全文获取类型
收费全文 | 16824篇 |
免费 | 1311篇 |
国内免费 | 20篇 |
出版年
2023年 | 63篇 |
2022年 | 209篇 |
2021年 | 309篇 |
2020年 | 198篇 |
2019年 | 248篇 |
2018年 | 346篇 |
2017年 | 328篇 |
2016年 | 475篇 |
2015年 | 871篇 |
2014年 | 879篇 |
2013年 | 1070篇 |
2012年 | 1406篇 |
2011年 | 1241篇 |
2010年 | 796篇 |
2009年 | 748篇 |
2008年 | 1014篇 |
2007年 | 987篇 |
2006年 | 866篇 |
2005年 | 782篇 |
2004年 | 745篇 |
2003年 | 646篇 |
2002年 | 577篇 |
2001年 | 428篇 |
2000年 | 396篇 |
1999年 | 297篇 |
1998年 | 138篇 |
1997年 | 119篇 |
1996年 | 85篇 |
1995年 | 109篇 |
1994年 | 65篇 |
1993年 | 73篇 |
1992年 | 126篇 |
1991年 | 122篇 |
1990年 | 117篇 |
1989年 | 129篇 |
1988年 | 115篇 |
1987年 | 101篇 |
1986年 | 89篇 |
1985年 | 104篇 |
1984年 | 72篇 |
1983年 | 61篇 |
1982年 | 40篇 |
1979年 | 58篇 |
1978年 | 56篇 |
1977年 | 39篇 |
1976年 | 47篇 |
1975年 | 37篇 |
1974年 | 44篇 |
1973年 | 40篇 |
1972年 | 35篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
Masayuki Nakamichi Fumiaki Cho Tetsuhiro Minami 《Primates; journal of primatology》1990,31(2):213-224
This study documents age-related changes in the interactions of wild-born cynomolgus macaque mothers and their infants living
in individual cages during the first 14 weeks of infant life. Body contact between mother and infant, maternal holding, and
infant sucking were found to decrease, and the mothers showed an increased frequency of aggression toward their infants with
age. These results were broadly similar to those reported for mother-infant interactions in other macaques living in social
groups. Nevertheless, a clear difference between the present cynomolgus macaques and other macaques in social groups was apparent.
The cynomolgus macaque mothers tended to permit their infants to move about freely without displaying maternal protectiveness
such as restraint or retrieval, unlike other macaque mothers in social groups. Such maternal behaviors might derive from the
experience of living in individual cages for many years and the relative safety of living in individual cages. The lack of
maternal restraint and retrieval could be responsible for the observed sex differences in behavior: male infants moved more
actively, and broke, and made contact with their mothers more frequently than did female infants. Moreover, mothers of female
infants held and groomed them more frequently and were less aggressive toward them. 相似文献
162.
S Krzemień J Ho?owiecki K Jagoda B Ho?owiecka 《Polski tygodnik lekarski (Warsaw, Poland : 1960)》1990,45(51-52):1032-1036
Retrospective analysis has included 323 patients with acute nonlymphocytic leukaemia. The comparable patient groups were treated since 1981, according to protocols used by the Polish Acute Leukaemia Group. The prognostic value for achieving complete remission and survival of 67 pre-treatment factors (42 quantitative and 25 qualitative) was evaluated. The most important 9 parameters were scored according to their prognostic value as follows: age, percent of blasts in bone marrow, peripheral blood blast count, morphological subtype, percent of granulocytes in bone marrow, percent of blasts with CD-15 antigen, thrombocyte count, spleen/liver enlargement, CSF protein levels. Proposed scoring system enables classification of ANLL patients to a standard and high risk groups. 相似文献
163.
Y E Earm W K Ho I S So 《Proceedings of the Royal Society of London. Series B, Containing papers of a Biological character. Royal Society (Great Britain)》1990,240(1297):61-81
To investigate the underlying ionic mechanism of the late plateau phase of the action potential in rabbit atrium the whole-cell patch-clamp technique with intracellular perfusion was used. We recorded the inward current during repolarizations following a brief 2 ms depolarizing pulse to +40 mV from a holding potential of between -70 and -80 mV. The development of this current coincides with the onset of the late plateau phase of the action potential. Peak activation of the current occurs about 10 ms from the beginning of the depolarizing pulse, and it decays spontaneously with a slow timecourse. Its voltage dependency from -40 mV to +40 mV shows very steep activation (-40 to -20 mV) and shows almost the same maximum magnitude between -10 mV and +40 mV. This behaviour is quite different from that of the calcium current. The inward current and the late plateau phase of the action potential were both abolished by the application of 5 mM EGTA, 1 microM ryanodine and by reducing the Na+ gradient. The fully activated current-voltage relation of the inward current was plotted as the difference current before and after treatment with Ryanodine, Diltiazem, 20 mM Na+ inside or 30% Na+ outside and shows an exponential voltage dependence with the largest magnitude of the current occurring at negative potentials. The current-voltage (I-V) curve was well fitted by the Na-Ca exchange equation, i = A exp (-(1 - r)EF/RT). The results suggest that the inward current contributes to the generation of the late plateau phase of the rabbit atrial action potential, and is activated by intracellular calcium released from the sarcoplasmic reticulum. Sarcoplasmic reticulum calcium release appears to be triggered both by the membrane voltage and by the calcium current. It is concluded that the inward current is generated by Na-Ca exchange. 相似文献
164.
Purification, characterization, and amino terminal sequence of xylose reductase from Candida shehatae. 总被引:1,自引:0,他引:1
D-Xylose is a major component of the carbohydrates derived from agricultural residues and forest products. Among more than two hundred known xylose-utilizing yeasts, only a few species are known to be able to ferment xylose anaerobically. Candida shehatae is one of such xylose-fermenting yeasts. Xylose reductase (E.C. 1.1.1.21) is a key enzyme responsible for xylose metabolism in xylose-utilizing as well as xylose-fermenting yeasts. In this paper, we report the development of a convenient and reliable procedure for the purification of xylose reductase from C. shehatae to near homogeneity. The amino acid composition and N-terminal sequence of the enzyme have also been analyzed. C. shehatae seems to contain only a single xylose reductase, but the enzyme has a dual coenzyme specificity for both NADPH and NADH. The enzyme is remarkably stable at room temperature and 4 degrees C. 相似文献
165.
M F Ho A P Monaco L A Blonden G J van Ommen N A Affara M A Ferguson-Smith H Lehrach 《American journal of human genetics》1992,50(2):317-330
McLeod syndrome, characterized by acanthocytosis and the absence of a red-blood-cell Kell antigen (Kx), is a multisystem disorder involving a late-onset myopathy, splenomegaly, and neurological defects. The locus for this syndrome has been mapped, by deletion analysis, to a region between the loci for Duchenne muscular dystrophy (DMD) and chronic granulomatous disease (CGD). In this study, we describe a new marker, 3BH/R 0.3 (DXS 709), isolated by cloning the deletion breakpoint of a DMD patient. A long-range restriction map of Xp21, encompassing the gene loci for McLeod and CGD, was constructed, and multiple CpG islands were found clustered in a 700-kb region. Using the new marker, we have limited the McLeod syndrome critical region to 150-380-kb. Within this interval, two CpG-rich islands which may represent candidate sites for the McLeod gene were identified. 相似文献
166.
H Cho S E Ramer M Itoh E Kitas W Bannwarth P Burn H Saito C T Walsh 《Biochemistry》1992,31(1):133-138
The cytoplasmic domains of two human transmembrane protein tyrosine phosphatases (PTPases), LAR and CD45, have been expressed in Escherichia coli, purified to near-homogeneity, and compared for catalytic efficiency toward several phosphotyrosine-containing peptide substrates. A 615-residue LAR fragment (LAR-D1D2) containing both tandemly repeated PTPase domains shows almost identical specific activity and high catalytic efficiency as the 40-kDa single-domain LAR-D1 fragment, consistent with a single functional active site in the 70-kDa LAR-D1D2 enzyme. A 90-kDa fragment of the human leukocyte CD45 PTPase, containing two similar tandemly repeated PTPase domains, shows parallel specificity to LAR-D1 and LAR-D1D2 with a high kcat/Km value for a phosphotyrosyl undecapeptide. Sufficient purified LAR-D1 and LAR-D1D2 PTPases were available to demonstrate enzymatic exchange of 18O from 18O4 inorganic phosphate into H2(16)O at rates of approximately 1 x 10(-2) s-1. The oxygen-18 exchange probably proceeds via a phosphoenzyme intermediate. Brief incubation of all three PTPase fragments with a [32P]phosphotyrosyl peptide substrate prior to quench with SDS sample buffer and gel electrophoresis led to autoradiographic detection of 32P-labeled enzymes. Pulse/chase studies on the LAR 32P-enzyme showed turnover of the labeled phosphoryl group. 相似文献
167.
Summary The effect ofin situ extraction and elicitor treatment on shikonin production was studied with the suspension cultures ofLithospermum erythrorhizon. Shikonin concentration of 60 mg/L was achieved by the use of both techniques which was 24 times higher than that of control culture, and 65 times higher in terms of shikonin productivity. The host-pathogen effect of elicitor treatment andin situ extraction for product removal were effective for shikonin production. 相似文献
168.
P. G. Arnison P. Donaldson L. C. C. Ho W. A. Keller 《Plant Cell, Tissue and Organ Culture》1990,20(3):147-155
Embryo formation by cultured broccoli (Brassica oleracea L. var. italica) anthers was best in the pH range of 5.5 to 5.8. Manipulation of the initial medium pH showed, however, that embryos could be recovered throughout the entire pH range tested. Experiments designed to test the influence of anther density on embryo production exhibited an apparent population effect. Comparison of anthers cultured with and without filaments showed a significantly lower level of embryo formation with filaments attached. The importance of anther orientation with the adaxial surface up was also demonstrated. Detailed studies of the effect of temperature on anther response showed the importance of 35°C treatments. Other temperatures and a variety of temperature manipulations were either comparatively ineffective or inhibitory. The duration of 35°C exposure required for optimal response varied widely between 18 and 48 h. Wide variation in plant to plant response was observed despite attempts to optimize the manipulation of physical parameters. Individual plants were identified that reliably formed many thousands of embryos, whereas other plants failed to form embryos under all tested conditions. 相似文献
169.
Aliquotes of human amniotic fluid (AF), fetal serum (FS), and cord blood (CB) were obtained as by-products of routine clinical diagnostic procedures at term or in the second trimester of pregnancy. When samples of CB were applied to a pH 5.5-4 chromatofocusing gradient, three isoforms of AFP could be resolved; a pl 4.57 form (isoform IA, 52% AFP), a pl 4.27 form (isoform IB, 43% AFP), and one species that was bound to the column but could be eluted with 1.0 M NaCl (isoform II, pl less than 4.00, 5% AFP). Term AF displayed a profile similar to that observed in term CB. When samples of 15-20-week gestation AF were chromatofocused, the immunoreactive AFP recovered was distributed between isoform IA and IB (60%) and isoform II (40%). FS and AF obtained from same pregnancy (23-26 weeks) displayed an identical chromatofocusing profile. Aliquotes of AF subjected to conA revealed 83% reactive variants compared with greater than 95% reactive variants for CB. FS displayed a conA profile identical to CB. When individual CB charge isoforms were isolated and subjected to conA analysis, greater than 97% of the AFP bound to conA. In contrast, when AFP isoform IA and IB were isolated from midgestation AF, approximately 22% of the AFP did not bind to the lectin while 100% of isolated AFP isoform II eluted as the reactive variant. These data suggest that human AFP exists as at least three charge and two lectin variants and that the charge profile may change during fetal development. 相似文献
170.
Hormonal regulation, processing, and secretion of cysteine proteinases in barley aleurone layers. 总被引:14,自引:3,他引:11
下载免费PDF全文
![点击此处可从《The Plant cell》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Barley aleurone layers synthesize and secrete several proteases in response to gibberellic acid (GA3). Two major cysteine proteinases designated EP-A (37,000 M(r)) and EP-B (30,000 M(r)) have been described [Koehler and Ho (1988). Plant Physiol. 87, 95-103]. We now report the cDNA cloning of EP-B and describe the post-translational processing and hormonal regulation of both cysteine proteinases. Three cDNAs for cysteine proteinases were cloned from GA3-induced barley aleurone layers. Genomic DNA gel blot analysis indicated that these are members of a small gene family with no more than four to five different genes. The proteins encoded by two of these clones, pHVEP1 and 4, are 98% similar to each other and are isozymes of EP-B. The proteins contain large preprosequences followed by the amino acid sequence described as the mature N terminus of purified EP-B, and are antigenic to EP-B antiserum. The results of pulse-chase experiments indicated that the post-translational processing of large prosequences proceeds in a multistep fashion to produce the mature enzymes. Processing intermediates for EP-B are observed both in the aleurone layers and surrounding incubation medium, but only mature EP-A is secreted. The regulation of synthesis of EP-A, EP-B, and other aleurone cysteine proteinases was compared at the protein and mRNA levels. We conclude that barley aleurone cysteine proteinases are differentially regulated with respect to their temporal and hormonally induced expression. 相似文献