Spontaneous mutation rates to new length alleles at tandem-repetitive hypervariable loci in human DNA

Tandem-repetitive minisatellite regions in vertebrate DNA frequently show substantial allelic variation in the number of repeat units. This variation is thought to arise through processes such as unequal crossover or replication slippage. We show here that the spontaneous mutation rate to new length alleles at extremely variable human minisatellites is sufficiently high to be directly measurable in human pedigrees. The mutation rate at different loci increases with variability in accord with the neutral mutation/random drift hypothesis, and rises to 5% per gamete for the most unstable human minisatellite isolated. Mutations are sporadic, occur with similar frequencies in sperm and oocytes, and can involve the gain or loss of substantial numbers of repeat units, consistent with length changes arising primarily by unequal exchange at meiosis. Germline instability must therefore be taken into account when using hypervariable loci as genetic markers, particularly in pedigree analysis and parenthood testing.     

Coevolution with viruses drives the evolution of bacterial mutation rates

Bacteria with greatly elevated mutation rates (mutators) are frequently found in natural and laboratory populations, and are often associated with clinical infections. Although mutators may increase adaptability to novel environmental conditions, they are also prone to the accumulation of deleterious mutations. The long-term maintenance of high bacterial mutation rates is therefore likely to be driven by rapidly changing selection pressures, in addition to the possible slow transition rate by point mutation from mutators to non-mutators. One of the most likely causes of rapidly changing selection pressures is antagonistic coevolution with parasites. Here we show whether coevolution with viral parasites could drive the evolution of bacterial mutation rates in laboratory populations of the bacterium Pseudomonas fluorescens. After fewer than 200 bacterial generations, 25% of the populations coevolving with phages had evolved 10- to 100-fold increases in mutation rates owing to mutations in mismatch-repair genes; no populations evolving in the absence of phages showed any significant change in mutation rate. Furthermore, mutator populations had a higher probability of driving their phage populations extinct, strongly suggesting that mutators have an advantage against phages in the coevolutionary arms race. Given their ubiquity, bacteriophages may play an important role in the evolution of bacterial mutation rates.     

Differentiated parental DNA strands confer developmental asymmetry on daughter cells in fission yeast

The two strands of the DNA molecule are complementary but not identical. Hence, upon semiconservative replication, different parental DNA strands are segregated to daughter cells. A molecular analysis suggests that the process of fission yeast mating-type interconversion uses asymmetry of the DNA strands to generate a regular lineage of cellular differentiation.     

一个涉及两个三角形的三元二次型不等式

应用三角形重要的加权正弦和不等式，建立了一个新的涉及两个三角形的三元二次三角不等式，讨论了它的一则应用，提出了一个有关的猜想。     

Single strands induce recA protein to unwind duplex DNA for homologous pairing.

Single-stranded DNA, whether homologous or not, stimulates purified Escherichia coli recA protein to unwind duplex DNA. This helps to explain how recA promotes a search for homology in genetic recombination. As oligodeoxynucleotide also stimulate unwinding, a common mechanism may relate the function of recA protein in recombination to other functions (SOS) induced by oligonucleotides.     

Direct estimation of per nucleotide and genomic deleterious mutation rates in Drosophila

Spontaneous mutations are the source of genetic variation required for evolutionary change, and are therefore important for many aspects of evolutionary biology. For example, the divergence between taxa at neutrally evolving sites in the genome is proportional to the per nucleotide mutation rate, u (ref. 1), and this can be used to date speciation events by assuming a molecular clock. The overall rate of occurrence of deleterious mutations in the genome each generation (U) appears in theories of nucleotide divergence and polymorphism, the evolution of sex and recombination, and the evolutionary consequences of inbreeding. However, estimates of U based on changes in allozymes or DNA sequences and fitness traits are discordant. Here we directly estimate u in Drosophila melanogaster by scanning 20 million bases of DNA from three sets of mutation accumulation lines by using denaturing high-performance liquid chromatography. From 37 mutation events that we detected, we obtained a mean estimate for u of 8.4 x 10(-9) per generation. Moreover, we detected significant heterogeneity in u among the three mutation-accumulation-line genotypes. By multiplying u by an estimate of the fraction of mutations that are deleterious in natural populations of Drosophila, we estimate that U is 1.2 per diploid genome. This high rate suggests that selection against deleterious mutations may have a key role in explaining patterns of genetic variation in the genome, and help to maintain recombination and sexual reproduction.     

Cauchy不等式在不等式推广中的应用

Cauchy不等式的应用非常广泛.利用Cauchy不等式及其推广形式,对一些重要不等式进行推广,得到相应的结论.     

一个Hardy-Hilbert类不等式的一个改进

利用改进了的Cauchy不等式对1个类似于Hardy-Hilbert不等式的不等式作了改进.建立了1个新的不等式：〖ＤＤ（〗∞〖〗ｎ＝１〖ＤＤ）〗〖ＤＤ（〗∞〖〗ｍ＝１〖ＤＤ）〗〖ＳＸ（〗ａｍｂｎ〖〗ｌｎ　ｍ＋ｌｎ　ｎ＋１〖ＳＸ）〗＜π〖ＪＢ（｛〗〖ＤＤ（〗∞〖〗ｎ＝１〖ＤＤ）〗ｎａ２ｎ〖ＤＤ（〗∞〖〗ｎ＝１〖ＤＤ）〗ｎｂ２ｎ〖ＪＢ）｝〗１／２（１－Ｒ）１／２．其中Ｒ＝〖ＪＢ（（〗〖ＳＸ（〗（α,γ）〖〗‖α‖〖ＳＸ）〗－〖ＳＸ（〗（β,γ）〖〗‖β‖〖ＳＸ）〗〖ＪＢ））〗２.     

浅探算术-几何均值不等式在不等式证明中的应用

均值不等式是数学中几个经典不等式之一,在生产和生活中具有重要作用,是证明不等式及求解各类最值问题的一个重要依据和方法。其中算术一几何均值不等式应用曩为广泛,具有变通灵活性和条件约束性等特点,在不等式证明方面具有不可怠视的作用。本文分别从内容的突破和形式的构造两个方面,探索算术一几何均值不等式在不等式证明中的应用。     

线粒体DNA突变与母系遗传病

近几年来,线粒体DNA(mtDNA）突变与人类疾病及母系遗传病的关系愈来愈受到人们的关注。从分子水平上探讨mtDNA突变的发生情况及mtDNA突变所引起的遗传病的表达及传递方式,有助于揭开线粒体遗传病的本质,最终为人类防治线粒体遗传病提供理论基础。