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Clark GR 《Science (New York, N.Y.)》1974,183(4128):968-970
Observations of the growing margin of Pecten diegensis show that calcification can occur on an unstable substrate, although this initial shell material is quite disordered. Later growth on the inner surface of the disordered material soon becomes ordered, which suggests that the calcification process benefits from the stability of a mineralized substrate, but does not utilize the substrate as a physical or stereochemical template. 相似文献
104.
Two G protein oncogenes in human endocrine tumors 总被引:55,自引:0,他引:55
J Lyons C A Landis G Harsh L Vallar K Grünewald H Feichtinger Q Y Duh O H Clark E Kawasaki H R Bourne 《Science (New York, N.Y.)》1990,249(4969):655-659
Somatic mutations in a subset of growth hormone (GH)-secreting pituitary tumors convert the gene for the alpha polypeptide chain (alpha s) of Gs into a putative oncogene, termed gsp. These mutations, which activate alpha s by inhibiting its guanosine triphosphatase (GTPase) activity, are found in codons for either of two amino acids, each of which is completely conserved in all known G protein alpha chains. The likelihood that similar mutations would activate other G proteins prompted a survey of human tumors for mutations that replace either of these two amino acids in other G protein alpha chain genes. The first gene so far tested, which encodes the alpha chain of Gi2, showed mutations that replaced arginine-179 with either cysteine or histidine in 3 of 11 tumors of the adrenal cortex and 3 of 10 endocrine tumors of the ovary. The mutant alpha i2 gene is a putative oncogene, referred to as gip2. In addition, gsp mutations were found in 18 of 42 GH-secreting pituitary tumors and in an autonomously functioning thyroid adenoma. These findings suggest that human tumors may harbor oncogenic mutations in various G protein alpha chain genes. 相似文献
105.
Creatine reacts with nitrite under acid conditions to produce first sarcosine and then N-nitrososarcosine, which is a weak carcinogen in the rat. Creatinine reacts with acidified nitrite to produce either creatinine-5-oxime or 1-methylhydantoin-5-oxime, depending on reaction conditions. The toxicity and environmental significance of these compounds is not yet known. 相似文献
106.
Hovis WA Clark DK Anderson F Austin RW Wilson WH Baker ET Ball D Gordon HR Mueller JL El-Sayed SZ Sturm B Wrigley RC Yentsch CS 《Science (New York, N.Y.)》1980,210(4465):60-63
The Coastal Zone Color Scanner (CZCS) on Nimbus-7, launched in October 1978, is the only sensor in orbit that is specifically designed to study living marine resources. The initial imagery confirms that CZCS data can be processed to a level that reveals subtle variations in the concentration of phytoplankton pigments. This development has potential applications for the study of large-scale patchiness in phytoplankton distributions, the evolution of spring blooms, water mass boundaries, and mesoscale circulation patterns. 相似文献
107.
Interferometer experiment with independent local oscillators 总被引:1,自引:0,他引:1
We have operated an interferometer with independent local oscillators and without any communication link of wide bandwidth between the elements of the interferometer. This makes operation possible at very long base lines because, heretofore, construction of the communications link has been the factor limiting the separation of the elements. In our system, coherence at the two elements is maintained through the use of two highly stable, atomic oscillators. The intermediate-frequency output signals are recorded at each element on a high-speed digital tape recorder. Interference fringes are produced later by cross-correlating the two tape records in a digital computer. 相似文献
108.
X-ray scattering studies were performed to understand the structure and correlations in the lamellar phases of thick, freely suspeded films of (i) the hydrated phospholipid dimyristoylphosphotidylcholine (DMPC) and (ii) the ternar system consisting of the surfactant sodium dodecyl sulfate (SDS), cosurfactant (pentanol), and water. The films were drawn in a temperature- and humidity-controlled environment, where the layers were oriented to within 0.1 degrees . In the DMPC system, this made it possible to directly observe the orientation of the P(beta') modulation and to identify phase L(beta') as three distinct phases distinguished by the direction of chain tilt with respect to the lattice. In the L(alpha) phase of the ternary system, power law behavior of the (0,0,L) structure factor arising from the algebraic decay of layer correlatios was observed in single crystals. 相似文献
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