强非线性受迫振动的多尺度分析

考虑形式为的强非线性振动响应,f(u)是奇非线性的,ε不需要是小的,且m=1,0,或-1。可用两步多尺度逼近获得近似解,与通常的多尺度法不同的是以下两个步骤:(1)失谐采用激励频率的平方Ω~2,并将它作为对所谓“主干曲线”的偏差,(2)定义新的展开参数α=α(ε)。     

Crystal structure of the transfer-RNA domain of transfer-messenger RNA in complex with SmpB

Accurate translation of genetic information into protein sequence depends on complete messenger RNA molecules. Truncated mRNAs cause synthesis of defective proteins, and arrest ribosomes at the end of their incomplete message. In bacteria, a hybrid RNA molecule that combines the functions of both transfer and messenger RNAs (called tmRNA) rescues stalled ribosomes, and targets aberrant, partially synthesized, proteins for proteolytic degradation. Here we report the 3.2-A-resolution structure of the tRNA-like domain of tmRNA (tmRNA(Delta)) in complex with small protein B (SmpB), a protein essential for biological functions of tmRNA. We find that the flexible RNA molecule adopts an open L-shaped conformation and SmpB binds to its elbow region, stabilizing the single-stranded D-loop in an extended conformation. The most striking feature of the structure of tmRNA(Delta) is a 90 degrees rotation of the TPsiC-arm around the helical axis. Owing to this unusual conformation, the SmpB-tmRNA(Delta) complex positioned into the A-site of the ribosome orients SmpB towards the small ribosomal subunit, and directs tmRNA towards the elongation-factor binding region of the ribosome. On the basis of this structure, we propose a model for the binding of tmRNA on the ribosome.     

Experience-dependent plasticity of dendritic spines in the developing rat barrel cortex in vivo

Do changes in neuronal structure underlie cortical plasticity? Here we used time-lapse two-photon microscopy of pyramidal neurons in layer 2/3 of developing rat barrel cortex to image the structural dynamics of dendritic spines and filopodia. We found that these protrusions were highly motile: spines and filopodia appeared, disappeared or changed shape over tens of minutes. To test whether sensory experience drives this motility we trimmed whiskers one to three days before imaging. Sensory deprivation markedly (approximately 40%) reduced protrusive motility in deprived regions of the barrel cortex during a critical period around postnatal days (P)11-13, but had no effect in younger (P8-10) or older (P14-16) animals. Unexpectedly, whisker trimming did not change the density, length or shape of spines and filopodia. However, sensory deprivation during the critical period degraded the tuning of layer 2/3 receptive fields. Thus sensory experience drives structural plasticity in dendrites, which may underlie the reorganization of neural circuits.     

Mutation and sex in a competitive world

How do deleterious mutations interact to affect fitness? The answer to this question has substantial implications for a variety of important problems in population biology, including the evolution of sex, the rate of adaptation and the conservation of small populations. Here we analyse a mathematical model of competition for food in which deleterious mutations affect competitive ability. We show that, if individuals usually compete in small groups, then competition can easily lead to a type of genetic interaction known as synergistic epistasis. This means that a deleterious mutation is most damaging in a genome that already has many other deleterious mutations. We also show that competition in small groups can produce a large advantage for sexual populations, both in mean fitness and in ability to resist invasion by asexual lineages. One implication of our findings is that experimental efforts to demonstrate synergistic epistasis may not succeed unless the experiments are redesigned to make them much more naturalistic.