全文获取类型
收费全文 | 421101篇 |
免费 | 43655篇 |
国内免费 | 199篇 |
专业分类
464955篇 |
出版年
2016年 | 4544篇 |
2015年 | 6010篇 |
2014年 | 7127篇 |
2013年 | 10697篇 |
2012年 | 11563篇 |
2011年 | 12066篇 |
2010年 | 8209篇 |
2009年 | 7523篇 |
2008年 | 10762篇 |
2007年 | 11411篇 |
2006年 | 10729篇 |
2005年 | 10191篇 |
2004年 | 10285篇 |
2003年 | 10060篇 |
2002年 | 9837篇 |
2001年 | 17357篇 |
2000年 | 17395篇 |
1999年 | 14058篇 |
1998年 | 4983篇 |
1997年 | 5294篇 |
1996年 | 5019篇 |
1995年 | 4731篇 |
1994年 | 4633篇 |
1993年 | 4631篇 |
1992年 | 12267篇 |
1991年 | 12071篇 |
1990年 | 12084篇 |
1989年 | 11857篇 |
1988年 | 11165篇 |
1987年 | 10500篇 |
1986年 | 9820篇 |
1985年 | 10238篇 |
1984年 | 8439篇 |
1983年 | 7276篇 |
1982年 | 5613篇 |
1981年 | 5020篇 |
1980年 | 4694篇 |
1979年 | 8082篇 |
1978年 | 6300篇 |
1977年 | 5940篇 |
1976年 | 5672篇 |
1975年 | 6130篇 |
1974年 | 6697篇 |
1973年 | 6568篇 |
1972年 | 6122篇 |
1971年 | 5535篇 |
1970年 | 4767篇 |
1969年 | 4782篇 |
1968年 | 4453篇 |
1967年 | 3808篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
Conjugal transfer of plasmid pAM beta 1 in Lactobacillus reuteri and between lactobacilli and Enterococcus faecalis. 总被引:6,自引:4,他引:2 下载免费PDF全文
G W Tannock 《Applied microbiology》1987,53(11):2693-2695
The broad-host-range plasmid pAM beta 1 (erythromycin resistance) was transferred conjugally from Streptococcus lactis to Lactobacillus reuteri, L. murinus, and L. fermentum. Transfer of pAM beta 1 between two L. reuteri strains occurred, and lactobacillus transconjugants could act as donors of pAM beta 1 in crosses with Enterococcus faecalis JH2-2. 相似文献
112.
Partial monosomy 22 as result of an X/22 translocation in a newborn with DiGeorge syndrome 总被引:5,自引:0,他引:5
The case of a neonate with clinical symptoms of DiGeorge syndrome is reported. During pregnancy the measurements by ultrasonography revealed already a significant growth retardation of the fetus, for the first time obvious in the 20th week. The child died immediately after birth. A de novo translocation X/22 was observed with the translocation chromosome being late replicating in all mitoses analysed. The own observation is discussed regarding other cases with DiGeorge syndrome and taking the differential diagnoses into account. count. 相似文献
113.
Membrane-bound phosphotyrosyl-protein phosphatase activity in human erythrocytes. Dephosphorylation of membrane band 3 protein 总被引:3,自引:0,他引:3
G Clari A M Brunati V Moret 《Biochemical and biophysical research communications》1987,142(2):587-594
Human erythrocyte membranes exhibit, in addition to "acid" p-nitrophenyl-phosphatase activity, remarkable phosphotyrosyl-protein phosphatase activity, assayed on synthetic polymer poly (Glu-Tyr) 4:1, previously phosphorylated on Tyr residues by rat spleen tyrosine-protein kinase. The results reported here indicate that such a 32P-Tyr-phosphatase activity, rather than p-nitrophenyl-phosphatase, is involved in the dephosphorylation of transmembrane band 3 protein on 32P-tyrosine residues. 相似文献
114.
A cDNA library was constructed from chick aorta poly(adenylic acid)-containing RNA in the expression vector pEX1. Several clones were identified by screening the library with a polyclonal antiserum raised against chick tropoelastin and confirmed by DNA sequencing. Analysis of the deduced amino acid sequence, corresponding to the mature tropoelastin and most of the signal peptide, revealed that the molecule is composed of at least 8, and possibly 13, repeating units. The common features of each unit include an N-terminal region composed largely of alanines and lysines and ending with an aromatic amino acid, followed by a GAG span and then a C-terminal region consisting mostly of valines, prolines, and glycines often present in several copies of the sequence (VPGV). This structure is discussed in terms of the functional properties of the molecule. 相似文献
115.
Sterol-phospholipid interaction in model membranes: role of C5-C6 double bond in cholesterol 总被引:2,自引:0,他引:2
Several double-bond isomers of cholesterol where the normal C5-C6 double bond (delta 5) has been moved to different positions in the ring skeleton, i.e., delta 1, delta 4, delta 7, delta 8(9), delta 8(14), and delta 14, have been synthesized and incorporated in phosphotidylcholine vesicles. In addition, dienes like delta 5,7, delta 7,14, and delta 8,14 have also been studied. Many of these cholesterol analogues are intermediates in the sterol biosynthesis in different organisms. The incorporation studied indicated that more than 90% of the sterol was present in the vesicles. The effect of these cholesterol analogues was studied by glucose permeability, electron spin resonance, and fluorescence polarization spectroscopy. These studies indicated that delta 14-cholesten-3 beta-ol was most effective in restricting glucose permeability or in increasing the order parameter but was still not as effective as cholesterol. This was followed by delta 8(14)- and delta 8(9)-cholesten-3 beta-ol. The delta 1, delta 4, and delta 7 analogues and the dienols were relatively less effective in condensing the membrane. These studies indicate that the double bond at C5-C6 in cholesterol is most effective for optimal sterol-phospholipid interaction and may have formed the basis of the migration of the double bond from rings C and D in sterols to C5-C6 during the evolution of cholesterol. 相似文献
116.
The epsilon subunit of Escherichia coli F1-ATPase is a tightly bound but dissociable partial inhibitor of ATPase activity. The effects of epsilon on the enzyme were investigated by comparing the ATPase activity and aurovertin binding properties of the epsilon-depleted F1-ATPase and the epsilon-replete complex. Kinetic data of multisite ATP hydrolysis were analyzed to give the best fit for one, two, or three kinetic components. Each form of F1-ATPase contained a high-affinity component, with a Km near 20 microM and a velocity of approximately 1 unit/mg. Each also exhibited a component with a Km in the range of 0.2 mM. The velocity of this component was 25 units/mg for epsilon-depleted ATPase but only 4 units/mg for epsilon-replete enzyme. The epsilon-depleted enzyme also contained a very low affinity component not present in the epsilon-replete enzyme. In unisite hydrolysis studies, epsilon had no effect on the equilibrium between substrate ATP and product ADP.P1 at the active site but reduced the rate of product release 15-fold. These results suggest that epsilon subunit slows a conformational change that is required to reduce the affinity at the active site, allowing dissociation of product. It is suggested that inhibition of multisite hydrolysis by epsilon is also due to a reduced rate of product release. epsilon-depleted F1-ATPase showed little of no modulation of aurovertin fluorescence by added ADP and ATP. Aurovertin fluorescence titrations in buffer containing ethylenediaminetetraacetic acid (EDTA) revealed that epsilon-depleted enzyme had high affinity for aurovertin (Kd less than 0.1 microM) regardless of the presence of nucleotides.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
117.
We investigated the denaturation of tetrameric 20 beta-hydroxysteroid dehydrogenase (20R)-17 beta,20 beta,21-trihydroxysteroid:NAD+ oxidoreductase, EC 1.1.1.53) to find out whether intermediate states are formed during the process. The denaturation process was studied in the presence and absence of stabilizers, both specific, such as NADH, and non-specific, such as the salting-out anion phosphate. Changes in enzymatic activity, intrinsic protein fluorescence and far-ultraviolet circular dichroism were monitored. When NADH was present, denaturation of the enzyme by urea was a one-step transition between the native and the completely denatured state. In dilute phosphate, and even more so in concentrated phosphate, the existence of intermediate states with different stability is evidenced by the noncoincidence of the transition curves that probe for different functional and conformational aspects of the enzyme. Therefore, for 20 beta-hydroxysteroid dehydrogenase the formation of intermediates can be prevented by adding NADH, or enhanced by adding concentrated phosphate. 相似文献
118.
Laser-Raman and infrared spectroscopic studies reveal abundant beta-pleated sheet conformation in the eggshell proteins of the fish Salmo gairdneri. This secondary structure is the underlying molecular conformation, dictating the formation of the helicoidal architecture of the eggshell. Disulphide bonds crosslink the eggshell proteins of the fertilized eggs and are apparently found in g-g-g (gauche-gauche-gauche), g-g-t (gauche-gauche-trans) and t-g-t (trans-gauche-trans) conformation. There is no evidence for the existence of free sulphydryls. The tyrosines appear to act as hydrogen-bond acceptors, whereas the aromatic residues phenylalanine and tryptophan are also eggshell protein constituents. 相似文献
119.
LINE-1: a mammalian transposable element 总被引:36,自引:0,他引:36
120.
Mass (TM) and relative mass (organ mass/body mass; RTM) of the right testis and epididymis (EM and REM, respectively) were determined every 14 days from 10 to 122 days of age for intact boars (I) and boars hemicastrated on Day 10 (HC) in two crossbred herds (Trial 1 and Trial 2). Plasma follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin, growth hormone (GH), and testosterone were determined in four blood samples from each pig, three collected 24 h prior to castration and one immediately prior to castration. Values for TM and RTM of HC boars were approximately double (p less than 0.0001) those of I boars by 38 days of age, and these differences were maintained through Day 122. Both EM and REM were greater (p less than 0.05) in HC than in I boars from Day 52 to Day 122. The TM, RTM, EM and REM were greater (p less than 0.05) in Trial 1 than in Trial 2 for both I and HC boars from Day 80 to Day 122, indicating an earlier onset of pubertal testicular growth in the Trial-1 boars. Plasma GH concentration was greater (p less than 0.05) in HC than in I boars from Day 16 to Day 38. A transient increase in plasma FSH (p less than 0.05) was observed from Day 24 to Day 38. After Day 38, there was no difference (p greater than 0.05) in FSH or GH between HC and I boars, or between trials. Plasma LH, prolactin, and testosterone concentrations were also similar in HC and I boars.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献