全文获取类型
收费全文 | 7973篇 |
免费 | 894篇 |
国内免费 | 3篇 |
出版年
2021年 | 112篇 |
2020年 | 68篇 |
2019年 | 99篇 |
2018年 | 87篇 |
2017年 | 75篇 |
2016年 | 132篇 |
2015年 | 203篇 |
2014年 | 248篇 |
2013年 | 310篇 |
2012年 | 426篇 |
2011年 | 383篇 |
2010年 | 237篇 |
2009年 | 211篇 |
2008年 | 301篇 |
2007年 | 336篇 |
2006年 | 297篇 |
2005年 | 308篇 |
2004年 | 302篇 |
2003年 | 303篇 |
2002年 | 299篇 |
2001年 | 214篇 |
2000年 | 226篇 |
1999年 | 164篇 |
1998年 | 117篇 |
1997年 | 82篇 |
1996年 | 86篇 |
1995年 | 87篇 |
1994年 | 80篇 |
1993年 | 89篇 |
1992年 | 159篇 |
1991年 | 142篇 |
1990年 | 148篇 |
1989年 | 173篇 |
1988年 | 141篇 |
1987年 | 131篇 |
1986年 | 106篇 |
1985年 | 128篇 |
1984年 | 125篇 |
1983年 | 97篇 |
1982年 | 86篇 |
1981年 | 85篇 |
1980年 | 90篇 |
1979年 | 116篇 |
1978年 | 93篇 |
1977年 | 68篇 |
1976年 | 75篇 |
1975年 | 71篇 |
1974年 | 92篇 |
1973年 | 84篇 |
1972年 | 58篇 |
排序方式: 共有8870条查询结果,搜索用时 250 毫秒
61.
Fifteen classical linkage groups were identified in two salmonid species (Salmo trutta and Salmo gairdneri) and three fertile, interspecific hybrids (S. gairdneri X Salmo clarki, Salvelinus fontinalis X Salvelinus namaycush and S. fontinalis X Salvelinus alpinus) by backcrossing multiply heterozygous individuals. These linkage relationships of electrophoretically detected, protein coding loci were highly conserved among species. The loci encoding the enzymes appeared to be randomly distributed among the salmonid chromosomes. Recombination frequencies were generally greater in females than in males. In males, certain linkage groups were pseudolinked with other linkage groups, presumably because of facultative multivalent pairing and directed disjunction of chromosomes. Five such pseudolinkage groups were identified and they also appeared to be common among species and hybrids. Duplicate loci were never classically linked with each other, although some exhibited pseudolinkage and some showed evidence of exchanging alleles. Gene-centromere recombination frequencies estimated from genotypic distributions of gynogenetic offspring were consistent with map locations inferred from female intergenic recombination frequencies. These linkage relationships support the contention that all extant salmonids arose from a common tetraploid progenitor and that this progenitor may have been a segmental allotetraploid. 相似文献
62.
Cytogenetic analyses of a Salvelinus hybrid reveal an evolutionary relationship between the parental species 总被引:1,自引:0,他引:1
Mitotic analyses of the brook trout (Salvelinus fontinalis) x arctic char (S. alpinus) hybrids (sparctic trout) revealed a mode of 2n = 82 with 18 metacentric and 64 acrocentric chromosomes. The brook trout had 2n = 84 with 16 metacentric chromosomes and the arctic char had 2n = 80 with 20 metacentric chromosomes; both species are derivatives of a single tetraploid event. Variable multivalent-like configurations that may be centromeric associations of bivalents were observed in C-banded pachytene figures of female sparctic trout. Metaphase I analyses of sparctic trout males indicated that two fusions of nonhomologous acrocentric chromosomes representing two duplicated chromosome sets must have occurred in the arctic char after its evolutionary divergence from the brook trout. A mode of seven tetravalent rods per cell suggests that preferential multivalent pairing occurs in the sparctic hybrid; metaphase I analyses of S. alpinus males revealed a mode of only five tetravalent rods per cell. The presence of multivalents implies that the arctic char, like the brook trout, is still undergoing diploidization. Cytochemical detection of the nucleolar organizer region (NOR) revealed intra- and interspecific as well as intraindividual variability in the numbers and types of chromosomes (metacentric or acrocentric) on which NORs appeared in arctic char and sparctic trout. Brook trout only had NORs on acrocentric chromosomes. This may indicate that different chromosomal fusions occurred in the evolution of brook trout from arctic char. 相似文献
63.
G. C. Wright 《Plant and Soil》1989,116(1):111-114
Very little research has been done to investigate the effect of a dry podding zone on reproductive development in peanut plants
that are otherwise well hydrated via subsoil moisture extraction. The influence of podding zone moisture content on reproductive
development and growth of three peanut cultivars (McCubbin, Gajah and Robut 33-1) was investigated in pots grown in the glasshouse.
In two cultivars (McCubbin and Gajah) seed yield was reduced in a dry (air-dry) compared to a wet (field capacity) podding
zone. Seed yield of Robut 33-1 was unaffected by podding zone moisture content, indicating that cultivar variation in reproductive
performance in response to podding zone moisture may exist. 相似文献
64.
The highly fluorescent dye Lucifer Yellow CH (LYCH), now in common use in microinjection studies, has been shown to enter the vacuole of a range of plant-cell protoplasts from the external medium. Uptake was quantified by lysing the protoplasts following incubation and determining the amount of LYCH incorporated by spectrofluorimetry. Uptake was biphasic with respect to both time and substrate concentration, enhanced at low pH and inhibited by low temperature and metabolic inhibitors. The kinetics of uptake showed several similarities with those reported for the fluid-phase endocytosis of LYCH in animal cells and yeast cells. A calculated membrane permeability coefficient for LYCH, based on the observed rates of uptake, was too high to be consistent with simple diffusion of the undissociated form of the molecule and inconsistent with the membrane-impermeant properties of the dye. The data are discussed in the light of the possibility of fluid-phase endocytosis versus active transmembrane transport.Abbreviations CCCP
carbonyl cyanide M-chlorophenyl hydrazone
- LYCH
Lucifer Yellow CH 相似文献
65.
66.
Myogenin, a factor regulating myogenesis, has a domain homologous to MyoD 总被引:191,自引:0,他引:191
67.
The total glucose metabolism of 48-h spherical trophoblastic vesicles, Day-60 trophoblastic vesicles sections and Day-14 porcine blastocyst sections was measured by the method of O'Fallon and Wright (1). Trophoblastic vesicles were formed by enzyme dispersal in Day-14 porcine blastocysts. Glucose was based on DNA content of the tissue measured by diamino benzoic acid reaction with DNA (2). Slope of the lines (PMoles glucose utilized/4 h x DNA content) was different between Day-14 blastocyst sections and 48 h trophoblastic vesicles (P = 0.062) and between Day-14 blastocyst sections and Day-60 trophoblastic vesicles sections (P >/= 0.05). Slopes of the lines were identical between 48-h trophoblastic vesicles and Day-60 trophoblastic vesicles sections (P >/= 0.87). Average glucose utilization on a per ng DNA basis was calculated. Day-14 blastocyst sections utilized 0.67 Pmoles glucose/4 h per ng DNA, Day-60 trophoblastic vesicles sections; 0.57; and 48-h sperical trophoblastic vesicles used 0.29. It is hypothesized that the change in glucose utilization between the Day-14 porcine blastocyst and enzymatically formed trophoblastic vesicles may be due to a decrease in metabolism as a consequence of in vitro culture. Further, it is theorized that Day-60 trophoblastic vesicles sections used higher quantities of glucose than 48-h sperical trophoblastic vesicles on a per ng DNA basis due to the increased availability of glucose to the cells of the inner layers, caused by the sectioning of the tissue. The results of this study identify changes in glucose metabolism of enzymatically formed porcine trophoblastic vesicles during culture. It is proposed that enzymatically-formed trophoblastic vesicles be used as a model system for the study of embyro metabolism. 相似文献
68.
Eighty Holstein dairy cows were treated with 25 mg of prostaglandin F2alpha (PGF2alpha) on Days 14 to 16 post calving. Eighty-four herd mates served as saline controls in the double blind study. The reproductive parameters used to measure fertility were mean days to first service on all cows and mean days open, first service conception rates and services per pregnancy on cows that became pregnant. Mean days to first service was similar in both groups (71.8 +/- 27, treated; 68.5 +/- 28.6, controls; P = 0.5352). The mean days open was 98.6 +/- 52.0 d for treated cows compared to 118.8 +/- 71.2 d for controls (P = 0.0680). First service pregnancy rates (treated, 41.3%; control, 35.7%) were not significantly different (P = 0.0630); however, the mean services per pregnancy (treated, 1.64; control, 2.33) were significantly (P = 0.0021) improved in the treated group. Ten cows in the treated group and twelve cows in the control group were diagnosed as having retained fetal membranes and/or metritis. For treated and control cows mean days to first service were 82.2 +/- 34.8 d and 82.8 +/- 58.7 d (P = 0.5652). Days open were 97.0 +/- 32.5 d and 133.4 +/-58.4 d (P = 0.3636); services per conception were 1.83 and 2.50 (P = 0.3178), respectively. In all treated cows reproductive parameters were similar whether cows had high serum progesterone ( > 1 ng/ml) or low serum progesterone ( < 1 ng/ml) on the treatment day. This study suggests that early postpartum treatment of lactating dairy cows with prostaglandin produces an improvement in fertility. 相似文献
69.
Evolutionary conservation of homeodomain-binding sites and other sequences upstream and within the major transcription unit of the Drosophila segmentation gene engrailed. 总被引:8,自引:2,他引:6 下载免费PDF全文
J A Kassis C Desplan D K Wright P H O''''Farrell 《Molecular and cellular biology》1989,9(10):4304-4311
The engrailed (en) gene functions throughout Drosophila development and is expressed in a succession of intricate spatial patterns as development proceeds. Normal en function relies on an extremely large cis-acting regulatory region (70 kilobases). We are using evolutionary conservation to help identify en sequences important in regulating patterned expression. Sequence comparison of 2.6 kilobases upstream of the en coding region of D. melanogaster and D. virilis (estimated divergence time, 60 million years) showed that 30% of this DNA occurs in islands of near perfect sequence conservation. One of these conserved islands contains binding sites for homeodomain-containing proteins. It has been shown genetically that homeodomain-containing proteins regulate en expression. Our data suggested that this regulation may be direct. The remaining conserved islands may contain binding sites for other regulatory proteins. 相似文献
70.
Reversible cellular senescence: implications for immortalization of normal human diploid fibroblasts. 总被引:28,自引:6,他引:22 下载免费PDF全文
IMR-90 normal human diploid fibroblasts, transfected with a steroid inducible mouse mammary tumor virus-driven simian virus 40 T antigen, were carried through crisis to yield an immortal cell line. Growth was dependent on the presence of the inducer (dexamethasone) during both the extended precrisis life span of the cells and after immortalization. After dexamethasone removal, immortal cells divided once or twice and then accumulated in G1. These results are best explained by a two-stage model for cellular senescence. Mortality stage 1 (M1) causes a loss of mitogen responsiveness and arrest near the G1/S interface and can be bypassed or overcome by the cellular DNA synthesis-stimulating activity of T antigen. Mortality stage 2 (M2) is an independent mechanism that is responsible for the failure of cell division during crisis. The inactivation of M2 is a rare event, probably of mutational origin in human cells, independent of or only indirectly related to the expression of T antigen. Under this hypothesis, T-antigen-immortalized cells contain an active but bypassed M1 mechanism and an inactivated M2 mechanism. These cells are dependent on the continued expression of T antigen for the maintenance of immortality for the same reason that precrisis cells are dependent on T antigen for growth: both contain an active M1 mechanism. 相似文献