全文获取类型
收费全文 | 4225篇 |
免费 | 232篇 |
国内免费 | 12篇 |
出版年
2023年 | 37篇 |
2022年 | 79篇 |
2021年 | 161篇 |
2020年 | 86篇 |
2019年 | 97篇 |
2018年 | 134篇 |
2017年 | 122篇 |
2016年 | 163篇 |
2015年 | 203篇 |
2014年 | 266篇 |
2013年 | 302篇 |
2012年 | 321篇 |
2011年 | 320篇 |
2010年 | 220篇 |
2009年 | 162篇 |
2008年 | 210篇 |
2007年 | 176篇 |
2006年 | 140篇 |
2005年 | 159篇 |
2004年 | 125篇 |
2003年 | 132篇 |
2002年 | 96篇 |
2001年 | 60篇 |
2000年 | 56篇 |
1999年 | 44篇 |
1998年 | 31篇 |
1997年 | 18篇 |
1996年 | 19篇 |
1995年 | 20篇 |
1994年 | 28篇 |
1993年 | 22篇 |
1992年 | 45篇 |
1991年 | 39篇 |
1990年 | 24篇 |
1989年 | 21篇 |
1988年 | 19篇 |
1986年 | 21篇 |
1985年 | 29篇 |
1984年 | 13篇 |
1983年 | 11篇 |
1982年 | 21篇 |
1980年 | 18篇 |
1979年 | 12篇 |
1977年 | 11篇 |
1976年 | 17篇 |
1975年 | 13篇 |
1974年 | 20篇 |
1973年 | 15篇 |
1972年 | 16篇 |
1970年 | 12篇 |
排序方式: 共有4469条查询结果,搜索用时 15 毫秒
51.
Lisa Beltz Opendra Narayan Robert J. Adams Stephen J. Noga Albert D. Donnenberg 《Journal of medical primatology》1991,20(4):144-151
Rhesus progenitor-enriched BM was exposed overnight to SIV and cultured in a limiting dilution assay where the potential for progenitor interaction with lymphocytes or macrophages was low. Virus was consistently isolated late in culture, detection being aided by coculture with CEM174 lymphoblasts. Although infected cells had reduced clonogenic activity, colonies were indistinguishable from those derived from uninfected BM with respect to proliferative potential, morphology, and longevity in culture. Primate immunodeficiency viruses, therefore, may infect immature BM populations, directly affecting hematopoietic activity. 相似文献
52.
Clark E. Hartsock III Jeffrey K. Lewis Ian Leslie Joseph A. Pope Jr. Larry B. Tsai Raj Sachdev Shi-Yuan Meng 《Biotechnology letters》1995,17(10):1025-1030
Summary A synthetic lactose-inducible promoter was chosen to study host cell responses to the over-expression of heterologous genes. Fermentations were conducted to compare the effect of induction strategies on the synthesis of -galactosidase versus the production of recombinant protein. The levels of lactose, IPTG and glucose during induction were manipulated to adjust the utilization of lactose as the inducer and/or the carbon source. In addition, the involvement of the gal operon in lactose metabolism was also explored in order to optimize lactose transport and utilization during induction. 相似文献
53.
Post-translational modifications are fundamental to processes controlling behaviour, including cellular signaling, growth and transformation. As the molecular basis of protein modifications in normal and disease processes are becoming better defined, so new strategies for designing therapeutic entities to control complex disease processes are emerging. 相似文献
54.
Summary A two-step procedure was used for plant regeneration from in vitro grown leaf strips (2–3 mm wide) of cv. Bintje. Step I medium was designed with 2,4-dichlorophenoxycetic acid (2,4-D) at 0.0 or 9.0 M, in combination with 2.28 M kinetin (K), benzyl adenine (BA), zeatin (Z) or zeatin riboside (ZR). Step II media were 2,4-D-free media containing 5.78 M gibberellic acid (GA3) and growth regulators similar to those of step I media. Leaf explants cultured in medium I containing zeatin riboside or zeatin for 6 days and then subcultured in medium II containing zeatin riboside produced numerous shoots without callus formation. Zeatin riboside containing step I and II media caused shoot regeneration in a high number (97.5±2.2) of explants. Approximately, 33.7±8.4 shoots were regenerated from each leaf explant.Abbreviations BA
benzyladenine
- Z
zeatin
- ZR
zeatin riboside (trans isomer)
- 2,4-D
2,4-dichlorophenoxyacetic acid 相似文献
55.
Among diploid Lathyrus species a threefold variation in nuclear DNA amount is attributable to differences in the amount of repetitive DNA. Cross reassociation among repetitive and among non-repetitive DNA fractions from different species shows substantial divergence in DNA composition. The divergence in base composition is correlated with nuclear DNA amount. The degree of divergence is of the same order of magnitude in both the repetitive and nonrepetitive fractions. 相似文献
56.
Vasoactive intestinal peptide (VIP) caused a dose-dependent fall in lower esophageal sphincter (LES) pressure and dose-dependent contractions in the body of the esophagus. The response to VIP in the esophagus or LES was not modified by atropine, phentolamine, haloperidol, pyrilamine, methysergide, indomethacin and tetrodotoxin, showing that it exerts direct action at the esophageal smooth muscle. These studies suggest that VIP causes contraction in the esophageal body and relaxation of the LES by a direct action on the smooth muscle. It is possible that VIP may be the common mediator of noncholinergic, nonadrenergic neurons that cause relaxation of the lower esophageal sphincter and contraction in the esophageal body. 相似文献
57.
In previous experiments rats pretreated with slow-release d-amphetamine (d-Amp) pellets for days, given a 12-hr drug-free period, and then injected with d-Amp have been found to show a behavioral syndrome which has similarities to that induced by acute injections of the hallucinogens LSD and mescaline. The present results indicate that rats administered this same drug regimen have large decreases in Dopamine (DA), dihydroxyphenyl acetic acid (Dopac), and homovanillic acid (HVA) in caudate nucleus, smaller decreases in DA with no changes in Dopac and HVA levels in nucleus accumbens, but no alterations in 5-hydroxytryptamine (5HT) and 5-hydroxyindole acetic acid (5HIAA) levels in caudate, accumbens, brainstem and hippocampus. Increased 5HIAA levels are found in rats sacrificed with pellets intact following 3 days of continuous d-Amp administration, while sleep deprived and in motor stereotypies. The late and hallucinatory stage following continuous d-amp is correlated more closely with alterations in dopamine than of 5HT. 相似文献
58.
A green house study was conducted on the effect of P and Zn on nodulation and N fixation in chickpea (Cicer arietinum L.) in a loamy sand (Typic Torripsamments) using treatment combinations of five levels of P (0, 25, 50, 100 and 250 ppm),
and six levels of Zn (0, 5, 10, 20, 40 and 100 ppm). The number, dry matter and leghaemoglobin content of nodules, and amount
of N fixed generally increased with Zn alone upto 19 ppm and P alone upto 50 ppm, and decreased with their higher levels.
Application of 25 to 50 ppm P and 5 to 10 ppm Zn counteracted to a greater extent the adverse effect of 40 and 100 ppm Zn,
and 250 ppm P, resp. Maximum nodulation and N fixation (91 to 145% over zero P and Zn, at maturity) was recorded with 25 to
50 ppm P applied along with 5 to 10 ppm Zn. At 64 days, depletion in soil-N was noted, particularly when P was applied, whereas
at maturity there was a gain in soil-N, ranging from 10.5 to 44.5 kg/2×106 kg soil depending upon P and Zn treatments. The increase in nodulation and N fixation with balanced P and Zn nutrition might
be attributed to an increase in leghaemoglobin, and K and Fe concentration in nodules, and increased plant growth, resulting
into enhanced activity of N fixing organisms. The results showed that balanced P and Zn nutrition is essential not only for
plant growth but also for maximum activity of Rhizobium for N fixation.
Work done at Harvana Agricultural University, Hissar, India. 相似文献
59.
Naturally occurring antigenic variants of visna virus isolated from two persistently infected sheep have been compared using RNAase T1 fingerprint analysis of the viral RNAs. All the viruses examined have oligonucleotide changes in the 3′-terminal region of the viral genome and many of the alterations are common among the variants of both sheep. There is progressive accumulation of point mutations in the 3′-terminal region of antigenically altered viruses isolated from persistently infected animals. In addition, antigenic variants isolated independently from another sheep contain the same altered oligonucleotides; the nucleotide sequences of these oligonucleotides are identical. These data strongly suggest that the alterations observed in the nucleotide sequence of the variants are related to the change in their antigenic character. 相似文献
60.
Progesterone receptors were determined in the cytosol from the ampulla, ampullaryisthmic junction and isthmus of rabbit fallopian
tube and uterus of estrus and pregnant rabbits. The receptor levels when compared among its various anatomical segments, were
the same in ampulla, isthums and uterus but maximum in ampullary-isthmic junction. Significant differences were observed in
mated animals at 14, 24, 34, 48, 70 and 144 h after coitus. The receptor concentrations in portions of the fallopian tube
showed no significant change between 14 and 24 h after coitus, except for a decrease in ampullary-isthmic junction at 24 h.
At 34 h the concentration of receptor further decreased in all parts of the tube. At 48 and 70 h after coitus, receptor concentrations
decreased gradually in ampulla and ampullary-isthmic junction, while isthmus showed a gradual increase. At 144 h, the receptor
concentration showed no further change in ampulla and ampullary-isthmic junction; however, isthmus showed a decline. The uterine
receptor concentration declined steadily from estrus till 70 h after coitus, however, it was increased at 144 h. The dissociation
constant (Kd) of cytosol receptor in all the tissues at estrus and during early pregnancy was found similar. The implications of these
changes in relation to the normal ovum transport have been correlated in this paper. 相似文献