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51.
似哺乳类爬行动物和哺乳类动物脑颅侧壁构造类型的演替 总被引:1,自引:1,他引:0
本文对从原始似哺乳爬行动物进化到高级兽类哺乳动物过程中脑颅侧壁所发生的形态演化进行了分析,根据构造上的不同,将这些动物的脑颅侧壁的构造方式划分为四个构造类型,代表四个进化阶段。 相似文献
52.
X. -R. Wang A. E. Szmidt 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1990,80(5):635-640
Summary Restriction fragment analysis and heterologous hybridization of chloroplast (cp) DNA was used to develop species-specific markers for P. tabulaeformis, P. yunnanensis and P. massoniana. Fragment patterns created by the BclI and DraI restriction enzymes and hybridization patterns to the psbC and psbD probes were distinctive among the three species. No intraspecific variation was detected with respect to any of the cpDNA markers developed in this study. The cpDNA markers obtained were subsequently used to examine the parentage of P. densata, a putative Tertiary hybrid between P. tabulaeformis and P. yunnanensis. The analysis demonstrated for the first time that P. densata populations accommodate chloroplast genomes of P. tabulaeformis and P. yunnanensis, which strongly supports earlier suggestions of the hybrid origin of this species. It appears that P. densata represents a stabilized natural hybrid that has become adapted to high mountain environments where neither of the parental species can normally grow. 相似文献
53.
Sequence variability of simian immunodeficiency virus in a persistently infected rhesus monkey 总被引:2,自引:0,他引:2
A juvenile rhesus monkey that was inoculated intravenously with molecularly cloned SIVmac239 became persistently infected. A modified polymerase chain reaction (PCR) procedure was used to specifically amplify full-length envelope (env) gene sequences from DNA extracted from peripheral blood mononuclear cells (PBMC), lymph node tissue, and cells infected with recovered virus at 69 and 93 weeks post-infection. Extensive sequence variability accumulated in vivo in spite of infection with molecularly cloned virus. In the central portion of env. sequence variability was largely confined to three discrete regions. 相似文献
54.
S. F. Perry 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1990,159(6):761-769
Summary The respiratory surface area (SAR) per kilogram body mass (MB), the harmonic mean thickness of the air-blood barrier (htR) in the gas exchange tissue, and the anatomical diffusion factor (ADF=SAR/htR per MB) were calculated for four juvenile Nile crocodiles. The ADF of three small specimens (mean MB=3.59 kg) was 625 cm2·m–1·kg–1. The values varied considerably among individuals and were similar to that of a 5.68-kg specimen (593 cm2·m–1·kg–1). Only 9% of the ADF is located in the anterior third of the lung, which because of its conical shape makes up only 14 percent of the total lung volume. Particularly in the middle third of the lung, the proximal region near the intrapulmonary bronchus displays a greater ratio of respiratory/non-respiratory surface areas than do more distally located sampling sites. The htR is also significantly smaller proximally than distally. The cumulative ADF per unit MB is greater than that previously reported for this species on the basis of overall estimates of SAR and htR, but is still less than that of lizards and testudinids. The disposition of ADF between distal air storage region and the intrapulmonary bronchus is consistent with a bidirectional cross-current gas exchange model.Abbreviations
ADF
anatomical diffusion factor
- %AR
percent of SA included in the effective respiratory zone
-
M
B
body mass
-
NVP
non-ventilatory period
- %P
percent of total lung volume containing parenchyma
-
S
A
total surface area of intrapulmonary septa
-
S
ANR
that portion ofS
A
lying out the effective respiratory zone
-
S
V
surface-to-volume ratio in the parenchyma
- htR
harmonic mean thickness of the air-blood tissue barrier within the respiratory zone
-
V
P
parenchymal volume
-
VP
ventilatory period 相似文献
55.
Randolph M. Nesse M.D. 《Human nature (Hawthorne, N.Y.)》1990,1(3):261-289
Emotions can be explained as specialized states, shaped by natural selection, that increase fitness in specific situations. The physiological, psychological, and behavioral characteristics of a specific emotion can be analyzed as possible design features that increase the ability to cope with the threats and opportunities present in the corresponding situation. This approach to understanding the evolutionary functions of emotions is illustrated by the correspondence between (a) the subtypes of fear and the different kinds of threat; (b) the attributes of happiness and sadness and the changes that would be advantageous in propitious and unpropitious situations; and (c) the social emotions and the adaptive challenges of reciprocity relationships. In addition to addressing a core theoretical problem shared by evolutionary and cognitive psychology, explicit formulations of the evolutionary functions of specific emotions are of practical importance for understanding and treating emotional disorders. 相似文献
56.
Martijn A. Huynen 《Journal of molecular evolution》1996,43(3):165-169
RNA secondary-structure folding algorithms predict the existence of connected networks of RNA sequences with identical secondary
structures. Fitness landscapes that are based on the mapping between RNA sequence and RNA secondary structure hence have many
neutral paths. A neutral walk on these fitness landscapes gives access to a virtually unlimited number of secondary structures
that are a single point mutation from the neutral path. This shows that neutral evolution explores phenotype space and can
play a role in adaptation.
Received: 23 December 1995 / Accepted: 17 March 1996 相似文献
57.
Ana M. Moura-da-Silva R. David G. Theakston Julian M. Cramptonl 《Journal of molecular evolution》1996,43(3):263-269
The evolution of the Metalloproteinase Disintegrin Cysteine-rich (MDC) gene family and that of the mammalian Matrix-degrading
Metalloproteinases (MMPs) are compared. The alignment of snake venom and mammalian MDC and MMP precursor sequences generated
a phylogenetic tree that grouped these proteins mainly according to their function. Based on this observation, a common ancestry
is suggested for mammalian and snake venom MDCs; it is also possible that gene duplication of the already-assembled domain
structure, followed by divergence of the copies, may have significantly contributed to the evolution of the functionally diverse
MDC proteins. The data also suggest that the structural resemblance of the zinc-binding motif of venom MDCs and MMPs may best
be explained by common ancestry and conservation of the proteolytic motifs during the divergence of the proteins rather than
through convergent evolution.
Correspondence to: J.M. Crampton 相似文献
58.
We have determined the nucleotide sequences of sevenlacY alleles isolated from natural isolates ofEscherichia coli. Nucleotide heterozygosity estimates for this locus were compared to those obtained from previous studies of intraspecific variation at chromosomal loci, revealing thatlacY has unusually low synonymous site variation. The average pairwise heterozygosity of synonymous sites (Ks=0.0112+/-0.0100) is the second lowest reported and the lowest for loci that have an equivalent level of nonsynonymous variation. We consider several hypotheses to explain how different forces in evolution could act to create the observed pattern of polymorphism, including selection for translational efficiency and positive selection. Our analysis most strongly supports the hypothesis that positive selection has acted on thelacY locus inE. coli. 相似文献
59.
Ferritin, a protein widespread in nature, concentrates iron ∼1011–1012-fold above the solubility within a spherical shell of 24 subunits; it derives in plants and animals from a common ancestor
(based on sequence) but displays a cytoplasmic location in animals compared to the plastid in contemporary plants. Ferritin
gene regulation in plants and animals is altered by development, hormones, and excess iron; iron signals target DNA in plants
but mRNA in animals. Evolution has thus conserved the two end points of ferritin gene expression, the physiological signals
and the protein structure, while allowing some divergence of the genetic mechanisms. Comparison of ferritin gene organization
in plants and animals, made possible by the cloning of a dicot (soybean) ferritin gene presented here and the recent cloning
of two monocot (maize) ferritin genes, shows evolutionary divergence in ferritin gene organization between plants and animals
but conservation among plants or among animals; divergence in the genetic mechanism for iron regulation is reflected by the
absence in all three plant genes of the IRE, a highly conserved, noncoding sequence in vertebrate animal ferritin mRNA. In
plant ferritin genes, the number of introns (n= 7) is higher than in animals (n= 3). Second, no intron positions are conserved when ferritin genes of plants and animals are compared, although all ferritin
gene introns are in the coding region; within kingdoms, the intron positions in ferritin genes are conserved. Finally, secondary
protein structure has no apparent relationship to intron/exon boundaries in plant ferritin genes, whereas in animal ferritin
genes the correspondence is high. The structural differences in introns/exons among phylogenetically related ferritin coding
sequences and the high conservation of the gene structure within plant or animal kingdoms suggest that kingdom-specific functional
constraints may exist to maintain a particular intron/exon pattern within ferritin genes. In the case of plants, where ferritin
gene intron placement is unrelated to triplet codons or protein structure, and where ferritin is targeted to the plastid,
the selection pressure on gene organization may relate to RNA function and plastid/nuclear signaling.
Received: 25 July 1995 / Accepted: 3 October 1995 相似文献
60.
Hae-Jin Chung Christopher Shaffer Ross MacIntyre 《Molecular & general genetics : MGG》1996,250(5):635-646
InDrosophila, unlike humans, the lysosomal acid phosphatase (Acph-1) is a non-essential enzyme. It is also one of the most rapidly evolving gene-enzyme systems in the genus. In order to determine which parts of the enzyme are conserved and which parts are apparently under little functional constraint, we cloned the gene fromDrosophila melanogaster via a chromosomal walk. Fragments from the gene were used to recover an apparently full-length cDNA. The cDNA was subcloned into aDrosophila transformation vector where it was under the control of the 5 promoter sequence of thehsp-70 gene. Three independent transformants were obtained; in each, Acph-1 expression from the cDNA was constitutive and not dependent on heat shock, as determined by densitometric analyses of the allozymic forms of the enzyme. The pattern of expression indicates thehsp-70 and endogenousAcph-1 promoters act together in some, but not all, tissues. The sequence of the cDNA was determined using deletions made with exonuclease III, and primers deduced from the cDNA sequence were used to sequence the genomic clone. Five introns were found, and putative 5 up-stream regulatory sequences were identified. Amino acid sequence comparisons have revealed several highly conserved motifs betweenDrosophila Acph-1 and vertebrate lysosomal and prostatic acid phosphatases. 相似文献