全文获取类型
收费全文 | 91668篇 |
免费 | 4124篇 |
国内免费 | 8332篇 |
专业分类
104124篇 |
出版年
2023年 | 919篇 |
2022年 | 1361篇 |
2021年 | 1741篇 |
2020年 | 1961篇 |
2019年 | 3278篇 |
2018年 | 2296篇 |
2017年 | 1969篇 |
2016年 | 2424篇 |
2015年 | 3517篇 |
2014年 | 4617篇 |
2013年 | 6350篇 |
2012年 | 3864篇 |
2011年 | 5285篇 |
2010年 | 3893篇 |
2009年 | 4084篇 |
2008年 | 4338篇 |
2007年 | 4650篇 |
2006年 | 4246篇 |
2005年 | 3738篇 |
2004年 | 3198篇 |
2003年 | 2815篇 |
2002年 | 2522篇 |
2001年 | 1980篇 |
2000年 | 1760篇 |
1999年 | 1736篇 |
1998年 | 1653篇 |
1997年 | 1384篇 |
1996年 | 1274篇 |
1995年 | 1512篇 |
1994年 | 1379篇 |
1993年 | 1282篇 |
1992年 | 1316篇 |
1991年 | 1083篇 |
1990年 | 1004篇 |
1989年 | 944篇 |
1988年 | 927篇 |
1987年 | 912篇 |
1986年 | 613篇 |
1985年 | 1041篇 |
1984年 | 1411篇 |
1983年 | 1009篇 |
1982年 | 1368篇 |
1981年 | 974篇 |
1980年 | 984篇 |
1979年 | 940篇 |
1978年 | 549篇 |
1977年 | 449篇 |
1976年 | 368篇 |
1975年 | 281篇 |
1973年 | 293篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Uncertainties surrounding the structures of the Δ7-sterols in the seeds of Cucurbita maxima have been resolved. Seven components were found by TLC, GLC, HPLC, mass spectrometry and 1H NMR. They were 24β-ethyl-5α-cholesta-7,22,25(27)-trien-3β-ol, 24β-ethyl-5α-cholesta-7,25(27)-dien-3gb-ol, avenasterol, spinasterol, 24-dihydrospinasterol, 24ζ-methyllathosterol and 25(27)-dehydrofungisterol. The 1H NMR spectra indicated that the sterols with an ethyl substituent at C-24 occurred in the absence of their C-24 epimers. This seems to be the first instance of the detection of 25(27)-dehydrofungisterol in a higher plant. 相似文献
992.
Long poly(dA).poly(dT) tracts (poly(A) tracts), regions of DNA containing at least 20 contiguous dA residues on one strand and dT residues on the complementary strand, are found in about 2 X 10(4) copies interspersed throughout the human genome. Using poly(dA).poly(dA) as a hybridization probe, we identified recombinant lambda phage that contained inserts of human DNA with poly(A) tracts. Three such tracts have been characterized by restriction mapping and sequence analysis. One major poly(A) tract is present within each insert and is composed of from 28 to 35 A residues. In each case, the poly(A) tract directly abuts the 3' end of the human Alu element, indicating that the major class of poly(A) tracts in the human genome is associated with this family of repeats. The poly(A) tracts are also adjacent to A-rich sequences and, in one case, to a polypurine tract, having the structure GA3-GA3-GA4-GA6-GA5-GA4. We suggest that repetitive cycles of unequal crossing over may give rise to both the long poly(A) and polypurine tracts observed in this study. 相似文献
993.
994.
Native oxymyoglobin was isolated directly from the radular muscle of Aplysia kurodai with complete separation from metmyoglobin on a DEAE-cellulose column. It was examined for its spectral and stability properties. The spectrum of Aplysia MbO2 , which lacks the distal histidine, is very similar to those of mammalian oxymyoglobins , the alpha-peak being higher than the beta-peak and the absorbance ratio being 1.03. Its stability, however, is quite different from those of the mammalian oxymyoglobins , and Aplysia MbO2 is found to be extremely susceptible to autoxidation. Its rate is one-hundred times higher at pH 9.0, and its pH dependence is unusual and much less steep, when compared with sperm whale MbO2 as reference. 相似文献
995.
Microsecond rotational motions of eosin-labeled myosin measured by time-resolved anisotropy of absorption and phosphorescence 总被引:3,自引:0,他引:3
We have studied submicrosecond and microsecond rotational motions within the contractile protein myosin by observing the time-resolved anisotropy of both absorption and emission from the long-lived triplet state of eosin-5-iodoacetamide covalently bound to a specific site on the myosin head. These results, reporting anisotropy data up to 50 microseconds after excitation, extend by two orders of magnitude the time range of data on time-resolved site-specific probe motion in myosin. Optical and enzymatic analyses of the labeled myosin and its chymotryptic digests show that more than 95% of the probe is specifically attached to sulfhydryl-1 (SH1) on the myosin head. In a solution of labeled subfragment-1 (S-1) at 4 degrees C, absorption anisotropy at 0.1 microseconds after a laser pulse is about 0.27. This anisotropy decays exponentially with a rotational correlation time of 210 ns, in good agreement with the theoretical prediction for end-over-end tumbling of S-1, and with times determined previously by fluorescence and electron paramagnetic resonance. In aqueous glycerol solutions, this correlation time is proportional to viscosity/temperature in the microsecond time range. Furthermore, binding to actin greatly restricts probe motion. Thus the bound eosin is a reliable probe of myosin-head rotational motion in the submicrosecond and microsecond time ranges. Our submicrosecond data for myosin monomers (correlation time 400 ns) also agree with previous results using other techniques, but we also detect a previously unresolvable slower decay component (correlation time 2.6 microseconds), indicating that the faster motions are restricted in amplitude. This restriction is not consistent with the commonly accepted free-swivel model of S-1 attachment in myosin. In synthetic thick filaments of myosin, both fast (700 ns) and slow (5 microseconds) components of anisotropy decay are observed. In contrast to the data for monomers, the anisotropy of filaments has a substantial residual component (26% of the initial anisotropy) that does not decay to zero even at times as long as 50 microseconds, implying significant restriction in overall rotational amplitude. This result is consistent with motion restricted to a cone half-angle of about 50 degrees. The combined results are consistent with a model in which myosin has two principal sites of segmental flexibility, one giving rise to submicrosecond motions (possibly corresponding to the junction between S-1 and S-2) and the other giving rise to microsecond motions (possibly corresponding to the junction between S-2 and light meromyosin).(ABSTRACT TRUNCATED AT 400 WORDS) 相似文献
996.
L. Tabe P. Krieg R. Strachan D. Jackson E. Wallis A. Colman 《Journal of molecular biology》1984,180(3):645-666
The intramolecular signals for chicken ovalbumin secretion were examined by producing mutant proteins in Xenopus oocytes. An ovalbumin complementary DNA clone was manipulated in vitro, and constructs containing altered protein-coding sequences and either the simian virus 40 (SV40) early promoter or Herpes simplex thymidine kinase promoter, were microinjected into Xenopus laevis oocytes. The removal of the eight extreme N-terminal amino acids of ovalbumin had no effect on the segregation of ovalbumin with oocyte membranes nor on its secretion. A protein lacking amino acids 2 to 21 was sequestered in the endoplasmic reticulum but remained strongly associated with the oocyte membranes rather than being secreted. Removal of amino acids 231 to 279, a region previously reported to have membrane-insertion function, resulted in a protein that also entered the endoplasmic reticulum but was not secreted. Hybrid proteins containing at their N terminus amino acids 9 to 41 or 22 to 41 of ovalbumin fused to the complete chimpanzee α-globin polypeptide were also sequestered by oocyte membranes. We conclude that the ovalbumin “signal” seque?ce is internally located within amino acids 22 to 41, and we speculate that amino acids 9 to 21 could be important for the completion of ovalbumin translocation through membranes. 相似文献
997.
Complete nucleotide sequences of all three poliovirus serotype genomes. Implication for genetic relationship, gene function and antigenic determinants 总被引:91,自引:0,他引:91
H Toyoda M Kohara Y Kataoka T Suganuma T Omata N Imura A Nomoto 《Journal of molecular biology》1984,174(4):561-585
The complete nucleotide sequences of the genomes of the type 2 ( P712 , Ch, 2ab ) and type 3 (Leon 12a1b ) poliovirus vaccine strains were determined. Comparison of the sequences with the previously established genome sequence of type 1 (LS-c, 2ab ) poliovirus vaccine strain revealed that 71% of the nucleotides in the genome RNAs were common, that the 5' and 3' termini of the genomes were highly homologous, and that more than 80% of the nucleotide differences in the coding region occurred in the third letter position of in-phase codons, resulting in a low frequency of amino acid difference. These results strongly suggested that the serotypes of poliovirus derived from a common prototype. A comparison of the amino acid sequences predicted from the genome sequences showed highest variation in the capsid protein region, whereas non-structural proteins are highly conserved. Initiation of polyprotein synthesis occurs in all three strains more than 740 nucleotides downstream from the 5' end. An analysis of the non-coding region suggests that small peptides that could potentially originate from this region are conserved. The amino acid sequences immediately surrounding the cleavage signals, however, show a higher than average degree of variation. The analysis of the amino acid sequences of the capsid protein VP1 of all serotypes has led to the prediction of potential antigenic sites on the virion involved in neutralization. 相似文献
998.
Dr. F. James Bailey Janet Blankenship Jon H. Condra Robert Z. Maigetter Ronald W. Ellis 《Journal of industrial microbiology & biotechnology》1987,2(1):47-52
Summary Studies are presented on the fermentation of recombinantEscherichia coli that express rat atrial natriuretic factor (ANF) as a fusion protein. Our objective was to achieve high cell density while maintaining ANF expression at the same level as observed in shake flasks. Improved fermentation conditions included: maintaining glucose concentrations at 1 g/l, using an enriched medium, adding concentrates of medium throughout the fermentation, and blending oxygen for adequate aeration. Cell densities of 12 g/l (dry weight) were achieved, which represented a 10-fold increase over non-improved conditions, while maintaining ANF levels at 7 mg/g of dry cell mass. When galactose was used as an initial carbon source or as a feed supplement, there was a 2-3-fold increase in the expression of ANF from these high-cell-density fermentations. The recombinant ANF was biologically active. 相似文献
999.
As a quantitative approach to the life histories of fishes, the present paper attempted to predict a relation among reproduction, growth and mortality numerically with a technique of control theory, the discrete maximum principle. A method for predicting the relation was derived on the postulate that natural selection maximized the net reproductive rate subject to a few constraints. The derived method was applied to Atlantic cod and Atlantic herring populations in the Southern Gulf of St. Lawrence as numerical examples. The examples demonstrated that the theoretical reproductive effort and body weight were well consistent with the observed ones every age but the theoretical survival rates were slightly different from the observed ones. For the reasons mentioned below, however, it should be interpreted that the examples rather support the adopted postulate to a certain degree. First, in general, it is very difficult to obtain good estimates of the rates with traditional methods. Second, intense fishing pressure possibly changes the life history parameters to some extent in fish populations. Moreover, the examples also suggested that, to examine the postulate in further detail, similar analyses had to be made with the data of many fish populations on which intense fishing pressure had not been exerted. 相似文献
1000.
接触二氧化硫后小麦叶片中逆境乙烯的生物合成 总被引:2,自引:0,他引:2
AVG和AOA强烈抑制二氧化硫处理小麦叶片中乙烯产生和ACC合成,对MACC的形成也有一定的抑制作用。CoGl_2明显抑制乙烯产生,而ACC大量积累,MACC含量则未因ACC增加而相应增加。DNP和CCCP也抑制乙烯产生,但前者引起ACC大量积累,后者引起ACC含量下降。CHI对乙烯产生和ACC形成均显示强烈的抑制作用,同时也明显抑制MACC形成。这表明小麦叶片接触SO_2引起的逆境乙烯也是循蛋氨酸→SAM→ACC→乙烯途径。 相似文献