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排序方式: 共有116条查询结果,搜索用时 62 毫秒
101.
Andresen GB Ashkezari MD Baquero-Ruiz M Bertsche W Bowe PD Butler E Cesar CL Chapman S Charlton M Deller A Eriksson S Fajans J Friesen T Fujiwara MC Gill DR Gutierrez A Hangst JS Hardy WN Hayden ME Humphries AJ Hydomako R Jenkins MJ Jonsell S Jørgensen LV Kurchaninov L Madsen N Menary S Nolan P Olchanski K Olin A Povilus A Pusa P Robicheaux F Sarid E el Nasr SS Silveira DM So C Storey JW Thompson RI van der Werf DP Wurtele JS Yamazaki Y 《Nature》2010,468(7324):673-676
Antimatter was first predicted in 1931, by Dirac. Work with high-energy antiparticles is now commonplace, and anti-electrons are used regularly in the medical technique of positron emission tomography scanning. Antihydrogen, the bound state of an antiproton and a positron, has been produced at low energies at CERN (the European Organization for Nuclear Research) since 2002. Antihydrogen is of interest for use in a precision test of nature's fundamental symmetries. The charge conjugation/parity/time reversal (CPT) theorem, a crucial part of the foundation of the standard model of elementary particles and interactions, demands that hydrogen and antihydrogen have the same spectrum. Given the current experimental precision of measurements on the hydrogen atom (about two parts in 10(14) for the frequency of the 1s-to-2s transition), subjecting antihydrogen to rigorous spectroscopic examination would constitute a compelling, model-independent test of CPT. Antihydrogen could also be used to study the gravitational behaviour of antimatter. However, so far experiments have produced antihydrogen that is not confined, precluding detailed study of its structure. Here we demonstrate trapping of antihydrogen atoms. From the interaction of about 10(7) antiprotons and 7?×?10(8) positrons, we observed 38 annihilation events consistent with the controlled release of trapped antihydrogen from our magnetic trap; the measured background is 1.4?±?1.4 events. This result opens the door to precision measurements on anti-atoms, which can soon be subjected to the same techniques as developed for hydrogen. 相似文献
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Humphray SJ Oliver K Hunt AR Plumb RW Loveland JE Howe KL Andrews TD Searle S Hunt SE Scott CE Jones MC Ainscough R Almeida JP Ambrose KD Ashwell RI Babbage AK Babbage S Bagguley CL Bailey J Banerjee R Barker DJ Barlow KF Bates K Beasley H Beasley O Bird CP Bray-Allen S Brown AJ Brown JY Burford D Burrill W Burton J Carder C Carter NP Chapman JC Chen Y Clarke G Clark SY Clee CM Clegg S Collier RE Corby N Crosier M Cummings AT Davies J Dhami P Dunn M Dutta I Dyer LW Earthrowl ME Faulkner L 《Nature》2004,429(6990):369-374
Chromosome 9 is highly structurally polymorphic. It contains the largest autosomal block of heterochromatin, which is heteromorphic in 6-8% of humans, whereas pericentric inversions occur in more than 1% of the population. The finished euchromatic sequence of chromosome 9 comprises 109,044,351 base pairs and represents >99.6% of the region. Analysis of the sequence reveals many intra- and interchromosomal duplications, including segmental duplications adjacent to both the centromere and the large heterochromatic block. We have annotated 1,149 genes, including genes implicated in male-to-female sex reversal, cancer and neurodegenerative disease, and 426 pseudogenes. The chromosome contains the largest interferon gene cluster in the human genome. There is also a region of exceptionally high gene and G + C content including genes paralogous to those in the major histocompatibility complex. We have also detected recently duplicated genes that exhibit different rates of sequence divergence, presumably reflecting natural selection. 相似文献
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Community structure and metabolism through reconstruction of microbial genomes from the environment 总被引:3,自引:0,他引:3
Tyson GW Chapman J Hugenholtz P Allen EE Ram RJ Richardson PM Solovyev VV Rubin EM Rokhsar DS Banfield JF 《Nature》2004,428(6978):37-43
Microbial communities are vital in the functioning of all ecosystems; however, most microorganisms are uncultivated, and their roles in natural systems are unclear. Here, using random shotgun sequencing of DNA from a natural acidophilic biofilm, we report reconstruction of near-complete genomes of Leptospirillum group II and Ferroplasma type II, and partial recovery of three other genomes. This was possible because the biofilm was dominated by a small number of species populations and the frequency of genomic rearrangements and gene insertions or deletions was relatively low. Because each sequence read came from a different individual, we could determine that single-nucleotide polymorphisms are the predominant form of heterogeneity at the strain level. The Leptospirillum group II genome had remarkably few nucleotide polymorphisms, despite the existence of low-abundance variants. The Ferroplasma type II genome seems to be a composite from three ancestral strains that have undergone homologous recombination to form a large population of mosaic genomes. Analysis of the gene complement for each organism revealed the pathways for carbon and nitrogen fixation and energy generation, and provided insights into survival strategies in an extreme environment. 相似文献
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