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51.
Observations of clear air turbulence by high power radar 总被引:1,自引:0,他引:1
Clear air turbulence is a hazard to aviation and is thought to have important effects on atmospheric dynamics. This article describes the structure and evolution of clear air turbulence at high altitudes as revealed by a high power radar and vertical soundings of wind and temperature. 相似文献
52.
Naish TR Woolfe KJ Barrett PJ Wilson GS Atkins C Bohaty SM Bücker CJ Claps M Davey FJ Dunbar GB Dunn AG Fielding CR Florindo F Hannah MJ Harwood DM Henrys SA Krissek LA Lavelle M van Der Meer J McIntosh WC Niessen F Passchier S Powell RD Roberts AP Sagnotti L Scherer RP Strong CP Talarico F Verosub KL Villa G Watkins DK Webb PN Wonik T 《Nature》2001,413(6857):719-723
Between 34 and 15 million years (Myr) ago, when planetary temperatures were 3-4 degrees C warmer than at present and atmospheric CO2 concentrations were twice as high as today, the Antarctic ice sheets may have been unstable. Oxygen isotope records from deep-sea sediment cores suggest that during this time fluctuations in global temperatures and high-latitude continental ice volumes were influenced by orbital cycles. But it has hitherto not been possible to calibrate the inferred changes in ice volume with direct evidence for oscillations of the Antarctic ice sheets. Here we present sediment data from shallow marine cores in the western Ross Sea that exhibit well dated cyclic variations, and which link the extent of the East Antarctic ice sheet directly to orbital cycles during the Oligocene/Miocene transition (24.1-23.7 Myr ago). Three rapidly deposited glacimarine sequences are constrained to a period of less than 450 kyr by our age model, suggesting that orbital influences at the frequencies of obliquity (40 kyr) and eccentricity (125 kyr) controlled the oscillations of the ice margin at that time. An erosional hiatus covering 250 kyr provides direct evidence for a major episode of global cooling and ice-sheet expansion about 23.7 Myr ago, which had previously been inferred from oxygen isotope data (Mi1 event). 相似文献
53.
Sequence diversity in CYP3A promoters and characterization of the genetic basis of polymorphic CYP3A5 expression 总被引:51,自引:0,他引:51
Kuehl P Zhang J Lin Y Lamba J Assem M Schuetz J Watkins PB Daly A Wrighton SA Hall SD Maurel P Relling M Brimer C Yasuda K Venkataramanan R Strom S Thummel K Boguski MS Schuetz E 《Nature genetics》2001,27(4):383-391
Variation in the CYP3A enzymes, which act in drug metabolism, influences circulating steroid levels and responses to half of all oxidatively metabolized drugs. CYP3A activity is the sum activity of the family of CYP3A genes, including CYP3A5, which is polymorphically expressed at high levels in a minority of Americans of European descent and Europeans (hereafter collectively referred to as 'Caucasians'). Only people with at least one CYP3A5*1 allele express large amounts of CYP3A5. Our findings show that single-nucleotide polymorphisms (SNPs) in CYP3A5*3 and CYP3A5*6 that cause alternative splicing and protein truncation result in the absence of CYP3A5 from tissues of some people. CYP3A5 was more frequently expressed in livers of African Americans (60%) than in those of Caucasians (33%). Because CYP3A5 represents at least 50% of the total hepatic CYP3A content in people polymorphically expressing CYP3A5, CYP3A5 may be the most important genetic contributor to interindividual and interracial differences in CYP3A-dependent drug clearance and in responses to many medicines. 相似文献