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排序方式: 共有429条查询结果,搜索用时 31 毫秒
1.
John Bower 《Evolutionary anthropology》1995,4(4):130-139
The study of early food production in sub-Saharan Africa is at least as challenging as it is rewarding. Problems arise in large degree from the scarcity of relevant archeological material, particularly the remains of domesticated plants from prehistoric sites. This is attributable to several factors, including poor preservation, difficulties in recovering such material, and the limited amount of work so far invested in obtaining it. But, problems notwithstanding, fresh data and new methodological approaches have revealed aspects of early African food production that are interesting in themselves, as well as in global perspective. For example, contrary to what occurred in most other parts of the world, livestock herding in Africa often predated the earliest evidence of cultivation of domesticated plants. Moreover, the initial spread of food production throughout much of sub-Saharan Africa was accompanied by iron, rather than lithic, technology. This overview of current knowledge about early African food production is aimed at highlighting developmental patterns while also exposing limitations in our understanding of these patterns. Because of Africa's vast size, uniform coverage in reasonable depth of all parts of the continent is not possible. Thus, for example, I will not explicitly cover the complex neolithic record from Africa's Mediterranean region. Instead, I will generally concentrate on bodies of data and lines of investigation that characterize distinctive features of the African version of initial steps in raising crops and animals. 相似文献
2.
Evolution of the 28S ribosomal RNA gene in anurans: regions of variability and their phylogenetic implications 总被引:1,自引:0,他引:1
Fifteen restriction sites were mapped to the 28S ribosomal RNA gene of
individuals representing 54 species of frogs, two species of salamanders, a
caecilian, and a lungfish. Eight of these sites were present in all species
examined, and two were found in all but one species. Alignment of these
conserved restriction sites revealed, among anuran 28S rRNA genes, five
regions of major length variation that correspond to four of 12 previously
identified divergent domains of this gene. One of the divergent domains
(DD8) consists of two regions of length variation separated by a short
segment that is conserved at least throughout tetrapods. Most of the
insertions, deletions, and restriction-site variations identified in the
28S gene will require sequence-level analysis for a detailed reconstruction
of their history. However, an insertion in DD9 that is coextensive with
frogs in the suborder Neobatrachia, a BstEII site that is limited to
representatives of two leptodactylid subfamilies, and a deletion in DD10
that is found only in three ranoid genera are probably synapomorphies.
相似文献
3.
D. Jane Bower 《Chromosoma》1987,95(1):76-80
At least one-third of mouse trophoblast cells undergo endoreduplication during the first half of gestation. It has been suggested that the endoreduplicated chromosomes may be polytenised. Here it is shown, using in situ hybridisation to the -1 antitrypsin genes, which map at a unique site, that while there is a tendency for duplicated chromosomes to cluster, this does not involve the complete fusion of replicated chromatids found in fully polytene chromosomes, and in a substantial proportion of homologues the sites on the chromosome arms corresponding to these genes are widely separated. The centromeres do not fuse into a single chromocentre but the possibility is not ruled out that individual chromosomes may be polytenised in the centromeric region. Evidence is also presented showing that endoreduplication in trophoblast nuclei is not accompanied by the formation of new prekinetochore structures, in contrast to the situation in polyploid mouse liver and C127 cells. 相似文献
4.
Characterization of cross-bridge elasticity and kinetics of cross-bridge cycling during force development in single smooth muscle cells 总被引:4,自引:2,他引:2 下载免费PDF全文
Force development in smooth muscle, as in skeletal muscle, is believed to reflect recruitment of force-generating myosin cross-bridges. However, little is known about the events underlying cross-bridge recruitment as the muscle cell approaches peak isometric force and then enters a period of tension maintenance. In the present studies on single smooth muscle cells isolated from the toad (Bufo marinus) stomach muscularis, active muscle stiffness, calculated from the force response to small sinusoidal length changes (0.5% cell length, 250 Hz), was utilized to estimate the relative number of attached cross-bridges. By comparing stiffness during initial force development to stiffness during force redevelopment immediately after a quick release imposed at peak force, we propose that the instantaneous active stiffness of the cell reflects both a linearly elastic cross-bridge element having 1.5 times the compliance of the cross-bridge in frog skeletal muscle and a series elastic component having an exponential length-force relationship. At the onset of force development, the ratio of stiffness to force was 2.5 times greater than at peak isometric force. These data suggest that, upon activation, cross-bridges attach in at least two states (i.e., low-force-producing and high-force-producing) and redistribute to a steady state distribution at peak isometric force. The possibility that the cross-bridge cycling rate was modulated with time was also investigated by analyzing the time course of tension recovery to small, rapid step length changes (0.5% cell length in 2.5 ms) imposed during initial force development, at peak force, and after 15 s of tension maintenance. The rate of tension recovery slowed continuously throughout force development following activation and slowed further as force was maintained. Our results suggest that the kinetics of force production in smooth muscle may involve a redistribution of cross-bridge populations between two attached states and that the average cycling rate of these cross-bridges becomes slower with time during contraction. 相似文献
5.
A second gene has been discovered at a previously studied Balbiani ring in Chironomus. Northern hybridizations demonstrated that cDNA clone pCt35 originated from a salivary gland specific 6.5-kilobase (kb) RNA that was abundant, nonribosomal, and apparently poly(A)+. pCt35 had a 120-base pair (bp) insert with 1.6 copies of a 75-bp sequence that contained two open reading frames. Southern hybridizations indicated that pCt35 was homologous to at least a 4-kb block of genomic DNA organized as a hierarchy of 150- and 300-bp tandem repeats. In situ hybridization localized these sequences to Balbiani ring 1. From these results we postulated that a 6.5-kb RNA gene may have evolved by stepwise duplication and amplification of a 75-bp ancestral sequence. 相似文献
6.
Studies of a plasma membrane steroid receptor in Xenopus oocytes using the synthetic progestin RU 486 总被引:1,自引:0,他引:1
A steroid binding protein (Mr = 110,000) has previously been identified in the plasma membrane of Xenopus laevis oocytes by photoaffinity labeling with [3H]R5020. In order to further characterize this steroid receptor, the photoaffinity labeled receptor protein was solubilized with 0.1% Brij 35. The solubilized labeled receptor yielded an approximate mol. wt of 102,000 +/- 2,000 by sucrose density gradient centrifugation, suggesting that the solubilized receptor exists as a monomer. RU 486, a synthetic progestin antagonist for mammalian cytosolic receptor systems, inhibited up to 70% of [3H] R5020 photoaffinity binding to the 110,000-Dalton receptor with an IC50 of 5 microM and induced germinal vesicle breakdown (GVBD) with an EC50 of 9.0 +/- 0.6 microM. GVBD induced by RU 486 was slower than with progesterone, and RU 486 was less powerful than progesterone. Micromolar concentrations of RU 486 also potentiated GVBD induced by sub-optimal concentrations of progesterone or R5020. Furthermore, RU 486 inhibited oocyte plasma membrane adenylate cyclase with an apparent IC50 of 7.5 +/- 2.5 microM. The close correlation of the EC50 value for RU 486 induction of GVBD with the IC50 values for inhibition of [3H]R5020 photoaffinity labeling of the 110,000-Dalton receptor and inhibition of adenylate cyclase activity further supports the physiological significance of the oocyte plasma membrane steroid receptor. 相似文献
7.
8.
Chicken lens delta-crystallin gene expression and methylation in several non-lens tissues. 总被引:5,自引:2,他引:3 下载免费PDF全文
D J Bower L H Errington D N Cooper S Morris R M Clayton 《Nucleic acids research》1983,11(9):2513-2527
RNA sequences coding for the most abundant chicken lens proteins, delta-crystallin, were detected at very low levels in day old post hatch chick lung, heart, kidney and liver, and in 6 day embryo headless bodies. The pattern of cytosine methylation within the CCGG sequences of the delta-crystallin genes was also examined and shown to vary in several non-lens tissues, from several stages of development. Embryonic neural retina, which expresses a higher level of delta-crystallin RNA than the above tissues, is no less methylated in the sites studied than the tissues which have no association with the eye, and is actually more heavily methylated than the kidney. Thus no obvious correlation was found between undermethylation and gene expression. 相似文献
9.
10.
S G Bower K W Harlow R L Switzer B Hove-Jensen 《The Journal of biological chemistry》1989,264(17):10287-10291
The prsA1 allele, specifying a mutant Escherichia coli phosphoribosylpyrophosphate (PRPP) synthetase, has been cloned. The mutation was shown by nucleotide sequence analysis to result from substitution of Asp-128 (GAT) in the wild type by Ala (GCT) in prsA1. This alteration was confirmed by chemical determination of the amino acid sequence of a tryptic peptide derived from the purified mutant enzyme. The mutation lies at the N-terminal end of a 16 residue sequence that is highly conserved in E. coli, Bacillus subtilis, and rat PRPP synthetases and has the following consensus sequence: DLHAXQIQGFFDI/VPI/VD. There was little alteration in the Km for ribose 5-phosphate. The Km for ATP of the mutant enzyme was increased 27-fold when Mg2+ was the activating cation but only 5-fold when Mn2+ was used. Maximal velocities of the wild type and mutant enzymes were the same. The mutant enzyme has a 6-fold lower affinity for Ca2+, as judged by the ability of Ca2+ to inhibit the reaction in the presence of 10 mM Mg2+. Wild type PRPP synthetase is subject to product inhibition by AMP, but AMP inhibition of the prsA1 mutant enzyme could not be detected. It has been previously proposed that a divalent cation binds to PRPP synthetase and serves as a bridge to the alpha-phosphate of ATP and AMP at the active site. The prsA1 mutation appears to alter this divalent cation site. 相似文献