全文获取类型
收费全文 | 4668篇 |
免费 | 283篇 |
国内免费 | 926篇 |
出版年
2024年 | 4篇 |
2023年 | 68篇 |
2022年 | 108篇 |
2021年 | 125篇 |
2020年 | 101篇 |
2019年 | 162篇 |
2018年 | 212篇 |
2017年 | 123篇 |
2016年 | 144篇 |
2015年 | 145篇 |
2014年 | 381篇 |
2013年 | 437篇 |
2012年 | 273篇 |
2011年 | 257篇 |
2010年 | 249篇 |
2009年 | 271篇 |
2008年 | 298篇 |
2007年 | 348篇 |
2006年 | 243篇 |
2005年 | 272篇 |
2004年 | 207篇 |
2003年 | 192篇 |
2002年 | 181篇 |
2001年 | 138篇 |
2000年 | 117篇 |
1999年 | 103篇 |
1998年 | 88篇 |
1997年 | 62篇 |
1996年 | 72篇 |
1995年 | 61篇 |
1994年 | 60篇 |
1993年 | 45篇 |
1992年 | 28篇 |
1991年 | 40篇 |
1990年 | 33篇 |
1989年 | 31篇 |
1988年 | 18篇 |
1987年 | 20篇 |
1986年 | 21篇 |
1985年 | 23篇 |
1984年 | 25篇 |
1983年 | 16篇 |
1982年 | 17篇 |
1981年 | 12篇 |
1980年 | 12篇 |
1979年 | 11篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1972年 | 4篇 |
排序方式: 共有5877条查询结果,搜索用时 46 毫秒
21.
22.
23.
Partial digestion of a target DNA fragment with 4-bp-recognition restriction enzymes followed by a forced ligation to an M13 vector was employed for the construction of a subfragment library. The library can be used for either shotgun or non-random nucleotide sequencing. Application of the partial digests generated with the 4-bp recognition restriction enzymes instead of DNase I in the improved non-random strategy for nucleotide sequencing (Li and Wu, 1987) made the procedure as easy as that of the random strategy. The library can also be used in shotgun nucleotide sequencing directly, and few self-ligated subfragments were found. The usefulness of this procedure was demonstrated by the sequencing of a goat 6.5-kb EcoRI fragment, which is located 5' to the globin gene. 相似文献
24.
alpha-Lytic protease, an extracellular bacterial serine protease, is synthesized with a large pro region that is required in vivo for the proper folding of the protease domain. To allow detailed mechanistic study, we have reconstituted pro region-dependent folding in vitro. The pro region promotes folding of the protease domain in the absence of other protein factors or exogenous energy sources. Surprisingly, we find that the pro region is a high affinity inhibitor of the mature protease. The pro region also inhibits the closely related Streptomyces griseus protease B, but not the more distantly related, yet structurally similar protease, elastase. Based on these data, we suggest a mechanism in which pro region binding reduces the free energy of a late folding transition state having native-like structure. 相似文献
25.
The influence of a catadromous life-strategy on levels of spatial genetic structuring in fish is poorly understood. In an effort to gain a better appreciation of how this specialized life-strategy determines population genetic structuring, we assessed variation in the mitochondrial DNA (mtDNA) control region in a catadromous perciform, the Australian bass Macquaria novemaculeata . Nineteen putative haplotypes were resolved using temperature gradient gel electrophoresis from 10 geographically distinct populations. Significant heterogeneity was revealed in haplotype frequencies and their spatial distributions among many locales. Gene partitioning statistics ( AMOVA ) for both raw haplotype frequency data and frequency data with sequence divergences were concordant, indicating that M. novemaculeata populations were moderately genetically structured (ΦST = 0.05, 0.06; P < 0.001, respectively). Isolation by distance seems to be a strong structuring force in M. novemaculeata , culminating in no detectable phylogeographic structuring among haplotypes. Low sequence divergences were observed among many haplotypes and it is suggested that these are the result of pruning of maternal lineages by cyclical variations in female reproductive success. This study highlights the importance of life-history patterns and, in particular, spawning locality, in determining spatial structuring of mtDNA variation in catadromous species. 相似文献
26.
To understand the origin and genetic diversity of Italian horses, mitochondrial DNA D‐loop sequences were generated for 163 horses from seven breeds. Sequence analysis of a 480‐bp segment revealed a total of 84 haplotypes with 57 polymorphic sites, indicating multiple maternal origins and high genetic diversity. Comparison of the haplotypes with the equine mtDNA haplotype/haplogroup nomenclature showed a haplogroup distribution in the Italian breeds more similar to that found in the Middle East breeds than in the European breeds, probably due to the economic and cultural relationship with the Middle East in the past centuries. 相似文献
27.
T. Žuna Pfeiffer S. Petrošanec I. Štolfa Čamagajevac F. Stević 《Plant biosystems》2018,152(5):1059-1066
Habitat fragmentation and modification have been recognized as the most serious threats to biodiversity. In Croatia, the intensive human activities resulted in the landscape transformation, reduction of grassland areas and loss of species number. Increasingly threatened are the wild edible and medicinal plants which contribute remarkably to the biodiversity of different grassland types. In northeastern Croatia, one of the last remnants of the steppe-like grassland remained preserved in a very complex anthropogenic landscape. We examined the diversity of vascular plants occurring on this small habitat patch. Among the relatively high number of plant taxa (in a total of 177) found during the two years of investigations, most could be used both for human nutrition (28 taxa), and for the treatment of a wide range of diseases (60 taxa), while some taxa have nutritional value for livestock (17 taxa). All 23 recorded taxa with edible and medicinal properties are threatened in Croatia, 21 are threatened across Europe, and two are Red Listed taxa. Our results showed that small and isolated steppe-like grassland represents an important refuge for variety of valuable plant species in intensively used landscapes. Accordingly, such small habitats should be, at least locally, effectively protected. 相似文献
28.
Mengnan Wang Dongjie Li Mingyue Zhang Wenzhi Yang Yali Cui Shijie Li 《Animal genetics》2015,46(4):354-360
The CDKN1C gene encodes a cyclin‐dependent kinase inhibitor and is one of the key genes involved in the development of Beckwith–Wiedemann syndrome and cancer. In this study, using a direct sequencing approach based on a single nucleotide polymorphism (SNP) at genomic DNA and cDNA levels, we show that CDKN1C exhibits monoallelic expression in all seven studied organs (heart, liver, spleen, lung, kidney, muscle and subcutaneous fat) in cattle. To investigate how methylation regulates imprinting of CDKN1C in cattle, allele‐specific methylation patterns in two putative differential methylation regions (DMRs), the CDKN1C DMR and KvDMR1, were analyzed in three tissues (liver, spleen and lung) using bisulfite sequencing PCR. Our results show that in the CDKN1C DMR both parental alleles were unmethylated in all three analyzed tissues. In contrast, KvDMR1 was differentially methylated between the two parental alleles in the same tissues. Statistical analysis showed that there is a significant difference in the methylation level between the two parental alleles (P < 0.01), confirming that this region is the DMR of KvDMR1 and that it may be correlated with CDKN1C imprinting. 相似文献
29.
30.
Adventitious root formation in vitro in 1-mm stem slices cut from microshoots of apple cv. Jork 9 was studied using light
and electron microscopy. When indole-3-butyric acid (IBA) had been added to the medium, starch grains accumulated during the
first 24 h of culture in cells of the cambial region and in cells in the vicinity of vascular tissue and in the primary rays.
This accumulation occurred only in the basal part of explants. After that, the nuclei in these cells were activated, and the
density of the cytoplasm and the number of cell organelles increased, whereas starch was broken down. Cambium cells started
to divide transversely and at 96 h, after several divisions, a continuous ring of isodiametric cytoplasmic cells had appeared
around the xylem near the basal cutting surface. The cells in this ring were rich in cell structures, and did not contain
large starch grains and a central vacuole. Root meristemoids regenerated from the portions of the ring that were localized
in the primary rays. From the other cells in the ring, callus developed. The meristemoids did not grow into the direction
of the epidermis as in shoots, but along the vascular bundles. After emergence from the cutting surface, the meristemoids
were transformed into small, dome-like primordia. They developed a typical root apex with root cap, root ground meristem and
tracheid connection with shoot vascular tissue.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献