全文获取类型
收费全文 | 1170篇 |
免费 | 127篇 |
国内免费 | 32篇 |
出版年
2022年 | 12篇 |
2021年 | 12篇 |
2020年 | 9篇 |
2019年 | 9篇 |
2018年 | 21篇 |
2017年 | 18篇 |
2016年 | 24篇 |
2015年 | 34篇 |
2014年 | 43篇 |
2013年 | 56篇 |
2012年 | 76篇 |
2011年 | 66篇 |
2010年 | 49篇 |
2009年 | 47篇 |
2008年 | 53篇 |
2007年 | 41篇 |
2006年 | 56篇 |
2005年 | 44篇 |
2004年 | 35篇 |
2003年 | 44篇 |
2002年 | 32篇 |
2001年 | 32篇 |
2000年 | 32篇 |
1999年 | 32篇 |
1998年 | 27篇 |
1997年 | 26篇 |
1996年 | 13篇 |
1995年 | 14篇 |
1994年 | 19篇 |
1993年 | 8篇 |
1992年 | 26篇 |
1991年 | 15篇 |
1990年 | 22篇 |
1989年 | 21篇 |
1988年 | 25篇 |
1987年 | 24篇 |
1986年 | 10篇 |
1984年 | 16篇 |
1983年 | 9篇 |
1982年 | 12篇 |
1981年 | 10篇 |
1980年 | 12篇 |
1979年 | 10篇 |
1977年 | 11篇 |
1976年 | 8篇 |
1975年 | 7篇 |
1973年 | 7篇 |
1972年 | 8篇 |
1971年 | 8篇 |
1970年 | 10篇 |
排序方式: 共有1329条查询结果,搜索用时 31 毫秒
91.
Phylogenetic Relationships of Acanthocephala Based on Analysis of 18S Ribosomal RNA Gene Sequences 总被引:3,自引:0,他引:3
García-Varela M Pérez-Ponce de León G de la Torre P Cummings MP Sarma SS Laclette JP 《Journal of molecular evolution》2000,50(6):532-540
Acanthocephala (thorny-headed worms) is a phylum of endoparasites of vertebrates and arthropods, included among the most
phylogenetically basal tripoblastic pseudocoelomates. The phylum is divided into three classes: Archiacanthocephala, Palaeacanthocephala,
and Eoacanthocephala. These classes are distinguished by morphological characters such as location of lacunar canals, persistence
of ligament sacs in females, number and type of cement glands in males, number and size of proboscis hooks, host taxonomy,
and ecology. To understand better the phylogenetic relationships within Acanthocephala, and between Acanthocephala and Rotifera,
we sequenced the nearly complete 18S rRNA genes of nine species from the three classes of Acanthocephala and four species
of Rotifera from the classes Bdelloidea and Monogononta. Phylogenetic relationships were inferred by maximum-likelihood analyses
of these new sequences and others previously determined. The analyses showed that Acanthocephala is the sister group to a
clade including Eoacanthocephala and Palaeacanthocephala. Archiacanthocephala exhibited a slower rate of evolution at the
nucleotide level, as evidenced by shorter branch lengths for the group. We found statistically significant support for the
monophyly of Rotifera, represented in our analysis by species from the clade Eurotatoria, which includes the classes Bdelloidea
and Monogononta. Eurotatoria also appears as the sister group to Acanthocephala.
Received: 12 October 1999 / Accepted: 8 February 2000 相似文献
92.
High rate of DNA loss in the Drosophila melanogaster and Drosophila virilis species groups 总被引:6,自引:3,他引:3
We recently proposed that patterns of evolution of non-LTR
retrotransposable elements can be used to study patterns of spontaneous
mutation. Transposition of non-LTR retrotransposable elements commonly
results in creation of 5' truncated, "dead-on-arrival" copies. These
inactive copies are effectively pseudogenes and, according to the neutral
theory, their molecular evolution ought to reflect rates and patterns of
spontaneous mutation. Maximum parsimony can be used to separate the
evolution of active lineages of a non-LTR element from the fate of the
"dead-on-arrival" insertions and to directly assess the relative
frequencies of different types of spontaneous mutations. We applied this
approach using a non-LTR element, Helena, in the Drosophila virilis group
and have demonstrated a surprisingly high incidence of large deletions and
the virtual absence of insertions. Based on these results, we suggested
that Drosophila in general may exhibit a high rate of spontaneous large
deletions and have hypothesized that such a high rate of DNA loss may help
to explain the puzzling dearth of bona fide pseudogenes in Drosophila. We
also speculated that variation in the rate of spontaneous deletion may
contribute to the divergence of genome size in different taxa by affecting
the amount of superfluous "junk" DNA such as, for example, pseudogenes or
long introns. In this paper, we extend our analysis to the D. melanogaster
subgroup, which last shared a common ancestor with the D. virilis group
approximately 40 MYA. In a different region of the same transposable
element, Helena, we demonstrate that inactive copies accumulate deletions
in species of the D. melanogaster subgroup at a rate very similar to that
of the D. virilis group. These results strongly suggest that the high rate
of DNA loss is a general feature of Drosophila and not a peculiar property
of a particular stretch of DNA in a particular species group.
相似文献
93.
94.
95.
Erin E. Cummings Linda P. O’Reilly Dale E. King Richard M. Silverman Mark T. Miedel Cliff J. Luke David H. Perlmutter Gary A. Silverman Stephen C. Pak 《PloS one》2015,10(10)
α1-antitrypsin deficiency (ATD) predisposes patients to both loss-of-function (emphysema) and gain-of-function (liver cirrhosis) phenotypes depending on the type of mutation. Although the Z mutation (ATZ) is the most prevalent cause of ATD, >120 mutant alleles have been identified. In general, these mutations are classified as deficient (<20% normal plasma levels) or null (<1% normal levels) alleles. The deficient alleles, like ATZ, misfold in the ER where they accumulate as toxic monomers, oligomers and aggregates. Thus, deficient alleles may predispose to both gain- and loss-of-function phenotypes. Null variants, if translated, typically yield truncated proteins that are efficiently degraded after being transiently retained in the ER. Clinically, null alleles are only associated with the loss-of-function phenotype. We recently developed a C. elegans model of ATD in order to further elucidate the mechanisms of proteotoxicity (gain-of-function phenotype) induced by the aggregation-prone deficient allele, ATZ. The goal of this study was to use this C. elegans model to determine whether different types of deficient and null alleles, which differentially affect polymerization and secretion rates, correlated to any extent with proteotoxicity. Animals expressing the deficient alleles, Mmalton, Siiyama and S (ATS), showed overall toxicity comparable to that observed in patients. Interestingly, Siiyama expressing animals had smaller intracellular inclusions than ATZ yet appeared to have a greater negative effect on animal fitness. Surprisingly, the null mutants, although efficiently degraded, showed a relatively mild gain-of-function proteotoxic phenotype. However, since null variant proteins are degraded differently and do not appear to accumulate, their mechanism of proteotoxicity is likely to be different to that of polymerizing, deficient mutants. Taken together, these studies showed that C. elegans is an inexpensive tool to assess the proteotoxicity of different AT variants using a transgenic approach. 相似文献
96.
Carlos Cepeda Damian M Cummings Véronique M André Sandra M Holley Michael S Levine 《ASN neuro》2010,2(2)
The discovery of the HD (Huntington’s disease) gene in 1993 led to the creation of genetic mouse models of the disease and opened the doors for mechanistic studies. In particular, the early changes and progression of the disease could be followed and examined systematically. The present review focuses on the contribution of these genetic mouse models to the understanding of functional changes in neurons as the HD phenotype progresses, and concentrates on two brain areas: the striatum, the site of most conspicuous pathology in HD, and the cortex, a site that is becoming increasingly important in understanding the widespread behavioural abnormalities. Mounting evidence points to synaptic abnormalities in communication between the cortex and striatum and cell–cell interactions as major determinants of HD symptoms, even in the absence of severe neuronal degeneration and death. 相似文献
97.
LEANDRO AMBRÓSIO CAMPOS HÉLIO RICARDO DA SILVA ANTONIO SEBBEN 《Biological journal of the Linnean Society. Linnean Society of London》2010,99(4):752-767
We describe the extra bony elements, plates, and osteoderms present in species of the genus Brachycephalus. Samples of eight species of Brachycephalus, including seven populations of Brachycephalus ephippium, were examined. The large additional elements associated with the skull (parotic plate) and vertebrae (vertebral and paravertebral plates) all comprise intramembranous bone, similar to that of the frontoparietal or nasal bones of the skull of most of frogs. Additionally, in the dermis of one unnamed species, we discovered and described true osteoderms. We discuss the morphological nature and diversity of theses elements and their importance as evidence of phylogenetic relationship within Brachycephalus. In summary, three distinct conditions of extra bony elements occur in the genus Brachycephalus: (1) bony plates may be present or absent in species of the genus; (2) a few, small bony plates may be developed and these may be represented by (a) paravertebral plates small and restricted to the distal ends of the transverse processes of the presacral IV, (b) parotic plates small and not covering the tops of the squamosals, and (c) ornamented spinal plates on all vertebrae; and (3) well‐developed bony plates may be present as (a) paravertebral plates forming a ‘bone‐shield’ on the dorsal surface of the trunk, ornamented, and visible through the integument, (b) parotic plates covering the tops of the squamosals, and (c) spinal plates associated with all vertebrae, and ornamented on vertebrate I–VI. Although the phenomenon of miniaturization may be associated with the appearance of new elements in at least some of the species in the genus, the traditional rule may not be universally applicable. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 752–767. 相似文献
98.
PATRICIA FERNANDES NERES VERÔNICA GOMES DA FONSECA‐GENEVOIS RODRIGO AUGUSTO TORRES MARIANA DA FONSECA CAVALCANTI FRANCISCO JOSÉ VICTOR DE CASTRO NEYVAN RENATO RODRIGUES DA SILVA TANIA TASSINARI RIEGER WILFRIDA DECRAEMER 《Zoological Journal of the Linnean Society》2010,158(1):1-15
A new species of Daptonema is described based upon morphological characters and 18S rRNA sequence. Daptonema matrona sp. nov. was collected in Pina Basin (north‐eastern Brazil). It differs from all other species of the genus by the presence of reduced cephalic setae and straight spicules. These features require an adaptation of the generic diagnosis. Moreover, the females are characterized by intra‐uterine development of the offspring, considered herein as their major autapomorphic feature. Molecular systematic analyses supported Daptonema matrona sp. nov. as a distinct genetic and evolutionary lineage. The data also indicate hypotheses of taxonomic synonymies amongst some related taxa from Xyalidae as well as the paraphyly of Daptonema. © 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 158 , 1–15. 相似文献
99.
Mitochondrial (Mt) DNA from Podospora anserina was isolated and characterized with respect to density in CsCl, contour length and endonuclease restriction enzymes. The density of Mt DNA for four races examined was 1.694 g/cm3, compared with 1.712 g/cm3 for nuclear DNA. Extraction in the presence of a nuclease inhibitor, aurintricarboxylic acid and isolation in DAPI CsCl gradients allowed us to isolate high molecular weight DNA. Mt DNA isolated by total DNA extraction contained ca. 1% of circular molecules, 31 micron in contour length; Mt DNA isolated from purified mitochondria contained 2--4% of these 31 micron circles. Analysis with Eco RI restriction endonuclease revealed that each of the four races examined, s, A, T and E had a characteristic fragment pattern. Races s and A Mt DNA differed by only one fragment after Eco RI enzymatic digestion; similarly, these two DNA differed by only one or two fragments after Hae III digestion. 相似文献
100.
Edward Saltzman Wendy Anderson Caroline M. Apovian Hannah Boulton Alison Chamberlain Diana Cullum-Dugan Susan Cummings Edward Hatchigian Barbara Hodges Christopher R. Keroack Mark Pettus Phyllis Thomason Linda Veglia Lorraine S. Young 《Obesity (Silver Spring, Md.)》2005,13(2):234-243
Objective: To provide evidence‐based guidelines for patient selection and to recommend the medical and nutritional aspects of multidisciplinary care required to minimize perioperative and postoperative risks in patients with severe obesity who undergo weight loss surgery (WLS). Research Methods and Procedures: Members of the Multidisciplinary Care Task Group conducted searches of MEDLINE and PubMed for articles related to WLS in general and medical and nutritional care in particular. Pertinent abstracts and literature were reviewed for references. Multiple searches were carried out for various aspects of multidisciplinary care published between 1980 and 2004. A total of 3000 abstracts were identified; 242 were reviewed in detail. Results: We recommended multidisciplinary screening of WLS patients to ensure appropriate selection; preoperative assessment for cardiovascular, pulmonary, gastrointestinal, endocrine, and other obesity‐related diseases associated with increased risk for complications or mortality; preoperative weight loss and cessation of smoking; perioperative prophylaxis for deep vein thrombosis and pulmonary embolism (PE); preoperative and postoperative education and counseling by a registered dietitian; and a well‐defined postsurgical diet progression. Discussion: Obesity‐related diseases are often undiagnosed before WLS, putting patients at increased risk for complications and/or early mortality. Multidisciplinary assessment and care to minimize short‐ and long‐term risks include: comprehensive medical screening; appropriate pre‐, peri‐, and postoperative preparation; collaboration with multiple patient care disciplines (e.g., anesthesiology, pulmonary medicine, cardiology, and psychology); and long‐term nutrition education/counseling. 相似文献