全文获取类型
收费全文 | 2853篇 |
免费 | 279篇 |
国内免费 | 1篇 |
出版年
2023年 | 16篇 |
2022年 | 29篇 |
2021年 | 73篇 |
2020年 | 46篇 |
2019年 | 56篇 |
2018年 | 75篇 |
2017年 | 70篇 |
2016年 | 78篇 |
2015年 | 149篇 |
2014年 | 163篇 |
2013年 | 220篇 |
2012年 | 232篇 |
2011年 | 239篇 |
2010年 | 133篇 |
2009年 | 116篇 |
2008年 | 192篇 |
2007年 | 189篇 |
2006年 | 161篇 |
2005年 | 136篇 |
2004年 | 131篇 |
2003年 | 103篇 |
2002年 | 111篇 |
2001年 | 22篇 |
2000年 | 31篇 |
1999年 | 27篇 |
1998年 | 21篇 |
1997年 | 12篇 |
1996年 | 8篇 |
1995年 | 10篇 |
1994年 | 7篇 |
1993年 | 14篇 |
1992年 | 17篇 |
1991年 | 12篇 |
1990年 | 10篇 |
1989年 | 13篇 |
1988年 | 14篇 |
1987年 | 16篇 |
1985年 | 11篇 |
1984年 | 12篇 |
1983年 | 10篇 |
1982年 | 10篇 |
1981年 | 8篇 |
1979年 | 13篇 |
1976年 | 8篇 |
1975年 | 10篇 |
1974年 | 9篇 |
1973年 | 13篇 |
1972年 | 11篇 |
1970年 | 9篇 |
1969年 | 9篇 |
排序方式: 共有3133条查询结果,搜索用时 468 毫秒
1.
Sexually selected infanticide is an important source of infant mortality in many mammalian species. In species with long-term male-female associations, females may benefit from male protection against infanticidal outsiders. We tested whether mountain gorilla (Gorilla beringei beringei) mothers in single and multi-male groups monitored by the Dian Fossey Gorilla Fund’s Karisoke Research Center actively facilitated interactions between their infants and a potentially protective male. We also evaluated the criteria mothers in multi-male groups used to choose a preferred male social partner. In single male groups, where infanticide risk and paternity certainty are high, females with infants <1 year old spent more time near and affiliated more with males than females without young infants. In multi-male groups, where infanticide rates and paternity certainty are lower, mothers with new infants exhibited few behavioral changes toward males. The sole notable change was that females with young infants proportionally increased their time near males they previously spent little time near when compared to males they had previously preferred, perhaps to encourage paternity uncertainty and deter aggression. Rank was a much better predictor of females’ social partner choice than paternity. Older infants (2–3 years) in multi-male groups mirrored their mothers’ preferences for individual male social partners; 89% spent the most time in close proximity to the male their mother had spent the most time near when they were <1 year old. Observed discrepancies between female behavior in single and multi-male groups likely reflect different levels of postpartum intersexual conflict; in groups where paternity certainty and infanticide risk are both high, male-female interests align and females behave accordingly. This highlights the importance of considering individual and group-level variation when evaluating intersexual conflict across the reproductive cycle. 相似文献
2.
Frankia is the diverse bacterial genus that fixes nitrogen within root nodules of actinorhizal trees and shrubs. Systematic and ecological
studies of Frankia have been hindered by the lack of morphological, biochemical, or other markers to readily distinguish strains. Recently,
nucleotide sequence of 16 S RNA from the small ribosomal subunit has been used to classify and identify a variety of microorganisms.
We report nucleotide sequences from portions of the 16 S ribosomal RNA from Frankia strains AcnI1 isolated from Alnus viridis ssp. crispa (Ait.) Turrill and PtI1 isolated from Purshia tridentata (Pursh) DC. The number of nucleotide base substitutions and gaps we find more than doubles the previously reported sequence
diversity for the same variable regions within other strains of Frankia. 相似文献
3.
4.
Salil K. Niyogi Thomas S. Soper Robert S. Foote Frank W. Larimer Richard J. Mural Sankar Mitra Eva H. Lee Richard Machanoff Fred C. Hartman 《Journal of biosciences》1987,11(1-4):203-214
Both Lys-166 and His-291 of ribulosebisphosphate carboxylase/oxygenase fromRhodospirillum rubrum have been implicated as the active-site residue that initiates catalysis. To decide between these two candidates, we resorted
to site-directed mutagenesis to replace Lys-166 and His-291 with several amino acids. All 7 of the position-166 mutants tested
are severely deficient in carboxylase activity, whereas the alanine and serine mutants at position 291 are ∼40% and ∼18% as
active as the native carboxylase, essentially ruling out His-291 in theRhodospirillum rubrum carboxylase (and by inference His-298 in the spinach enzyme) as a catalytically essential residue. The ability of some of
the mutant proteins to undergo carbamate formation or to bind either ribulosebisphosphate or a transition-state analogue remains
largely unimpaired. This implies that Lys-166 is not required for substrate binding; rather, the results corroborate the earlier
postulate that Lys-166 functions as an acid-base group in catalysis or in stabilizing a transition state in the reaction pathway. 相似文献
5.
R. Sankar P. S. Devamanoharan G. Raghupathi M. Krishnasamy C. S. Shyamala Devi 《Journal of biosciences》1987,12(3):267-271
Plumbagin was administered to rats at a concentration of 1,2,4,8 and 16 mg per kg body weight. After 24 h lipid peroxide levels
were found to decrease in subcellular fractions of liver. Plumbagin inhibited ascorbate and nicotinafde adenine dinucleotide
phosphate (reduced) dependent lipid peroxidation but was without any effect on cumene hydroperoxide dependent lipid peroxidation.
Injection of 16 mg of plumbagin per kg body weight was found to decrease liver total reduced glutathione and also fcrosomal
glucose-6-phosphatase. The results are discussed with reference to the anti- and prooxidant properties of plumbagin. 相似文献
6.
Forty cases of lymphoma were categorized as Burkitt-type lymphoma in a study of fine needle aspiration (FNA) smears. These constituted 14.3% of all cases of non-Hodgkin's lymphoma diagnosed between 1974 and 1982. The median age was 22 years in these cases, 81.8% of which had extranodal tumors. The majority of the cells in the smears (59.8% +/- 8.32%) were in the 11 micron to 15 micron size range and 60.3% +/- 10.3% had noncleaved nuclei. An average 71% of the cells contained cytoplasmic and/or nuclear vacuolizations. Nonneoplastic macrophages were present in the smears in 87.5% of the cases. A study of paraffin-embedded sections in 17 cases revealed the characteristic "starry-sky" appearance in 11; in 5 it was not clearly appreciated and in 1 the nonneoplastic macrophages were absent. FNA cytology was found to be quite reliable for arriving at a diagnosis of Burkitt-type lymphoma. More than 50% of the cases were managed without resort to subsequent surgical biopsy. Exploratory laparotomy was avoided in 69% of the cases having abdominal tumors. 相似文献
7.
Genetic mechanisms of tumor-specific loss of 11p DNA sequences in Wilms tumor. 总被引:12,自引:7,他引:5
下载免费PDF全文
![点击此处可从《American journal of human genetics》网站下载免费的PDF全文](/ch/ext_images/free.gif)
D D Dao W T Schroeder L Y Chao H Kikuchi L C Strong V M Riccardi S Pathak W W Nichols W H Lewis G F Saunders 《American journal of human genetics》1987,41(2):202-217
Wilms tumor, a common childhood renal tumor, occurs in both a heritable and a nonheritable form. The heritable form may occasionally be attributed to a chromosome deletion at 11p13, and tumors from patients with normal constitutional chromosomes often show deletion or rearrangement of 11p13. It has been suggested that a germinal or somatic mutation may occur on one chromosome 11 and predispose to Wilms tumor and that a subsequent somatic genetic event on the normal homologue at 11p13 may permit tumor development. To study the frequency and mechanism of such tumor-specific genetic events, we have examined the karyotype and chromosome 11 genotype of normal and tumor tissues from 13 childhood renal tumor patients with different histologic tumor types and associated clinical conditions. Tumors of eight of the 12 Wilms tumor patients, including all viable tumors examined directly, show molecular evidence of loss of 11p DNA sequences by somatic recombination (four cases), chromosome loss (two cases), and recombination (two cases) or chromosome loss and duplication. One malignant rhabdoid tumor in a patient heterozygous for multiple 11p markers did not show any tumor-specific 11p alteration. These findings confirm the critical role of 11p sequences in Wilms tumor development and reveal that mitotic recombination may be the most frequent mechanism by which tumors develop. 相似文献
8.
P C Joshi C Carraro M A Pathak 《Biochemical and biophysical research communications》1987,142(1):265-274
The role of reactive oxygen (1O2 and O2-.) in skin photosensitization and tanning reaction has been examined. Riboflavin (RF), hematoporphyrin (HP), 3-carbethoxypsoralen (3-CP), and 8-methoxypsoralen (8-MOP), upon photoexcitation under aerobic conditions, produced singlet O2 (1O2). RF, 3-CP, and 8-MOP also produced superoxide anion (O2-.). Reactive O2 produced by photosensitized RF, 3-CP, and 8-MOP was found to oxidize tyrosine and dopa to dopachrome and subsequently their conversion to melanin. HP did not oxidize tyrosine to dopachrome, and 3-CP and RF revealed substantial oxidation of tyrosine. Dopa was oxidized to dopachrome and subsequently to melanin by all photosensitizers tested at a variable rate as follows: RF greater than 3-CP greater than HPD greater than 8-MOP. UVA alone and to a lesser extent UVB also produced 1O2 which induced the oxidation of tyrosine and dopa to dopachrome and subsequently to melanin. The production of dopachrome was higher with dopa compared to tyrosine under all irradiation conditions. These observations appear to have relevance to the O2-requiring immediate tanning reaction of the skin stimulated by solar radiation and in the induction of skin photosensitization. 相似文献
9.
S.P. Pathak J.W. Bhattacherjee N. Kalra S. Chandra 《Journal of applied microbiology》1988,65(4):347-352
The seasonal distribution of Aeromonas hydrophila in water and recovery rate from live river fish was investigated. The highest isolation rates of A. hydrophila occurred in water during the late winter followed by a progressive decline in density during the summer and monsoon seasons. The organism was recovered from fish throughout the period from which it was concluded that they form a reservoir which is unrelated to their density in water. The enterotoxigenicity of some environmental strains was tested in suckling mice and rabbit ileal loop. 相似文献
10.
Sixty-nine nonmetrical morphological variants of the cranium have been studied in six samples of non-tribal, state populations
in India, and their incidence reported. Using C.A.B. Smith’s angular transformation of frequencies, the multivariate Thetasquare
distances and their respective standard deviations have been presented. On the basis of nonmetrical cranial variation, it
is clear that the samples from Uttar Pradesh, Andhra Pradesh and Bihar are closer to each other but distant from Madhya Pradesh,
Karnataka and Maharashtra. On the other hand, Karnataka and Maharashtra samples are quite close to each other and both, in
turn, are comparatively closer to Madhya Pradesh than to Uttar Pradesh, Andhra Pradesh and Bihar. Madhya Pradesh sample emerges
as the most divergent group among the six population samples studied. This, in general, is in conformity with the picture
that emerges from various analysis of morphometric and other biological data on various populations of India. 相似文献