全文获取类型
收费全文 | 1494篇 |
免费 | 184篇 |
专业分类
1678篇 |
出版年
2021年 | 33篇 |
2020年 | 18篇 |
2019年 | 21篇 |
2018年 | 25篇 |
2017年 | 16篇 |
2016年 | 40篇 |
2015年 | 50篇 |
2014年 | 58篇 |
2013年 | 70篇 |
2012年 | 69篇 |
2011年 | 86篇 |
2010年 | 48篇 |
2009年 | 41篇 |
2008年 | 54篇 |
2007年 | 69篇 |
2006年 | 64篇 |
2005年 | 46篇 |
2004年 | 49篇 |
2003年 | 51篇 |
2002年 | 47篇 |
2001年 | 36篇 |
2000年 | 33篇 |
1999年 | 36篇 |
1998年 | 17篇 |
1997年 | 18篇 |
1996年 | 25篇 |
1995年 | 19篇 |
1994年 | 16篇 |
1993年 | 17篇 |
1992年 | 25篇 |
1991年 | 16篇 |
1990年 | 19篇 |
1989年 | 20篇 |
1988年 | 18篇 |
1987年 | 25篇 |
1986年 | 21篇 |
1985年 | 27篇 |
1984年 | 18篇 |
1983年 | 19篇 |
1982年 | 22篇 |
1981年 | 22篇 |
1980年 | 11篇 |
1979年 | 17篇 |
1978年 | 22篇 |
1977年 | 20篇 |
1976年 | 12篇 |
1975年 | 21篇 |
1974年 | 13篇 |
1973年 | 17篇 |
1972年 | 18篇 |
排序方式: 共有1678条查询结果,搜索用时 0 毫秒
11.
Using transfection to study gene expression in trypanosomes 总被引:1,自引:0,他引:1
12.
Summary About half of the cases of Angelman syndrome arise from deletions of chromosome band 15q12. In 25 cases we have been able to determine the parental origin of the deletion and, in line with other reported cases, we have found the deletion to be of maternal origin. There were no exceptions. The parental origin was determined using cytogenetic markers in 13 of the cases, in nine by using the pattern of inheritance of restriction fragment length polymorphisms, and in three using both techniques. 相似文献
13.
The influence of parasites on host sexual selection 总被引:7,自引:0,他引:7
Clayton DH 《Parasitology today (Personal ed.)》1991,7(12):329-334
In 1982 Hamilton and Zuk(1) proposed a provocative solution for the unexplained fact that the males of many species exhibit 'showy' traits such as brightly coloured plumage or vigorous courtship displays. They suggested that showy traits are fully expressed only by males who are resistant to parasites and that females examine such traits in order to choose resistant males as mates. Hamilton and Zuk's proposal has been the topic of extensive research and vigorous debate for nearly a decade. This article reviews the research, relevant criticisms and unanswered questions pertaining to the influence of parasites on sexual selection. 相似文献
14.
M Singhofer-Wowra M Little L Clayton P Dawson K Gull 《Journal of molecular biology》1986,192(4):919-924
About 96% of the amino acid sequence of an alpha-tubulin from the slime mould Physarum polycephalum has been determined. Of 430 sequenced amino acids, 30 differ from the deduced amino acid sequence of a recently published alpha-tubulin complementary DNA from the plasmodial form of P. polycephalum. The myxamoebal alpha-tubulin differs from all other known alpha-tubulins in one of the last three C-terminal amino acids that are Gly-Glu-Tyr instead of the usual Glu-Glu-Tyr. These last three amino acids are preceded by 11 residues that appear to be particularly susceptible to mutation. No heterogeneity was found whilst sequencing the myxamoebal alpha-tubulin, indicating that only one type of alpha-tubulin is present in myxamoebae. This alpha-tubulin appears to be less conserved than the previously described plasmodial alpha-tubulin, supporting the hypothesis that the structural constraints on tubulin in axonemes have a significant effect on its rate of mutation. 相似文献
15.
William S. Clayton Jr. 《Helgoland Marine Research》1986,40(1-2):83-90
Effects of various treatments on prey capture, prey ingestion and ingestion time of individualArtemia salina nauplii by the sea anemoneAiptasia pallida Verrill were studied in the laboratory. Exposure to crudeArtemia homogenate, 5 × 10–4 M reduced glutathione or 5 × 10–4 M proline significantly decreased the number ofArtemia that were captured and ingested but had no significant effect on the ingestion time of individualArtemia. Multiple captures increased the total ingestion time but decreased ingestion time per prey item. Results suggest that, under these conditions, the prey capture phase of zooplankton feeding was somewhat distinct from the ingestion phase since chemical stimuli that significantly reduced prey capture had no significant effect on ingestion time. 相似文献
16.
17.
A Bergh H Nikula J E Damber R Clayton I Huhtaniemi 《Journal of reproduction and fertility》1985,74(1):279-286
Testicular descent was prevented unilaterally in newborn rats by cutting the gubernaculum testis. At 100 days of age, the number of Leydig and Sertoli cells per testis, the concentration of receptors for LH, FSH, prolactin and GnRH, and endogenous concentrations of progesterone and testosterone were determined. The weight of the abdominal testes was reduced by 80%, but in spite of this they contained as many Sertoli (32.8 +/- 1.3 X 10(6), mean +/- s.e.m., n = 6) and Leydig (28.2 +/- 1.7 X 10(6) cells as did scrotal testes (32.1 +/- 2.5 X 10(6) and 24.3 +/- 1.2 X 10(6) respectively). The numbers of receptors for LH (3.2 +/- 0.2 and 1.0 +/- 0.2 pmol/testis, mean +/- s.e.m., n = 11), FSH (358 +/- 11.0 and 96.3 +/- 12.6 fmol/testis) and prolactin (535 +/- 32.7 and 92.4 +/- 13.2 fmol/testis) were reduced (P less than 0.001) in abdominal testes, but the number of GnRH receptors was unaffected (8.9 +/- 1.4 and 12.1 +/- 1.8 fmol/testis, n = 6). Testicular testosterone concentration (30.9 +/- 4.4 vs 15.4 +/- 3.2 ng/g, n = 11, P less than 0.001), but not that of progesterone (0.87 +/- 0.10 vs 1.01 +/- 0.21 ng/g), was decreased in abdominal testes. The decreased receptor and androgen values reflect functional disturbances in the abdominal testes. The changed local milieu within abdominal testes may reduce hormone receptor concentrations which are then involved in the observed Leydig cell dysfunction. 相似文献
18.
In vitro replication of human mitochondrial DNA: accurate initiation at the origin of light-strand synthesis 总被引:18,自引:0,他引:18
Synthesis of human light-strand mitochondrial DNA was accomplished in vitro using DNA primase, DNA polymerase, and other accessory proteins isolated from human mitochondria. Replication begins with the synthesis of primer RNA on a T-rich sequence in the origin stem-loop structure of the template DNA and absolutely requires ATP. A transition from RNA synthesis to DNA synthesis occurs near the base of the stem-loop structure and a potential recognition site for signaling that transition has been identified. The start sites of the in vitro products were mapped at the nucleotide level and were found to be in excellent agreement with those of in vivo nascent light-strand DNA. Isolated human mitochondrial enzymes recognize and utilize the bovine, but not the mouse, origin of light-strand replication. 相似文献
19.
Steven S. Thoresen John R. Clayton S.I. Ahmed 《Journal of experimental marine biology and ecology》1984,83(2):149-157
Measurements of uptake rates, intracellular nitrogen pools, and other key intracellular constituents were made during exponential growth in Skeletonema costatum (Grev.) Cleve under varying pH levels. An understanding of the overall effects of extracellular pH on the above mentioned cellular parameters is crucial in order to ascertain the degree to which pH must be regulated and monitored in laboratory experiments with marine phytoplankton.It was found that uptake rates and intracellular pool sizes of NO?3 were directly influenced by the extracellular pH level, whereas, other cellular compounds remained relatively unchanged. Therefore, nitrogen uptake and intracellular nitrogen storage are dependent on key H+ and OH? ion transport mechanisms that are associated with phytoplankton metabolism. These findings reiterate the fact that investigators examining nitrogen uptake and assimilatory mechanisms in marine phytoplankton must be conscious of cellular H + and OH? fluxes that contribute to intracellular pH regulation and changes in extracellular pH levels, both of which interact to affect phytoplankton metabolic processes. 相似文献
20.
Chicken lens delta-crystallin gene expression and methylation in several non-lens tissues. 总被引:5,自引:2,他引:3 下载免费PDF全文
D J Bower L H Errington D N Cooper S Morris R M Clayton 《Nucleic acids research》1983,11(9):2513-2527
RNA sequences coding for the most abundant chicken lens proteins, delta-crystallin, were detected at very low levels in day old post hatch chick lung, heart, kidney and liver, and in 6 day embryo headless bodies. The pattern of cytosine methylation within the CCGG sequences of the delta-crystallin genes was also examined and shown to vary in several non-lens tissues, from several stages of development. Embryonic neural retina, which expresses a higher level of delta-crystallin RNA than the above tissues, is no less methylated in the sites studied than the tissues which have no association with the eye, and is actually more heavily methylated than the kidney. Thus no obvious correlation was found between undermethylation and gene expression. 相似文献