全文获取类型
收费全文 | 221篇 |
免费 | 9篇 |
国内免费 | 9篇 |
出版年
2021年 | 3篇 |
2019年 | 1篇 |
2018年 | 1篇 |
2017年 | 1篇 |
2016年 | 1篇 |
2015年 | 11篇 |
2014年 | 13篇 |
2013年 | 14篇 |
2012年 | 7篇 |
2011年 | 13篇 |
2010年 | 12篇 |
2009年 | 8篇 |
2008年 | 4篇 |
2007年 | 14篇 |
2006年 | 8篇 |
2005年 | 1篇 |
2004年 | 6篇 |
2003年 | 3篇 |
2002年 | 4篇 |
2001年 | 6篇 |
2000年 | 5篇 |
1999年 | 5篇 |
1998年 | 9篇 |
1997年 | 9篇 |
1996年 | 9篇 |
1995年 | 4篇 |
1994年 | 12篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1990年 | 4篇 |
1989年 | 1篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 1篇 |
1984年 | 5篇 |
1983年 | 5篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1977年 | 3篇 |
1955年 | 1篇 |
1954年 | 2篇 |
1949年 | 2篇 |
1948年 | 1篇 |
1937年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有239条查询结果,搜索用时 15 毫秒
161.
Restriction mapping and sequencing have shown that humans have
substantially lower levels of mitochondrial genome diversity (d) than
chimpanzees. In contrast, humans have substantially higher levels of
heterozygosity (H) at protein-coding loci, suggesting a higher level of
diversity in the nuclear genome. To investigate the discrepancy further, we
sequenced a segment of the mitochondrial genome control region (CR) from 49
chimpanzees. The majority of these were from the Pan troglodytes versus
subspecies, which was underrepresented in previous studies. We also
estimated the average heterozygosity at 60 short tandem repeat (STR) loci
in both species. For a total sample of 115 chimpanzees, d = 0.075 +/0
0.037, compared to 0.020 +/- 0.011 for a sample of 1,554 humans. The
heterozygosity of human STR loci is significantly higher than that of
chimpanzees. Thus, the higher level of nuclear genome diversity relative to
mitochondrial genome diversity in humans is not restricted to
protein-coding loci. It seems that humans, not chimpanzees, have an unusual
d/H ratio, since the ratio in chimpanzees is similar to that in other
catarrhines. This discrepancy in the relative levels of nuclear and
mitochondrial genome diversity in the two species cannot be explained by
differences in mutation rate. However, it may result from a combination of
factors such as a difference in the extent of sex ratio disparity, the
greater effect of population subdivision on mitochondrial than on nuclear
genome diversity, a difference in the relative levels of male and female
migration among subpopulations, diversifying selection acting to increase
variation in the nuclear genome, and/or directional selection acting to
reduce variation in the mitochondrial genome.
相似文献
162.
163.
164.
Charles H. Pickett Ryan Rodriguez - Present address: University of California Center for Comparative Medicine Davis CA USA Jim Brown Dominique Coutinot Kim A. Hoelmer - Present address: USDA ARS BIIR S. Chapel St. Newark DE USA Ulrich Kuhlmann Henri Goulet Michael D. Schwartz Peter B. Goodell 《Biocontrol Science and Technology》2007,17(3):261-272
Lygus hesperus Knight is native to the western United States and is a perennial pest of numerous crops in California. It is responsible for triggering the early season application of insecticides on cotton, Gossypium hirsutum L., and strawberries, Fragaria L. Despite several surveys conducted in alfalfa (Medicago sativa L.) grown in central California, nymphal parasitoids associated with L. hesperus and L. elisus have not been found. Two exotic parasitoids were released into California beginning in 1998. Peristenus relictus (Ruhte), formerly P. stygicus Loan, and P. digoneutis Loan were collected from several locations in southern Europe and released at up to six locations over a 6-year period. At the original release site in Sacramento, a 0.25-ha plot of alfalfa, parasitism by P. digoneutis and P. relictus combined increased from zero to 90%, 3 years after the last releases were made. Parasitoids have been recovered from vacant fields of weedy annuals within 2 km of this site. Recoveries at more southerly release sites in central California have been poor. 相似文献
165.
Eric D. Musselman Jake E. Cariello Warren M. Grill Nicole A. Pelot 《PLoS computational biology》2021,17(9)
Electrical stimulation and block of peripheral nerves hold great promise for treatment of a range of disease and disorders, but promising results from preclinical studies often fail to translate to successful clinical therapies. Differences in neural anatomy across species require different electrodes and stimulation parameters to achieve equivalent nerve responses, and accounting for the consequences of these factors is difficult. We describe the implementation, validation, and application of a standardized, modular, and scalable computational modeling pipeline for biophysical simulations of electrical activation and block of nerve fibers within peripheral nerves. The ASCENT (Automated Simulations to Characterize Electrical Nerve Thresholds) pipeline provides a suite of built-in capabilities for user control over the entire workflow, including libraries for parts to assemble electrodes, electrical properties of biological materials, previously published fiber models, and common stimulation waveforms. We validated the accuracy of ASCENT calculations, verified usability in beta release, and provide several compelling examples of ASCENT-implemented models. ASCENT will enable the reproducibility of simulation data, and it will be used as a component of integrated simulations with other models (e.g., organ system models), to interpret experimental results, and to design experimental and clinical interventions for the advancement of peripheral nerve stimulation therapies. 相似文献
166.
Gayathri Govindaraju CA. Jabeena Devadathan Valiyamangalath Sethumadhavan Nivethika Rajaram Arumugam Rajavelu 《Biochimica et Biophysica Acta (BBA) - Gene Regulatory Mechanisms》2017,1860(10):1047-1057
In eukaryotes, cytosine methylation regulates diverse biological processes such as gene expression, development and maintenance of genomic integrity. However, cytosine methylation and its functions in pathogenic apicomplexan protozoans remain enigmatic. To address this, here we investigated the presence of cytosine methylation in the nucleic acids of the protozoan Plasmodium falciparum. Interestingly, P. falciparum has TRDMT1, a conserved homologue of DNA methyltransferase DNMT2. However, we found that TRDMT1 did not methylate DNA, in vitro. We demonstrate that TRDMT1 methylates cytosine in the endogenous aspartic acid tRNA of P. falciparum. Through RNA bisulfite sequencing, we mapped the position of 5-methyl cytosine in aspartic acid tRNA and found methylation only at C38 position. P. falciparum proteome has significantly higher aspartic acid content and a higher proportion of proteins with poly aspartic acid repeats than other apicomplexan pathogenic protozoans. Proteins with such repeats are functionally important, with significant roles in host-pathogen interactions. Therefore, TRDMT1 mediated C38 methylation of aspartic acid tRNA might play a critical role by translational regulation of important proteins and modulate the pathogenicity of the malarial parasite. 相似文献
167.
168.
Lytton John Musselman 《The Botanical review》2002,68(4):474-487
More than any other taxonomic character, megaspores have been used in the genusIsoetes (known by the English common name of “quillwort”), despite the fallacy of a single-character taxonomy. Microspores, on the
other hand, have been largely neglected in taxonomic schemes. Like megaspores, terms for microspore ornamentation (also known
as “sculpturing”) have not been standardized. I examined microspore ornamentation, including both macroornamentation and microornamentation,
of 52 taxa from Africa, Asia, Australasia, Europe, North America, and South America with the scanning electron microscope.
Macroornamentation is discernible with light microscopy; microornamentation requires scanning electron microscopy. Ornately
sculptured spores were much more frequent than were laevigate or psilate patterns: 21 taxa had an echinate pattern; 19 had
an aculeate pattern; 6 were cristate; 5 were psilate; and 1 was laevigate. The proximal and distal ridges and surfaces may
vary in both the type and density of ornamentation. Distinct macroornamentation patterns characterize certain species groups.
Microornamentation types include granulate, bacillate, fimbriate, and filamentose: of the microspores I examined, virtually
all were partially granulate; 11 were bacillate; 4 were fimbriate; and 1 was filamentose. Based on this limited sampling,
species with a higher ploidy level often have larger microspores, but no clear relationship between microspore ornamentation
and ploidy level was established, nor were any geographical or ecological trends clear. Like megaspores, microspore ornamentation
is strongly convergent. Although microspores are often attached to megaspores, the role of spore ornamentation in coordinated
dispersal remains unclear. 相似文献
169.
170.
Some species of the genus Amanita are economically important gourmet mushrooms, while others cause dramatic poisonings or even deaths every year in China and in many other countries. A DNA barcode is a short segment or a combination of short segments of DNA sequences that can distinguish species rapidly and accurately. To establish a standard DNA barcode for poisonous species of Amanita in China, three candidate markers, the large subunit nuclear ribosomal RNA (nLSU), the internal transcribed spacer (ITS), and the translation elongation factor 1 alpha (tef1 α) were tested using the eukaryotic general primers for their feasibility as barcodes to identify seven species of lethal fungi and two species of edible ones which can easily be confused with the lethal ones known from China. In addition, A.phalloides—a European and North American species closely related to one of the seven taxa, A.subjunquillea was also included. PCR amplification and sequencing success rate, intra and inter specific variation and rate of species identification were considered as main criteria for evaluation of the candidate DNA barcodes. Although the nLSU had high PCR and sequencing success rates (100% and 100% respectively), occasional overlapping occurred between the intra and inter specific variations. The PCR amplification and sequencing success rates of ITS were 100% and 85.7% respectively. ITS showed high sequence variation among species group and low variation within a given species. There was a relatively high PCR amplification and sequencing success rate for tef1 α (85.7% and 100% respectively), and its intra and inter specific variation was higher than that of ITS or nLSU. All three candidate markers showed hight species resolution. ITS and tef1 α had a more clearly defined barcode gap than nLSU. Our study showed that the tef1 α and nLSU can be proposed as supplementary barcodes for the genus Amanita, while ITS can be used as a primary barcode marker considering that the ITS region may become a universal barcode marker for the fungal kingdom. 相似文献