全文获取类型
收费全文 | 33729篇 |
免费 | 1566篇 |
国内免费 | 1543篇 |
出版年
2024年 | 38篇 |
2023年 | 380篇 |
2022年 | 501篇 |
2021年 | 690篇 |
2020年 | 706篇 |
2019年 | 893篇 |
2018年 | 794篇 |
2017年 | 757篇 |
2016年 | 855篇 |
2015年 | 1136篇 |
2014年 | 1885篇 |
2013年 | 2783篇 |
2012年 | 1461篇 |
2011年 | 1489篇 |
2010年 | 1286篇 |
2009年 | 1494篇 |
2008年 | 1566篇 |
2007年 | 1568篇 |
2006年 | 1488篇 |
2005年 | 1388篇 |
2004年 | 1233篇 |
2003年 | 1201篇 |
2002年 | 1061篇 |
2001年 | 787篇 |
2000年 | 771篇 |
1999年 | 722篇 |
1998年 | 708篇 |
1997年 | 638篇 |
1996年 | 588篇 |
1995年 | 614篇 |
1994年 | 597篇 |
1993年 | 472篇 |
1992年 | 486篇 |
1991年 | 381篇 |
1990年 | 388篇 |
1989年 | 296篇 |
1988年 | 344篇 |
1987年 | 270篇 |
1986年 | 231篇 |
1985年 | 336篇 |
1984年 | 391篇 |
1983年 | 256篇 |
1982年 | 288篇 |
1981年 | 140篇 |
1980年 | 121篇 |
1979年 | 118篇 |
1978年 | 71篇 |
1977年 | 38篇 |
1976年 | 41篇 |
1973年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The effect of early postnatal undernutrition and subsequent rehabilitation on wet weight, DNA, RNA, protein and the activities
of acid and alkaline DNases in the cerebellar region of rat brain was studied. The cerebellar region was found to be affected
significantly during early undernutrition. Further, earlier the initiation of nutritional rehabilitation the better was the
recovery and in some cases timely nutritional rehabilitation resulted in better than normal biochemical composition of the
brain. The specific activities of acid and alkaline DNases were not affected by early undernutrition. However, the total activities
of these enzymes were significantly low in undernourished rats (R115 and R21) Rehabilitation of these deprived groups upto 150 days resulted in higher amounts of these enzymes as compared to those of
age-matched controls. It is concluded that the two DNases, are synthesized in a preferential manner during rehabilitation,
It is further concluded that cerebellar region, in terms of development schedule and response to imposed calorie restriction,
is intermediary between grey and white matter regions. 相似文献
992.
Approximately 43–60% of the total genome in bovine, goat and sheep consisted of interspersed repeated and single copy DNA
sequences. Most of the interspersed repeated DNA sequences were 1500–2400 nucleotide pair long while a minor portion was more
than 4000 nucleotide pair long in goat and sheep and 3200 nucleotide pair long in bovine. About 1/3rd of single copy sequence
were interspersed and their length was in the range of 1000–1500 nucleotide pairs. 相似文献
993.
The effect of gyrase inhibitors and cyclic AMP on induction and glucose repression of the 6-hydroxy-nicotine oxidases in Arthrobacter oxidans 总被引:1,自引:0,他引:1
The induction by d,l-nicotine of the enantiozymes 6-hydroxy-L-nicotine oxidase and 6-hydroxy-D-nicotine oxidase in Archrobacter oxidans was differently affected by the inhibitors of Escherichia coli gyrase, novobiocin and nalidixic acid. These compounds inhibited 6-hydroxy-L-nicotine oxidase induction slightly, but led to an increase in the level of 6-hydroxy-D-nicotine oxidase activity. Furthermore, the specific repression by glucose of 6-hydroxy-D-nicotine oxidase synthesis was not abolished by the addition of cAMP but by that of novobiocin.Abbreviations 6-HDNO
6-hydroxy-D-nicotine oxidase
- 6-HLNO
6-hydroxy-L-nicotine oxidase
- cAMP
cyclic 3,5-adenosine monophosphate
- Enzymes
Adenylate cyclase
- ATP
pyrophosphate-lyase (cyclizing) (EC 4.6.1.1)
- cAMP-phosphodiesterase
3:5-cyclic-nucleotide 5-nucleotido-hydrolase (EC 3.1.4.17)
- DNA gyrase
DNA topoisomerase II (EC 5.99)
- DNA polymerase
deoxynucleosidetriphosphate: DNA desoxynucleotidyl-transferase (EC 2.7.7.7)
- 6-hydroxy-L-nicotine oxidase
6-hydroxy-L-nicotine: oxygen oxidoreductase (EC 1.5.3.5)
- 6-hydroxy-D-nicotine oxidase
6-hydroxy-D-nicotine: oxygen oxidoreductase (EC 1.5.3.6)
- -lactamase
penicillin amido--lactamhydrolase (EC 3.5.2.6)
- nicotine dehydrogenase
nicotine: (acceptor)6-oxidoreductase (hydroxylating) (EC 1.5.99.4) 相似文献
994.
In Escherichia coli three major alkaline phosphatase isozymes are formed by molecular conversions depending on physiological conditions. A chromosomal gene, iap, is responsible for alkaline phosphatase isozyme conversion and is assumed to code for a proteolytic enzyme removing the arginine residue(s) from the N-terminal position of alkaline phosphatase subunits. A chromosomal fragment which complemented the Iap? phenotype was cloned into pBR322 by a shotgun method. Transducing phage λiap was constructed in vitro from the chromosomal fragment containing the iap gene and λtna DNA. The integration site of the phage on chromosome was identified as the iap locus by PI transduction, which meant that the cloned chromosomal DNA contained authentic iap gene.The restriction map of the hybrid plasmid was constructed. Based upon this information, several iap deletion plasmids as well as smaller iup+ plasmids were constructed. Analysis of the phenotypes conferred by these plasmids enabled us to locate iap gene within a 2-kb segment of the cloned DNA.The cells carrying the iap+ plasmid showed very efficient isozyme conversion even in medium containing arginine, an inhibitor for the isozyme conversion. This indicates overproduction of the iap gene product. 相似文献
995.
996.
The nucleotide sequence of a segment of mtDNA from Rattus norvegiens (rat) which contains the genes for tRNAile, tRNAgl and tRNAf-met has been determined. A detailed comparison has been made between this sequence and the corresponding sequences of mouse, human and bovine mtDNAs with regard to the primary and secondary structure of the tRNA genes, the regions connecting the tRNA genes, and the regions flanking the tRNA genes which code for the carboxyl terminus of URF-1 and the amino terminus of URF-2. No differences were found in the nucleotide sequences of the genes for tRNAile, tRNAgln and tRNAf-met in mtDNAs from three different female lines of rats (SASCO-1, SASCO-2 and Wild-UT) that differ by substitutions of 0.8% to 1.8% of their total nucleotides. 相似文献
997.
998.
Rat immunoglobulin δ heavy-chain mRNA has been isolated. RNA blot analysis revealed that this mRNA with a length of 1.8 kb encodes for the secreted form of IgD. The corresponding cDNA was cloned in plasmid pBR322 and its sequence was determined. The hybrid plasmid contains a 775-bp insert comprising a partial Cδ1 sequence and complete CδH, Cδ3, CδDC and 3' untranslated sequences. Rat and mouse IgD amino acid sequences show striking homology in Cδ3 and CδDC regions. 相似文献
999.
Dictyostelium discoideum DNA fragments have been inserted into the chimeric bacterium-yeast plasmid YEp13. Recombinant plasmids were used to transform yeast using a strain of Saccharomyces cerevisiae deficient in OMP decarboxylase activity. Several clones were selected for growth in uracil-free medium. One clone was further analysed and contains a plasmid with a segment of D. discoideum DNA which complements a yeast ura3 mutation. 相似文献
1000.
A plasmid derived from ColE1 is constructed so that the removal of one restriction endonuclease HindIII fragment allows the ends of the remaining single fragment (the replicator) to be joined, generating a palindromic sequence 2394 bp in length. The circular species thus produced gives rise to transformants of E. coli at very low frequency. Since the palindromic sequence is effectively lethal to a plasmid containing it, the replicator will give rise to more transformants when the restriction fragment originally removed from it is replaced by another. This principle can be exploited to allow the efficient molecular cloning of unselected restriction fragments. 相似文献