交流电路中电压的意义

本文通过用库场仑线积分定义电压和用总电场线积分定义电压的讨论，认为以库仑场线积分定义电压为恰当．     

广告是不是科学传播的死敌

广告是伴随着商品生产和商品交换而出现的,是商品经济发展的必然产物。广告在我国的命运经历过重大变迁。20世纪30～40年代的上海滩曾经是广告遍地。建国后,广告曾被认为是"摆噱头、吹牛皮,资本主义生意经",很难在媒体上出现。改革开放之初,电视广告的出现甚至被认为是     

Why the genome does not congeal.

Since the genes in any given organism have evolved together to produce an adaptive phenotype, one might expect that rearrangement by chromosomal recombination would be maladaptive and thus wiped out by natural selection. This article reviews various theories that explain why, on the contrary, recombination is almost universal.     

Evolutionary genomics: codon volatility does not detect selection

Plotkin et al. introduce a method to detect selection that is based on an index called codon volatility and that uses only the sequence of a single genome, claiming that this method is applicable to a large range of sequenced organisms. Volatility for a given codon is the ratio of non-synonymous codons to all sense codons accessible by one point mutation. The significance of each gene's volatility is assessed by comparison with a simulated distribution of 10(6) synonymous versions of each gene, with synonymous codons drawn randomly from average genome frequencies. Here we re-examine their method and data and find that codon volatility does not detect selection, and that, even if it did, the genomes of Mycobacterium tuberculosis and Plasmodium falciparum, as well as those of most sequenced organisms, do not meet the assumptions necessary for application of their method.     

Transposition of Tn554 does not generate a target duplication

Transposable elements from prokaryotic and eukaryotic organisms are discrete DNA segments bounded by inverted or directly repeated sequences that insert into non-homologous DNA in a reaction that is independent of the general recombination functions of the host. The mechanisms proposed generally involve a staggered double-stranded scission of the target DNA, ligation to the nicked ends of the transposable element, and replication of the element, resulting in the generation of a directly repeated oligonucleotide target sequence flanking the new copy of the element. Most transposons have a relatively low degree of target site specificity coupled with a low insertion frequency. Tn554, a Staphylococcus aureus transposon which specifies resistances to erythromycin and spectinomycin, displays an unusually high degree of insertion specificity. Tn554 transposes with high efficiency to a unique ('primary') site in the S. aureus chromosome and only rarely (less than 10(-6) per transductant) to other, secondary sites. We report here the nucleotide sequences surrounding the junctions of Tn554 in three independent 'primary' insertions and two 'secondary' insertions of the transposon. Two unusual features are revealed: first, the termini of Tn554 contain neither inverted nor directly repeated sequences. Second, transposition of Tn554 does not generate the short direct repeats of the target DNA that are characteristic of other transposable elements. These results suggest that the mechanism of Tn554 insertion may be significantly different from that of other transposons.     

洛仑兹力公式中v的寓意剖析

在电磁场量统一性的基础上，指出了洛仑兹力公式中的速度ｖ是电荷相对观察者的运动速度，顺便述及安培力的成因。     

一类集值映射在迭代下集值点个数不增的条件

集值点个数的增加是集值映射迭代之所以复杂的根本原因.本文研究一类只有一个集值点的集值映射的迭代,给出这类映射在迭代下集值点个数不增加的条件.