全文获取类型
收费全文 | 16212篇 |
免费 | 1317篇 |
国内免费 | 2篇 |
出版年
2023年 | 41篇 |
2022年 | 130篇 |
2021年 | 272篇 |
2020年 | 124篇 |
2019年 | 219篇 |
2018年 | 185篇 |
2017年 | 201篇 |
2016年 | 377篇 |
2015年 | 623篇 |
2014年 | 740篇 |
2013年 | 891篇 |
2012年 | 1233篇 |
2011年 | 1295篇 |
2010年 | 833篇 |
2009年 | 693篇 |
2008年 | 1093篇 |
2007年 | 1139篇 |
2006年 | 959篇 |
2005年 | 959篇 |
2004年 | 932篇 |
2003年 | 946篇 |
2002年 | 846篇 |
2001年 | 142篇 |
2000年 | 103篇 |
1999年 | 147篇 |
1998年 | 203篇 |
1997年 | 148篇 |
1996年 | 123篇 |
1995年 | 119篇 |
1994年 | 116篇 |
1993年 | 124篇 |
1992年 | 119篇 |
1991年 | 104篇 |
1990年 | 90篇 |
1989年 | 77篇 |
1988年 | 76篇 |
1987年 | 68篇 |
1986年 | 66篇 |
1985年 | 91篇 |
1984年 | 102篇 |
1983年 | 84篇 |
1982年 | 84篇 |
1981年 | 101篇 |
1980年 | 90篇 |
1979年 | 54篇 |
1978年 | 55篇 |
1977年 | 61篇 |
1976年 | 49篇 |
1975年 | 32篇 |
1973年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
Drosophila Centrosomin Protein is Required for Male Meiosis and Assembly of the Flagellar Axoneme 总被引:1,自引:0,他引:1 下载免费PDF全文
Kaijun Li Eugene Yujun Xu Jeffrey K. Cecil F. Rudolf Turner Timothy L. Megraw Thomas C. Kaufman 《The Journal of cell biology》1998,141(2):455-467
Centrosomes and microtubules play crucial roles during cell division and differentiation. Spermatogenesis is a useful system for studying centrosomal function since it involves both mitosis and meiosis, and also transformation of the centriole into the sperm basal body. Centrosomin is a protein localized to the mitotic centrosomes in Drosophila melanogaster. We have found a novel isoform of centrosomin expressed during spermatogenesis. Additionally, an anticentrosomin antibody labels both the mitotic and meiotic centrosomes as well as the basal body. Mutational analysis shows that centrosomin is required for spindle organization during meiosis and for organization of the sperm axoneme. These results suggest that centrosomin is a necessary component of the meiotic centrosomes and the spermatid basal body. 相似文献
972.
973.
Deborah Low Jeffrey A. Ahlgren Diane Horne Donald J. McMahon Craig J. Oberg Jeffery R. Broadbent 《Applied and environmental microbiology》1998,64(6):2147-2151
Recent work by our group has shown that an exopolysaccharide (EPS)-producing starter pair, Streptococcus thermophilus MR-1C and Lactobacillus delbrueckii subsp. bulgaricus MR-1R, can significantly increase moisture retention in low-fat mozzarella (D. B. Perry, D. J. McMahon, and C. J. Oberg, J. Dairy Sci. 80:799–805, 1997). The objectives of this study were to determine whether MR-1C, MR-1R, or both of these strains are required for enhanced moisture retention and to establish the role of EPS in this phenomenon. Analysis of low-fat mozzarella made with different combinations of MR-1C, MR-1R, and the non-EPS-producing starter culture strains S. thermophilus TA061 and Lactobacillus helveticus LH100 showed that S. thermophilus MR-1C was responsible for the increased cheese moisture level. To investigate the role of the S. thermophilus MR-1C EPS in cheese moisture retention, the epsE gene in this bacterium was inactivated by gene replacement. Low-fat mozzarella made with L. helveticus LH100 plus the non-EPS-producing mutant S. thermophilus DM10 had a significantly lower moisture content than did cheese made with strains LH100 and MR-1C, which confirmed that the MR-1C capsular EPS was responsible for the water-binding properties of this bacterium in cheese. Chemical analysis of the S. thermophilus MR-1C EPS indicated that the polymer has a novel basic repeating unit composed of d-galactose, l-rhamnose, and l-fucose in a ratio of 5:2:1.Lactic acid bacteria (LAB) are a diverse group of industrially important, gram-positive, non-spore-forming microbes that produce lactic acid as a major product of carbohydrate fermentation. Many strains of LAB produce extracellular polysaccharides which may be tightly associated with the bacterial cell wall as capsules or liberated into the growth medium as a loose slime (5). The term exopolysaccharide (EPS) has been used to refer to either type of external polysaccharide. EPSs may be homopolysaccharides, composed of a single type of sugar monomer, or heteropolysaccharides, containing several types of sugar monomers (25). Extracellular homopolysaccharides are made by such LAB as Leuconostoc mesenteroides and Streptococcus mutans, while extracellular heteropolysaccharides are produced by several other species of LAB, including Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus (6).The ability to produce EPS is unstable in LAB and may be lost following numerous transfers, prolonged periods of storage, or incubation at temperatures above that optimal for growth (6, 24). This instability of EPS production in mesophilic LAB has been attributed to the fact that the genes involved in polymer production are plasmid encoded. In contrast, genes for EPS production in thermophilic LAB, such as S. thermophilus and L. delbrueckii subsp. bulgaricus, are believed to be chromosomally encoded (6). Consequently, the unstable nature of the EPS phenotype in thermophilic strains is not understood, but it may be related to mobile genetic elements or genomic instability (24).Because of the ability of EPSs to act as viscosifying, stabilizing, or water-binding agents in various foods, these polymers can act as effective natural alternatives to commercial stabilizers (6). For example, EPS-producing (EPS+) LAB are commonly used as starter cultures for yogurt manufacture because EPS improves the viscosity and texture of yogurt and decreases its susceptibility to syneresis (loss of whey from the curd) (14, 28).Analysis of cheese microstructure has shown that in full-fat or part-skim mozzarella, the fat and a large portion of the water are located within channels that are formed by fat globules when the cheese curd is heated and stretched (18, 20). In low-fat mozzarella, however, there are very few fat globules to break up the protein matrix, resulting in less space for water retention (20). As a consequence, the cheese has a tough and rubbery texture and requires more heat for melting (19). Merrill et al. showed that procedures which increased moisture levels in reduced- and low-fat mozzarella improved the body, texture, and functional properties of the cheese (19). In addition to enhanced functionality, the ability to increase cheese moisture level (even by as little as 1%) gives processors an important economic advantage in the highly competitive mozzarella industry (27).Since EPS has the capacity to bind significant amounts of water, it was the hypothesis of our group that EPS+ LAB may be useful for the production of reduced- and low-fat mozzarella. Work by Perry et al. (21) recently showed that an EPS+ starter pair, S. thermophilus MR-1C and L. delbrueckii subsp. bulgaricus MR-1R, could be used to significantly increase moisture levels in low-fat mozzarella. The objectives of this study were to determine whether MR-1C, MR-1R, or both of these strains are required for enhanced moisture retention and to establish the role of EPS in this phenomenon. The results showed that S. thermophilus MR-1C was responsible for the increased cheese moisture level and demonstrated that this effect required the bacterium’s capsular EPS.(Part of this research was presented at the 92nd Annual Meeting of the American Dairy Science Association, Guelph, Ontario, Canada, 22 to 25 June 1997.) 相似文献
974.
Identification of Highly Attenuated Mutants of Simian Immunodeficiency Virus 总被引:16,自引:11,他引:5 下载免费PDF全文
Ronald C. Desrosiers Jeffrey D. Lifson James S. Gibbs Susan C. Czajak Anita Y. M. Howe Larry O. Arthur R. Paul Johnson 《Journal of virology》1998,72(2):1431-1437
Deletion mutants of the pathogenic clone of simian immunodeficiency virus isolate 239 (SIVmac239) were derived that are missing nef, vpr, and upstream sequences (US) in the U3 region of the LTR (SIVmac239Δ3), nef, vpx, and US (SIVmac239Δ3x), and nef, vpr, vpx, and US (SIVmac239Δ4). These multiply deleted derivatives replicated well in the continuously growing CEMx174 cell line and were infectious for rhesus monkeys. However, on the basis of virus load measurements, strength of antibody responses, and lack of disease progression, these mutants were highly attenuated. Measurements of cell-associated viral load agreed well with assays of plasma viral RNA load and with the strengths of the antibody responses; thus, these measurements likely reflected the extent of viral replication in vivo. A derivative of SIVmac239 lacking vif sequences (SIVmac239Δvif) could be consistently grown only in a vif-complementing cell line. This Δvif virus appeared to be very weakly infectious for rhesus monkeys on the basis of sensitive antibody tests only. The weak antibody responses elicited by SIVmac239Δvif were apparently in response to low levels of replicating virus since they were not elicited by heat-inactivated virus and the anti-SIV antibody responses persisted for greater than 1 year. These results, and the results of previous studies, allow a rank ordering of the relative virulence of nine mutant strains of SIVmac according to the following order: Δvpr > Δvpx > ΔvprΔvpx Δnef > Δ3 > Δ3x ≥ Δ4 > Δvif > Δ5. The results also demonstrate that almost any desired level of attenuation can be achieved, ranging from still pathogenic in a significant proportion of animals (Δvpr and Δvpx) to not detectably infectious (Δ5), simply by varying the number and location of deletions in these five loci. 相似文献
975.
The Epstein-Barr Virus BARF1 Gene Encodes a Novel, Soluble Colony-Stimulating Factor-1 Receptor 总被引:6,自引:1,他引:5 下载免费PDF全文
Laura D. Strockbine Jeffrey I. Cohen Terry Farrah Stewart D. Lyman Felecia Wagener Robert F. DuBose Richard J. Armitage Melanie K. Spriggs 《Journal of virology》1998,72(5):4015-4021
Epstein-Barr virus (EBV) is a ubiquitous herpesvirus associated with infectious mononucleosis and several tumors. The BARF1 gene is transcribed early after EBV infection from the BamHI A fragment of the EBV genome. Evidence shown here indicates that the BARF1 protein is secreted into the medium of transfected cells and from EBV-carrying B cells induced to allow lytic replication of the virus. Expression cloning identified colony-stimulating factor-1 (CSF-1) as a ligand for BARF1. Computer-assisted analyses indicated that subtle amino acid sequence homology exists between BARF1 and c-fms, the cellular proto-oncogene that is the receptor for CSF-1. Recombinant BARF1 protein was found to be biologically active, and it neutralized the proliferative effects of human CSF-1 in a dose-dependent fashion when assayed in vitro. Since CSF-1 is a pleiotropic cytokine best known for its differentiating effects on macrophages, these data suggest that BARF1 may function to modulate the host immune response to EBV infection. 相似文献
976.
Expression of the Oligodendrocyte-Myelin Glycoprotein by Neurons in the Mouse Central Nervous System 总被引:13,自引:2,他引:11
Amyn A. Habib Linda S. Marton Brenda Allwardt Jeffrey R. Gulcher Daniel D. Mikol Thorbergur Högnason †Naibedya Chattopadhyay ‡Kári Stefansson 《Journal of neurochemistry》1998,70(4):1704-1711
Abstract: The oligodendrocyte-myelin glycoprotein (OMgp) is a 110-kDa glycosylphosphatidylinositol-linked protein that was initially identified as a myelin-specific protein but whose precise function remains unknown. In this study, immunohistochemistry, western blots, in situ hybridization, and northern blots were used to determine the distribution of OMgp in the mouse brain. OMgp is present in a concentration detectable on western blots in the brains of newborn mice, and its concentration gradually increases until day 24 of life. OMgp mRNA is also present in amounts detectable on northern blots in the brains of newborn mice, and its concentration gradually increases until day 21 of life, after which the concentration diminishes a little. Most of the OMgp in the mouse brain appears to be expressed in diverse groups of neurons, but it is particularly prominent in large projection neurons such as the pyramidal cells of the hippocampus, the Purkinje cells of the cerebellum, motoneurons in the brainstem, and anterior horn cells of the spinal cord. However, OMgp is not confined to these cells and is expressed in cells in the white matter as well. The OMgp gene is placed within an intron of the neurofibromatosis type I gene and on the opposite strand. This organization raises the possibility that there may be a relationship between the functions of the products of the two genes. In support of this possibility, we show that within the mouse CNS OMgp and neurofibromin are expressed in the same cell types. 相似文献
977.
Despite abundant library resources for many organisms, physical mapping of these organisms has been seriously limited due to lack of efficient library screening techniques. We have developed a highly efficient strategy for large-scale screening of genomic libraries based on multiplex oligonucleotide hybridization on high-density genomic filters. We have applied this strategy to generate a bacterial artificial chromosome (BAC) anchored map of mouse chromosome 11. Using the MIT mouse SSLP data, 320 pairs of oligonucleotide probes were designed with an “overgo” computer program that selects new primer sequences that avoid the microsatellite repeat. BACs identified by these probes are automatically anchored to the chromosome. Ninety-two percent of the probes identified positive clones from a 5.9-fold coverage mouse BAC library with an average of 7 positive clones per marker. An average of 4.2 clones was confirmed for 204 markers by PCR. Our data show that a large number of clones can be efficiently isolated from a large genomic library using this strategy with minimal effort. This strategy will have wide application for large-scale mapping and sequencing of human and other large genomes. 相似文献
978.
Jeffrey A. Pogany Ephraim J. Simon Rebecca B. Katzman Bernadette M. De Guzman Lita Po Yu Amy E. Trotochaud Steven E. Clark 《Journal of plant research》1998,111(2):307-313
New organs are initiated throughout the life span of higher plants. This process occurs at the shoot meristem, which is initiated
during embryogenesis and is later responsible for generating the above-ground portion of the plant. The shoot meristem can
be thought of as having two zones, a central zone containing meristematic cells in an undifferentiated state, and a surrounding
peripheral zone where cells enter a specific developmental pathway toward a differentiated state. Recent advances have revealed
several genes that specifically regulate meristem development inArabidopsis. However, extensive mutagenesis by several labs have identified only a handful, of loci that appear to specifically regulate
shoot meristem development. We have undertaken an enhancer/suppressor mutagenesis of an existing meristem mutant (clv1) and have identified novel regulators of meristem development.
The extended abstract of a paper presented at the 13th International Symposium in Conjugation with Award of the International
Prize for Biology “Frontier of Plant Biology” 相似文献
979.
Jeffrey J. Wine Damjan Glavač Gregory Hurlock Clare Robinson Margaret Lee Uros Potočnik Metka Ravnik-Glavač Michael Dean 《Mammalian genome》1998,9(4):301-305
Cystic fibrosis is a common human genetic disease caused by mutations in CFTR, a gene that codes for a chloride channel that
is regulated by phosphorylation and cytosolic nucleotides. As part of a program to discover natural animal models for human
genetic diseases, we have determined the genomic sequence of CFTR in the Rhesus monkey, Macaca mulatta. The coding region of rhesus CFTR is 98.3% identical to human CFTR at the nucleotide level and 98.2% identical and 99.7% similar at the amino acid level. Partial
sequences of flanking introns (5582 base pair positions analyzed) revealed 91.1% identity with human introns. Relative to
rhesus intronic sequence, the human sequences had 27 insertions and 22 deletions. Primer sequences for amplification of rhesus
genomic CFTR sequences are provided. The accession number is AF013753 (all 27 exons and some flanking intronic sequence).
Received: 27 August 1992 / Accepted: 5 December 1997 相似文献
980.
Ulrike Teichmann Michael E. Ray Jane Ellison Caroline Graham Graeme Wistow Paul S. Meltzer Jeffrey M. Trent William J. Pavan 《Mammalian genome》1998,9(9):715-720
We report the isolation of the murine ortholog of AIM1, a human gene whose expression is associated with the reversal of
tumorigenicity in an experimental model of melanoma. Mouse and human AIM1 are more than 90% identical in amino acid sequence
in the βγ-crystallin repeats and the C-terminal domain, and more than 75% identical in the extended N-terminal domain. Consistent
with the isolated cDNA representing the authentic AIM1 ortholog, linkage analysis localized mouse Aim1 to proximal mouse Chromosome (Chr) 10 in a conserved linkage group with genes localized to human Chr band 6q21. Searches
of EST databases identified a second AIM1-like gene in both mouse and human, suggesting the existence of a gene family. Northern
analysis demonstrates Aim1 is expressed most abundantly in adult skin, lung, heart, liver, and kidney and is temporally regulated during embryogenesis.
Aim1 is expressed highly in the shaft region of the hair follicles and the presumptive ectoderm, but not at detectable levels
in melanocytes or melanocyte precursor cells.
Received: 18 February 1998 / Accepted: 8 May 1998 相似文献