全文获取类型
收费全文 | 2588篇 |
免费 | 433篇 |
出版年
2021年 | 28篇 |
2017年 | 26篇 |
2016年 | 39篇 |
2015年 | 57篇 |
2014年 | 62篇 |
2013年 | 86篇 |
2012年 | 103篇 |
2011年 | 97篇 |
2010年 | 74篇 |
2009年 | 63篇 |
2008年 | 104篇 |
2007年 | 84篇 |
2006年 | 91篇 |
2005年 | 88篇 |
2004年 | 91篇 |
2003年 | 83篇 |
2002年 | 95篇 |
2001年 | 85篇 |
2000年 | 81篇 |
1999年 | 77篇 |
1998年 | 41篇 |
1997年 | 46篇 |
1996年 | 44篇 |
1995年 | 38篇 |
1994年 | 27篇 |
1993年 | 29篇 |
1992年 | 48篇 |
1991年 | 57篇 |
1990年 | 68篇 |
1989年 | 62篇 |
1988年 | 44篇 |
1987年 | 57篇 |
1986年 | 57篇 |
1985年 | 73篇 |
1984年 | 49篇 |
1983年 | 60篇 |
1981年 | 27篇 |
1980年 | 28篇 |
1979年 | 39篇 |
1978年 | 38篇 |
1977年 | 28篇 |
1976年 | 31篇 |
1975年 | 32篇 |
1974年 | 29篇 |
1973年 | 27篇 |
1972年 | 37篇 |
1971年 | 43篇 |
1970年 | 34篇 |
1969年 | 27篇 |
1967年 | 30篇 |
排序方式: 共有3021条查询结果,搜索用时 227 毫秒
51.
J M Williamson E Inamine K E Wilson A W Douglas J M Liesch G Albers-Sch?nberg 《The Journal of biological chemistry》1985,260(8):4637-4647
Radioactive- and stable isotope-containing substrates were used to identify the biosynthetic precursors of the beta-lactam antibiotic, thienamycin, in Streptomyces cattleya. Acetate is utilized by the organism to form C(6) and C(7) of the beta-lactam ring. The two carbons of the hydroxyethyl group attached to C(6) are both derived from the methyl of methionine. The cysteaminyl side chain attached to C(2) is derived from cysteine. Selective inhibition of thienamycin and cephamycin C biosynthesis has been achieved either through the addition of metabolic inhibitors or through manipulation of the growth medium. These results suggest that the two beta-lactam antibiotics, thienamycin and cephamycin C, are formed by different biosynthetic pathways. 相似文献
52.
Southdown ewes in mid-seasonal anoestrus were exposed to rams for 0 h (control group), 2 h, 24 h, 40 h, 3 days, 10 days or 20 days. Serial blood samples were then taken to determine LH and FSH levels. Ewes with greater than 24 h ram exposure were ovariectomized immediately after bleeding, and all follicles greater than 1 mm diameter were dissected from the ovaries and measured. LH basal concentrations and pulse frequency increased significantly within 2 h of ram introduction, but by 24 h fell, and then remained low. FSH concentrations fell within 2 h of ram introduction and remained low. Control group ewes (isolated) had no follicles greater than 4 mm diameter, whereas all ewes exposed to rams had large follicles, with CL or preovulatory follicles present at 40 h after ram introduction. Ram introduction was also associated with follicle recruitment (antrum formation to less than 2 mm). Follicular recruitment and development to the large follicle stage therefore occurred during a period of low plasma gonadotrophin levels and suppressed LH pulsing. 相似文献
53.
The gene coding for coagulation factor X was studied in a family segregating chromosomal abnormalities involving chromosomes 13 and 6. An individual monosomic for 13q34 was deficient in levels of clotting factors VII and X, while her brother, who is trisomic for 13q34, had elevated levels. DNA dosage studies with a cloned human factor X gene demonstrated that the low levels of factor X expression in the individual with the chromosome 13q34 deletion were due to the absence of one copy of the factor X structural gene. This confirms the assignment of the human gene coding for factor X to 13q34. 相似文献
54.
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
55.
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
56.
J B Bristol P M Emmett K W Heaton R C Williamson 《BMJ (Clinical research ed.)》1985,291(6507):1467-1470
The habitual diet of 50 patients with large bowel cancer, as assessed by a dietary history method, was compared with that of 50 closely matched controls. Patients were included only if their symptoms were unlikely to have changed previous eating habits. The mean daily intakes of all major nutrient classes and of dietary fibre were estimated. Patients with large bowel cancer consumed 16% more energy than controls (mean (SEM) daily intake 9.92 (0.41) v 8.56 (0.32) MJ (2370 (98) v 2046 (76) kcal), respectively; p less than 0.0001), mainly in the form of carbohydrate (21% more; 282.6 (13.7) v 233.4 (10.5) g; p less than 0.0001) and fat (14% more; 100.8 (4.3) v 88.4 (3.2) g; p less than 0.001). The extra carbohydrate was largely in the form of sugars depleted in fibre and the extra fat as combinations of fat and such sugars. As the selection criteria used make it unlikely that this eating pattern was caused by the disease the data suggest that a high intake of sugars depleted in fibre and fat predisposes to the development of large bowel cancer. 相似文献
57.
The transbilayer distribution of phospholipids in right-side-out and inside-out vesicles derived from human erythrocytes was studied by phospholipase A2 digestion assays and by staining with the fluorescent dye merocyanine 540. In both types of vesicles, the normal asymmetric distribution of phospholipids characteristic of intact cells was disrupted. Because both types of vesicles are deficient in spectrin, the major protein of the cytoskeletal network which normally underlies the membrane, these results support the contention that spectrin is involved in the maintenance of phospholipid asymmetry. 相似文献
58.
59.
Identification of a deletion in the low density lipoprotein (LDL) receptor gene in a patient with familial hypercholesterolaemia 总被引:5,自引:1,他引:4
B. Horsthemke Anna M. Kessling Mary Seed V. Wynn R. Williamson S.E. Humphries 《Human genetics》1985,71(1):75-78
Summary DNA samples from 60 unrelated UK patients with familial hypercholesterolaemia (FH) were screened by Southern blot hybridisation to detect gross alterations in the low density lipoprotein (LDL) receptor gene. One patient was found to have a 2kb deletion in the 3 part of the gene. The deletion cosegregates with the FH phenotype in his family. This finding is compatible with the deletion being the cause of FH in this case and makes a presymptomatic test based on DNA analysis available for this family. The defects in most of the other patients are likely to be due to point mutations. 相似文献
60.