates large, cation-selective voltage-dependent pores. TheP
K/P
Cl ratio of these pores, estimated in KCl gradients, is close to 4. The pores display several conductance states and spend most of the time open at voltages near 0 mV, closing at more positive and negative voltages. At voltages near 0 mV the most frequent open state has a conductance of 2.4 nS in 0.3m KCl. At voltages more positive and more negative than 10 mV the most frequent open state displays a conductance of 1.2 nS in 0.3m KCl. With these results pore diameters of 3 and 1.5 nm, respectively, can be estimated. We suggest that these pores might account for the unusually high permeability of peroxisomes to low molecular weight solutes. Fusion also incorporates a perfectly anion-selective, two-open states channel with conductances of 50 and 100 pS in 0.1m KCl. 相似文献
27.
Carmelo B. Bruni M. Stella Carlomagno Silvestro Formisano Giovanni Paolella 《Molecular & general genetics : MGG》1986,203(3):389-396
Summary A detailed comparative analysis of the Escherichia coli and Salmonella typhimurium hisIE and hisD gene products and the functionally equivalent, single, HIS4 gene product of Saccharomyces cerevisiae permitted several insights concerning the relationship between these genes. Our analysis supports the idea that HIS4 results from the fusion of hisIE and hisD. The comparison permitted a more precise definition of the functional domains of hisI/HIS4A and hisE/HIS4B as well as the two functional domains of hisD/HIS4C. The homologies between the bacterial and yeast sequences suggest a region of the hisD/HIS4C protein that may constitute one of the active centres. A large fragment at the amino terminal region of the yeast protein is missing from the bacterial hisIE gene product and is probably not needed for catalytic activity. Another region of non-homology in the yeast protein is probably a peptide bridge connecting the HIS4AB domain to HIS4C. Although the overall homology at the level of amino acid sequence is modest (about 38%) there is a striking similarity when the hydropathic patterns and predicted secondary structural configurations of these proteins are compared. 相似文献
28.
Sperm-oocyte membrane fusion has been observed during monospermic fertilization of a human oocyte in vitro. Women were stimulated with both clomiphene citrate and human menopausal gonadotropin and were given human chorionic gonadotropin before a LH-surge. Twelve oocytes, collected at laparoscopy from six women who became pregnant by IVF, were allowed to mature for 7–14 hours in vitro and inseminated with preincubated sperm, fixed between 1–3 hours after insemination, and examined by transmission electron microscopy. Membrane fusion had occurred in one ovum 2 hours after insemination, and the oocyte had resumed maturation and was at anaphase II of meiosis. Cortical granules had been exocytosed, and some of their contents were visible at the surface close to the oolemma all around the oocyte. The sperm that fused with this oocyte was acrosome-reacted and had been partly incorporated into the ooplasm, while the anterior two-thirds of its head was phagocytosed by a tongue of cortical ooplasm. Membrane fusion had occurred between the oolemma and the plasma membrane overlying the postacrosomal segment of the sperm head, posterior to the equatorial vestige. Sperm chromatin had not decondensed, and serial sections revealed a midpiece attached to the basal plate and a tail located deeper in the ooplasm, all devoid of plasma membrane. Supplementary sperm penetrating the inner zona, approaching the perivitelline space, had undergone the acrosome reaction but had a persistent vestige of the equatorial segment of the acrosome with intact plasma membrane. Evidence of sperm chromatin decondensation was seen in other oocytes, 3 hours after insemination, which were at telophase II of meiosis. Eight oocytes penetrated by sperm were monospermic, while four were unfertilized. The general pattern of sperm fusion and incorporation appears to conform to that seen in most other mammals. The study also reveals that sperm have to complete the acrosome reaction before fusing with the egg. 相似文献
29.
Summary Factors affecting the polyethylene glycol (PEG)-induced membrane fusion were examined. Human erythrocyte membrane ghosts, cytoskeleton-free vesicles budded from erythrocytes, mechanically disrupted erythrocyte vesicles, and recombinant vesicles from glycophorin and egg phosphatidylcholine were used as models. Fusion was monitored by darkfield light microscopy and by freeze-fracture electron microscopy. Osmotic swelling was found necessary for fusion between membrane ghosts following PEG treatment. The sample with the highest fusion percentage was sealed ghosts incubated in hypotonic media after at least 5 min of treatment in <25% PEG. At similar osmolarity, glycerol, dextran and PEG produced progressively more pronounced intramembranous particle (IMP) patching, correlating with their increasing fusion percentages. The patching of IMP preceded cell-cell contact, and occurred without direct PEG-protein interaction. The presence of cytoskeletal elements in small vesicles had no significant effect on fusion, nor on the aggregation of intramembranous particle (IMP) upon PEG treatment. Disrupting the membrane by lysolecithin, dimethylsulfoxide, retinol or mild sonication resulted in the fragmentation of ghosts without an increase in fusion percentage. The purity of the commercial PEG used had no apparent effect on fusion. We concluded that the key steps in PEG-induced fusion of cell membrane are the creation of IMP-free zones, and the osmotic swelling of cells after the formation of bilayer contacts during the PEG treatment. Cell cytoskeleton affects PEG-induced fusion only to the extent of affecting IMP patching. 相似文献
30.
Summary During an earlier investigation, microtubules were observed at the periphery of invasion processes in the developing syncytial tapetum ofTradescantia virginiana L. They were also associated with membranous sacs that accumulate adjacent to tetrads, with putative fusion sites where the tapetal plasmodium is initiated, and, in postmeiotic stages, with the perispore membrane that encloses the developing spore cells. Colchicine was administered to developing flower buds to investigate the roles of these microtubules. The results indicate that microtubules neither initiate nor guide the tapetal invasion of the loculus. The treatments, however, resulted in absence of cell coat from invasion processes and prevention of cell fusion. They also inhibited polarized migration of membrane sacs and removed the associated microtubules. The development of an organized secretory apparatus at the perispore membrane was disrupted, with subsequent disordered deposition of sporopollenin in the extracellular spaces of the partially-fused plasmodium. The results suggest that microtubules participate in the formation and internal spatial organization of the tapetal plasmodium, and establishment of a secretory surface that normally produces sporopollenin at the tapetum-microspore interface. 相似文献