全文获取类型
收费全文 | 644篇 |
免费 | 102篇 |
出版年
2021年 | 8篇 |
2020年 | 5篇 |
2019年 | 8篇 |
2018年 | 7篇 |
2017年 | 10篇 |
2016年 | 15篇 |
2015年 | 25篇 |
2014年 | 19篇 |
2013年 | 25篇 |
2012年 | 27篇 |
2011年 | 31篇 |
2010年 | 14篇 |
2009年 | 23篇 |
2008年 | 19篇 |
2007年 | 28篇 |
2006年 | 16篇 |
2005年 | 24篇 |
2004年 | 21篇 |
2003年 | 24篇 |
2002年 | 18篇 |
2001年 | 21篇 |
2000年 | 10篇 |
1999年 | 21篇 |
1998年 | 19篇 |
1997年 | 14篇 |
1996年 | 23篇 |
1995年 | 13篇 |
1994年 | 8篇 |
1993年 | 13篇 |
1992年 | 14篇 |
1991年 | 9篇 |
1990年 | 12篇 |
1989年 | 7篇 |
1988年 | 12篇 |
1987年 | 5篇 |
1986年 | 8篇 |
1985年 | 16篇 |
1984年 | 8篇 |
1983年 | 9篇 |
1982年 | 7篇 |
1979年 | 6篇 |
1978年 | 8篇 |
1977年 | 9篇 |
1976年 | 7篇 |
1975年 | 9篇 |
1974年 | 7篇 |
1972年 | 9篇 |
1956年 | 7篇 |
1948年 | 4篇 |
1930年 | 4篇 |
排序方式: 共有746条查询结果,搜索用时 15 毫秒
1.
2.
The major albumin, a polypeptide of 21 kilodaltons (kDa), from the seeds of cocoa (Theobroma cacao L.), has been identified and partially purified by preparative gel electrophoresis. Some N-terminal sequence was obtained, permitting the construction of an oligonucleotide probe. This probe was used to isolate the corresponding copy DNA (cDNA) clone from a library made from poly(A)+ RNA from immature cocoa beans. The cDNA sequence has a single major open reading frame, that translates to give a 221-amino-acid polypeptide of Mr 24003. The existence of a precursor to the 21-kDa polypeptide of this size was confirmed by immunoprecipitation from total poly(A)+ RNA translation products. The polypeptide has a hydrophobic signal sequence of 26 amino acids before the mature start, and the mature polypeptide would have an Mr of 21223. The protein sequence is homologous with sequences of the Kunitz protease and -amylase inhibitor family, and the protein probably functions to defend the seed's protein reserves from the digestive enzymes of invading pests. However because the protein comprises 25–30% of the total seed protein it may itself also function as a storage protein. Electron micrographs of immunogold-labelled embryo sections show that the protein is located in membrane-enclosed organelles.Abbreviations cDNA
copy DNA
- IgG
immunoglobulin G
- kb
kilobase pairs
- kDa
kilodaltons
- Mr
relative molecular mass
- SDS-PAGE
sodium dodecyl sulphate-polyacylamide gel electrophoresis
The authors are very grateful to Dr R. Jennings of the Virology Department, Sheffield University Medical School, for help in raising antibodies, and to Dr G. Cope, of the Biological Sciences Electron Microscopy Unit, Sheffield University, for taking the electron micrographs.To whom correspondence should be addressed. 相似文献
3.
Conditioning on subsets of the data: applications to ascertainment and other genetic problems. 总被引:3,自引:3,他引:0 下载免费PDF全文
S E Hodge 《American journal of human genetics》1988,43(4):364-373
I here consider the question of when to formulate a likelihood over the whole data set, as opposed to conditioning the likelihood on subsets of the data (i.e., joint vs. conditional likelihoods). I show that when certain conditions are met, these two likelihoods are guaranteed to be equivalent, and thus that it is generally preferable to condition on subsets, since that likelihood is mathematically and computationally simpler. However, I show that when these conditions are not met, conditioning on subsets of the data is equivalent to introducing additional df into our genetic model, df that we may not have been aware of. I discuss the implications of these facts for ascertainment corrections and other genetic problems. 相似文献
4.
The effects of a known family-size distribution on the estimation of genetic parameters. 总被引:1,自引:1,他引:0 下载免费PDF全文
We consider the question: In a segregation analysis, can knowledge of the family-size distribution (FSD) in the population from which a sample is drawn improve the estimators of genetic parameters? In other words, should one incorporate the population FSD into a segregation analysis if one knows it? If so, then under what circumstances? And how much improvement may result? We examine the variance and bias of the maximum likelihood estimators both asymptotically and in finite samples. We consider Poisson and geometric FSDs, as well as a simple two-valued FSD in which all families in the population have either one or two children. We limit our study to a simple genetic model with truncate selection. We find that if the FSD is completely specified, then the asymptotic variance of the estimator may be reduced by as much as 5%-10%, especially when the FSD is heavily skewed toward small families. Results in small samples are less clear-cut. For some of the simple two-valued FSDs, the variance of the estimator in small samples of one- and two-child families may actually be increased slightly when the FSD is included in the analysis. If one knows only the statistical form of the FSD, but not its parameter, then the estimator is improved only minutely. Our study also underlines the fact that results derived from asymptotic maximum likelihood theory do not necessarily hold in small samples. We conclude that in most practical applications it is not worth incorporating the FSD into a segregation analysis. However, this practice may be justified under special circumstances where the FSD is completely specified, without error, and the population consists overwhelmingly of small families. 相似文献
5.
A comparative description of mitochondrial DNA differentiation in selected avian and other vertebrate genera 总被引:14,自引:1,他引:13
Levels of mitochondrial DNA (mtDNA) sequence divergence between species
within each of several avian (Anas, Aythya, Dendroica, Melospiza, and
Zonotrichia) and nonavian (Lepomis and Hyla) vertebrate genera were
compared. An analysis of digestion profiles generated by 13-18 restriction
endonucleases indicates little overlap in magnitude of mtDNA divergence for
the avian versus nonavian taxa examined. In 55 interspecific comparisons
among the avian congeners, the fraction of identical fragment lengths (F)
ranged from 0.26 to 0.96 (F = 0.46), and, given certain assumptions, these
translate into estimates of nucleotide sequence divergence (p) ranging from
0.007 to 0.088; in 46 comparisons among the fish and amphibian congeners, F
values ranged from 0.00 to 0.36 (F = 0.09), yielding estimates of P greater
than 0.070. The small mtDNA distances among avian congeners are associated
with protein-electrophoretic distances (D values) less than approximately
0.2, while the mtDNA distances among assayed fish and amphibian congeners
are associated with D values usually greater than 0.4. Since the
conservative pattern of protein differentiation previously reported for
many avian versus nonavian taxa now appears to be paralleled by a
conservative pattern of mtDNA divergence, it seems increasingly likely that
many avian species have shared more recent common ancestors than have their
nonavian taxonomic counterparts. However, estimates of avian divergence
times derived from mtDNA- and protein-calibrated clocks cannot readily be
reconciled with some published dates based on limited fossil remains. If
the earlier paleontological interpretations are valid, then protein and
mtDNA evolution must be somewhat decelerated in birds. The empirical and
conceptual issues raised by these findings are highly analogous to those in
the long-standing debate about rates of molecular evolution and times of
separation of ancestral hominids from African apes.
相似文献
6.
Methods for computing the standard errors of branching points in an evolutionary tree and their application to molecular data from humans and apes 总被引:23,自引:2,他引:21
Statistical methods for computing the standard errors of the branching
points of an evolutionary tree are developed. These methods are for the
unweighted pair-group method-determined (UPGMA) trees reconstructed from
molecular data such as amino acid sequences, nucleotide sequences,
restriction-sites data, and electrophoretic distances. They were applied to
data for the human, chimpanzee, gorilla, orangutan, and gibbon species.
Among the four different sets of data used, DNA sequences for an
895-nucleotide segment of mitochondrial DNA (Brown et al. 1982) gave the
most reliable tree, whereas electrophoretic data (Bruce and Ayala 1979)
gave the least reliable one. The DNA sequence data suggested that the
chimpanzee is the closest and that the gorilla is the next closest to the
human species. The orangutan and gibbon are more distantly related to man
than is the gorilla. This topology of the tree is in agreement with that
for the tree obtained from chromosomal studies and DNA-hybridization
experiments. However, the difference between the branching point for the
human and the chimpanzee species and that for the gorilla species and the
human-chimpanzee group is not statistically significant. In addition to
this analysis, various factors that affect the accuracy of an estimated
tree are discussed.
相似文献
7.
DNA sequence analysis of a 5.27-kb direct repeat occurring adjacent to the regions of S-episome homology in maize mitochondria. 总被引:8,自引:1,他引:7 下载免费PDF全文
J P Houchins H Ginsburg M Rohrbaugh R M Dale C L Schardl T P Hodge D M Lonsdale 《The EMBO journal》1986,5(11):2781-2788
The DNA sequence of the 5270-bp repeated DNA element from the mitochondrial genome of the fertile cytoplasm of maize has been determined. The repeat is a major site of recombination within the mitochondrial genome and sequences related to the R1(S1) and R2(S2) linear episomes reside immediately adjacent to the repeat. The terminal inverted repeats of the R1 and R2 homologous sequences form one of the two boundaries of the repeat. Frame-shift mutations have introduced 11 translation termination codons into the transcribed S2/R2 URFI gene. The repeated sequence, though recombinantly active, appears to serve no biological function. 相似文献
8.
Growth of Chikungunya Virus in Baby Hamster Kidney Cell (BHK-21-Clone 13) Suspension Cultures 总被引:2,自引:2,他引:0 下载免费PDF全文
This report describes the methods used to obtain high titers of chikungunya virus with suspension cultures of BHK-21-clone 13 cells. The cells were grown at 37 C to a cell concentration of 10(6) to 2 x 10(6) per ml. After maximum cell growth, the cells were inoculated with chikungunya virus at a multiplicity of 1 to 2 50% suckling mouse intracerebral lethal doses (SMICLD(50)) per cell in the spent Eagle's minimum essential medium for suspension cultures (MEMS), or the cell cultures were centrifuged at 200 x g and resuspended in either fresh MEMS or medium 199 prior to inoculation. The medium used had no effect on virus titer. The inoculated cultures were incubated at 34 C until the cell viability dropped to 30%, which usually occurred 28 to 30 hr postinoculation. After these procedures, chikungunya virus titers of log(10) 10.3 to 11.8 SMICLD(50) per ml were obtained. 相似文献
9.
The Habsburg jaw 总被引:3,自引:0,他引:3
10.