首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   165426篇
  国内免费   44篇
  完全免费   8935篇
  2020年   135篇
  2019年   538篇
  2018年   2385篇
  2017年   2148篇
  2016年   2067篇
  2015年   2113篇
  2014年   2988篇
  2013年   6464篇
  2012年   7001篇
  2011年   8469篇
  2010年   4766篇
  2009年   4952篇
  2008年   8654篇
  2007年   8891篇
  2006年   8433篇
  2005年   8551篇
  2004年   8798篇
  2003年   8462篇
  2002年   8297篇
  2001年   7988篇
  2000年   8544篇
  1999年   7074篇
  1998年   2442篇
  1997年   2329篇
  1996年   2078篇
  1995年   1983篇
  1994年   1720篇
  1993年   1840篇
  1992年   5563篇
  1991年   5085篇
  1990年   4632篇
  1989年   4224篇
  1988年   3864篇
  1987年   2059篇
  1986年   1005篇
  1985年   807篇
  1984年   863篇
  1983年   489篇
  1982年   705篇
  1981年   612篇
  1980年   510篇
  1979年   481篇
  1978年   433篇
  1977年   352篇
  1976年   375篇
  1975年   224篇
  1974年   196篇
  1973年   253篇
  1972年   225篇
  1971年   228篇
  1970年   215篇
  1969年   171篇
  1968年   71篇
  1967年   84篇
  1966年   74篇
  1965年   58篇
  1964年   28篇
  1963年   46篇
  1962年   38篇
  1961年   59篇
  1960年   27篇
  1959年   33篇
  1958年   18篇
  1957年   24篇
  1956年   11篇
  1955年   9篇
  1954年   6篇
  1953年   17篇
  1952年   7篇
  1950年   3篇
  1948年   4篇
  1940年   2篇
  1939年   4篇
  1938年   5篇
  1937年   8篇
  1936年   6篇
  1935年   8篇
  1934年   5篇
  1933年   10篇
  1932年   2篇
  1931年   8篇
  1930年   5篇
  1929年   3篇
  1928年   3篇
  1927年   4篇
  1926年   2篇
  1925年   2篇
  1924年   2篇
  1923年   3篇
  1921年   4篇
  1919年   2篇
  1916年   1篇
  1915年   1篇
  1913年   5篇
  1912年   1篇
  1908年   2篇
  1905年   1篇
  1904年   1篇
  1902年   1篇
  1888年   1篇
排序方式: 共有174405条查询结果,搜索用时 206 毫秒
1.
M Orita  Y Suzuki  T Sekiya  K Hayashi 《Genomics》1989,5(4):874-879
We report a rapid and sensitive method for the detection of base changes in given sequences of genomic DNA. This technique is based on the facts that specific regions of genomic sequences can be efficently labeled and amplified simultaneously by using labeled substrates in the polymerase chain reaction and that in nondenaturing polyacrylamide gels, the electrophoretic mobility of single-stranded nucleic acid depends not only on its size but also on its sequence. The process does not involve restriction enzyme digestion, blotting, or hybridization to probes. We found that most single base changes in up to 200-base fragments could be detected as mobility shifts. RAS oncogene activation was detected by this technique. We also show that the interspersed repetitive sequences of human, Alu repeats are highly polymorphic.  相似文献
2.
Mouse anti-Fas monoclonal antibody has a cytolytic activity on human cells that express the antigen. Complementary DNAs encoding the cell surface antigen Fas were isolated from a cDNA library of human T cell lymphoma KT-3 cells. The nucleotide sequence of the cDNAs revealed that the molecule coding for the Fas antigen determinant is a 319 amino acid polypeptide (Mr 36,000) with a single transmembrane domain. The extracellular domain is rich in cysteine residue, and shows a similarity to that of human tumor necrosis factor receptors, human nerve growth factor receptor, and human B cell antigen CD40. Murine WR19L cells or L929 cells transformed with the human Fas antigen cDNA were killed by the anti-Fas antibody in the process known as apoptosis.  相似文献
3.
F. Tajima 《Genetics》1989,123(3):585-595
The relationship between the two estimates of genetic variation at the DNA level, namely the number of segregating sites and the average number of nucleotide differences estimated from pairwise comparison, is investigated. It is found that the correlation between these two estimates is large when the sample size is small, and decreases slowly as the sample size increases. Using the relationship obtained, a statistical method for testing the neutral mutation hypothesis is developed. This method needs only the data of DNA polymorphism, namely the genetic variation within population at the DNA level. A simple method of computer simulation, that was used in order to obtain the distribution of a new statistic developed, is also presented. Applying this statistical method to the five regions of DNA sequences in Drosophila melanogaster, it is found that large insertion/deletion (greater than 100 bp) is deleterious. It is suggested that the natural selection against large insertion/deletion is so weak that a large amount of variation is maintained in a population.  相似文献
4.
Summary Some simple formulae were obtained which enable us to estimate evolutionary distances in terms of the number of nucleotide substitutions (and, also, the evolutionary rates when the divergence times are known). In comparing a pair of nucleotide sequences, we distinguish two types of differences; if homologous sites are occupied by different nucleotide bases but both are purines or both pyrimidines, the difference is called type I (or transition type), while, if one of the two is a purine and the other is a pyrimidine, the difference is called type II (or transversion type). Letting P and Q be respectively the fractions of nucleotide sites showing type I and type II differences between two sequences compared, then the evolutionary distance per site is K = — (1/2) ln {(1 — 2P — Q) }. The evolutionary rate per year is then given by k = K/(2T), where T is the time since the divergence of the two sequences. If only the third codon positions are compared, the synonymous component of the evolutionary base substitutions per site is estimated by K'S = — (1/2) ln (1 — 2P — Q). Also, formulae for standard errors were obtained. Some examples were worked out using reported globin sequences to show that synonymous substitutions occur at much higher rates than amino acid-altering substitutions in evolution.Contribution No. 1330 from the National Institute of Genetics, Mishima, 411 Japan  相似文献
5.
6.
Adiponectin is an adipocyte-derived hormone. Recent genome-wide scans have mapped a susceptibility locus for type 2 diabetes and metabolic syndrome to chromosome 3q27, where the gene encoding adiponectin is located. Here we show that decreased expression of adiponectin correlates with insulin resistance in mouse models of altered insulin sensitivity. Adiponectin decreases insulin resistance by decreasing triglyceride content in muscle and liver in obese mice. This effect results from increased expression of molecules involved in both fatty-acid combustion and energy dissipation in muscle. Moreover, insulin resistance in lipoatrophic mice was completely reversed by the combination of physiological doses of adiponectin and leptin, but only partially by either adiponectin or leptin alone. We conclude that decreased adiponectin is implicated in the development of insulin resistance in mouse models of both obesity and lipoatrophy. These data also indicate that the replenishment of adiponectin might provide a novel treatment modality for insulin resistance and type 2 diabetes.  相似文献
7.
8.
H Niwa  K Yamamura  J Miyazaki 《Gene》1991,108(2):193-199
We have developed a new expression vector which allows efficient selection for transfectants that express foreign genes at high levels. The vector is composed of a ubiquitously strong promoter based on the beta-actin promoter, a 69% subregion of the bovine papilloma virus genome, and a mutant neomycin phosphotransferase II-encoding gene driven by a weak promoter, which confers only marginal resistance to G418. Thus, high concentrations of G418 (approx. 800 micrograms/ml) effectively select for transfectants containing a high vector copy number (greater than 300). We tested this system by producing human interleukin-2 (IL-2) in L cells and Chinese hamster ovary (CHO) cells, and the results showed that high concentrations of G418 efficiently yielded L cell and CHO cell transfectants stably producing IL-2 at levels comparable with those previously attained using gene amplification. The vector sequences were found to have integrated into the host chromosome, and were stably maintained in the transfectants for several months.  相似文献
9.
Takahashi K  Yamanaka S 《Cell》2006,126(4):663-676
Differentiated cells can be reprogrammed to an embryonic-like state by transfer of nuclear contents into oocytes or by fusion with embryonic stem (ES) cells. Little is known about factors that induce this reprogramming. Here, we demonstrate induction of pluripotent stem cells from mouse embryonic or adult fibroblasts by introducing four factors, Oct3/4, Sox2, c-Myc, and Klf4, under ES cell culture conditions. Unexpectedly, Nanog was dispensable. These cells, which we designated iPS (induced pluripotent stem) cells, exhibit the morphology and growth properties of ES cells and express ES cell marker genes. Subcutaneous transplantation of iPS cells into nude mice resulted in tumors containing a variety of tissues from all three germ layers. Following injection into blastocysts, iPS cells contributed to mouse embryonic development. These data demonstrate that pluripotent stem cells can be directly generated from fibroblast cultures by the addition of only a few defined factors.  相似文献
10.
Comparative analysis of molecular sequence data is essential for reconstructing the evolutionary histories of species and inferring the nature and extent of selective forces shaping the evolution of genes and species. Here, we announce the release of Molecular Evolutionary Genetics Analysis version 5 (MEGA5), which is a user-friendly software for mining online databases, building sequence alignments and phylogenetic trees, and using methods of evolutionary bioinformatics in basic biology, biomedicine, and evolution. The newest addition in MEGA5 is a collection of maximum likelihood (ML) analyses for inferring evolutionary trees, selecting best-fit substitution models (nucleotide or amino acid), inferring ancestral states and sequences (along with probabilities), and estimating evolutionary rates site-by-site. In computer simulation analyses, ML tree inference algorithms in MEGA5 compared favorably with other software packages in terms of computational efficiency and the accuracy of the estimates of phylogenetic trees, substitution parameters, and rate variation among sites. The MEGA user interface has now been enhanced to be activity driven to make it easier for the use of both beginners and experienced scientists. This version of MEGA is intended for the Windows platform, and it has been configured for effective use on Mac OS X and Linux desktops. It is available free of charge from http://www.megasoftware.net.  相似文献
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号