全文获取类型
收费全文 | 77篇 |
免费 | 8篇 |
专业分类
85篇 |
出版年
2019年 | 3篇 |
2018年 | 2篇 |
2017年 | 2篇 |
2016年 | 2篇 |
2015年 | 1篇 |
2014年 | 6篇 |
2013年 | 5篇 |
2012年 | 3篇 |
2011年 | 3篇 |
2009年 | 2篇 |
2008年 | 1篇 |
2006年 | 2篇 |
2004年 | 2篇 |
2002年 | 1篇 |
2001年 | 5篇 |
2000年 | 5篇 |
1999年 | 2篇 |
1998年 | 3篇 |
1997年 | 6篇 |
1995年 | 2篇 |
1994年 | 1篇 |
1992年 | 2篇 |
1991年 | 3篇 |
1989年 | 3篇 |
1987年 | 1篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1978年 | 1篇 |
1977年 | 3篇 |
1976年 | 1篇 |
1967年 | 2篇 |
1966年 | 1篇 |
1962年 | 1篇 |
1939年 | 1篇 |
1935年 | 1篇 |
1918年 | 1篇 |
1917年 | 1篇 |
排序方式: 共有85条查询结果,搜索用时 15 毫秒
11.
Replication slippage may cause parallel evolution in the secondary structures of mitochondrial transfer RNAs 总被引:9,自引:4,他引:5
Presence of the dihydrouridine (D) stem in the mitochondrial cysteine tRNA
is unusually variable among lepidosaurian reptiles. Phylogenetic and
comparative analyses of cysteine tRNA gene sequences identify eight
parallel losses of the D-stem, resulting in D-arm replacement loops.
Sampling within the monophyletic Acrodonta provides no evidence for
reversal. Slipped-strand mispairing of noncontiguous repeated sequences
during replication or direct replication slippage can explain repeats
observed within cysteine tRNAs that contain a D-arm replacement loop. These
two mechanisms involving replication slippage can account for the loss of
the cysteine tRNA D-stem in several lepidosaurian lineages, and may
represent general mechanisms by which the secondary structures of
mitochondrial tRNAs are altered.
相似文献
12.
13.
Eric Alan Eager Richard Rebarber Brigitte Tenhumberg 《Bulletin of mathematical biology》2014,76(7):1809-1834
In many plant species dormant seeds can persist in the soil for one to several years. The formation of these seed banks is especially important for disturbance specialist plants, as seeds of these species germinate only in disturbed soil. Seed movement caused by disturbances affects the survival and germination probability of seeds in the seed bank, which subsequently affect population dynamics. In this paper, we develop a stochastic integral projection model for a general disturbance specialist plant-seed bank population that takes into account both the frequency and intensity of random disturbances, as well as vertical seed movement and density-dependent seedling establishment. We show that the probability measures associated with the plant-seed bank population converge weakly to a unique measure, independent of initial population. We also show that the population either persists with probability one or goes extinct with probability one, and provides a sharp criteria for this dichotomy. We apply our results to an example motivated by wild sunflower (Helianthus annuus) populations, and explore how the presence or absence of a “storage effect” impacts how a population responds to different disturbance scenarios. 相似文献
14.
Little is known about the introduced European woodwasp, Sirex noctilio F. (Hymenoptera: Siricidae), and its hymenopteran parasitoid complex in North America. To assess within-tree and landscape-level densities and distributions of these insects, and develop a more cost effective approach for sampling and monitoring S. noctilio, 18 infested pines (12 P. resinosa Aiton and six P. sylvestris L.) were felled from six stands within three regions in central New York and cut into 0.5-m bolts. Bolts were carefully split to recover all siricids (S. noctilio and native siricids) and parasitoids. In total, 2,558 siricids were recovered; 1,972 siricid larvae and 586 teneral adult S. noctilio. Parasitism of siricids, a majority of which were S. noctilio based on larval rearing results, was 16.4% with Ibalia leucospoides ensiger Norton causing 10.7% of the documented mortality. Numbers of siricids and parasitoids declined 33-86% from the northern to the southern sampling regions, peak insect densities occurred in sections of the bole 15-19 cm in diameter and numbers of insects were generally higher in P. sylvestris than P. resinosa according to the highest ranked zero-inflated poisson and probit regression models. Bark thickness was not correlated with siricid or parasitoid species densities. We also describe sampling plans where as few as two 0.5-m samples from infested trees provided reliable within-tree insect densities. 相似文献
15.
Eric Alan Eager Richard Rebarber Brigitte Tenhumberg 《Journal of mathematical biology》2014,69(1):1-37
Many plant populations have persistent seed banks, which consist of viable seeds that remain dormant in the soil for many years. Seed banks are important for plant population dynamics because they buffer against environmental perturbations and reduce the probability of extinction. Viability of the seeds in the seed bank can depend on the seed’s age, hence it is important to keep track of the age distribution of seeds in the seed bank. In this paper we construct a general density-dependent plant-seed bank model where the seed bank is age-structured. We consider density dependence in both seedling establishment and seed production, since previous work has highlighted that overcrowding can suppress both of these processes. Under certain assumptions on the density dependence, we prove that there is a globally stable equilibrium population vector which is independent of the initial state. We derive an analytical formula for the equilibrium population using methods from feedback control theory. We apply these results to a model for the plant species Cirsium palustre and its seed bank. 相似文献
16.
The diageotropica mutation alters auxin induction of a subset of the Aux/IAA gene family in tomato 总被引:5,自引:0,他引:5
The diageotropica (dgt) mutation has been proposed to affect either auxin perception or responsiveness in tomato plants. It has previously been demonstrated that the expression of one member of the Aux/IAA family of auxin-regulated genes is reduced in dgt plants. Here, we report the cloning of ten new members of the tomato Aux/IAA family by PCR amplification based on conserved protein domains. All of the gene family members except one (LeIAA7) are expressed in etiolated tomato seedlings, although they demonstrate tissue specificity (e.g. increased expression in hypocotyls vs. roots) within the seedling. The wild-type auxin-response characteristics of the expression of these tomato LeIAA genes are similar to those previously described for Aux/IAA family members in Arabidopsis. In dgt seedlings, auxin stimulation of gene expression was reduced in only a subset of LeIAA genes (LeIAA5, 8, 10, and 11), with the greatest reduction associated with those genes with the strongest wild-type response to auxin. The remaining LeIAA genes tested exhibited essentially the same induction levels in response to the hormone in both dgt and wild-type hypocotyls. These results confirm that dgt plants can perceive auxin and suggest that a specific step in early auxin signal transduction is disrupted by the dgt mutation. 相似文献
17.
Marcelo H Ratto Louis TJ Delbaere Yvonne A Leduc Roger A Pierson Gregg P Adams 《Reproductive biology and endocrinology : RB&E》2011,9(1):24
Background
The objective of the present study was to isolate and purify the protein fraction(s) of llama seminal plasma responsible for the ovulation-inducing effect of the ejaculate. 相似文献18.
Gallant EM Hart J Eager K Curtis S Dulhunty AF 《American journal of physiology. Cell physiology》2004,286(4):C821-C830
Enhanced sensitivity to caffeine is part of the standard tests for susceptibility to malignant hyperthermia (MH) in humans and pigs. The caffeine sensitivity of skeletal muscle contraction and Ca2+ release from the sarcoplasmic reticulum is enhanced, but surprisingly, the caffeine sensitivity of purified porcine ryanodine receptor Ca2+-release channels (RyRs) is not affected by the MH mutation (Arg615Cys). In contrast, we show here that native malignant hyperthermic pig RyRs (incorporated into lipid bilayers with RyR-associated lipids and proteins) were activated by caffeine at 100- to 1,000-fold lower concentrations than native normal pig RyRs. In addition, the results show that the mutant ryanodine receptor channels were less sensitive to high-affinity activation by a peptide (CS) that corresponds to a part of the IIIII loop of the skeletal dihydropyridine receptor (DHPR). Furthermore, subactivating concentrations of peptide CS enhanced the response of normal pig and rabbit RyRs to caffeine. In contrast, the caffeine sensitivity of MH RyRs was not enhanced by the peptide. These novel results showed that in MH-susceptible pig muscles 1) the caffeine sensitivity of native RyRs was enhanced, 2) the sensitivity of RyRs to a skeletal IIIII loop peptide was depressed, and 3) an interaction between the caffeine and peptide CS activation mechanisms seen in normal RyRs was lost. calcium ion homeostasis; excitation-contraction coupling; ryanodine receptor polymorphisms; muscle contraction 相似文献
19.
20.