The structure and chemistry of the TiO(2)-rich surface of SrTiO(3) (001)

Oxide surfaces are important for applications in catalysis and thin film growth. An important frontier in solid-state inorganic chemistry is the prediction of the surface structure of an oxide. Comparatively little is known about atomic arrangements at oxide surfaces at present, and there has been considerable discussion concerning the forces that control such arrangements. For instance, one model suggests that the dominant factor is a reduction of Coulomb forces; another favours minimization of 'dangling bonds' by charge transfer to states below the Fermi energy. The surface structure and properties of SrTiO(3)--a standard model for oxides with a perovskite structure--have been studied extensively. Here we report a solution of the 2 x 1 SrTiO(3) (001) surface structure obtained through a combination of high-resolution electron microscopy and theoretical direct methods. Our results indicate that surface rearrangement of TiO(6-x) units into edge-sharing blocks determines the SrO-deficient surface structure of SrTiO(3). We suggest that this structural concept can be extended to perovskite surfaces in general.     

A measurement of the cosmological mass density from clustering in the 2dF Galaxy Redshift Survey

The large-scale structure in the distribution of galaxies is thought to arise from the gravitational instability of small fluctuations in the initial density field of the Universe. A key test of this hypothesis is that forming superclusters of galaxies should generate a systematic infall of other galaxies. This would be evident in the pattern of recessional velocities, causing an anisotropy in the inferred spatial clustering of galaxies. Here we report a precise measurement of this clustering, using the redshifts of more than 141,000 galaxies from the two-degree-field (2dF) galaxy redshift survey. We determine the parameter beta = Omega0.6/b = 0.43 +/- 0.07, where Omega is the total mass-density parameter of the Universe and b is a measure of the 'bias' of the luminous galaxies in the survey. (Bias is the difference between the clustering of visible galaxies and of the total mass, most of which is dark.) Combined with the anisotropy of the cosmic microwave background, our results favour a low-density Universe with Omega approximately 0.3.     

Seasonal variations in testicular monoamine oxidase in the house sparrow (Passer domesticus) and uinta ground squirrels (Spermophilus armatus)

Résumé Des moinaux familiers (Passer domesticus) et des écureuils (Spermophilus armatus) des montagnes Uinta ont été capturés à différentes époques de l'année et l'activité de laMao a été mesurée dans leurs testicules. Chez les deux espèces l'activité de la MAO était la plus grande lorsque le poids des testicules était le plus élevé et la plus basse avant et après la saison du rut. Chez le moineau, l'augmentation de l'activité de MAO a précédé le développement des testicules.

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We thank the Atomic Energy Commission Grant No. At(11-1)-1602, Utah State University research project No. U-300 and Ecol. Grant No. 19 for their support of this research.R. L. Urry, present address: Division of Urology, University of Rochester, School of Medicine and Dentistry, Rochester, New York, 14642.J. L. Frehn was on sabbatical leave from his present position in the Department of Biology, Illinois State University, Normal, Illinois, 61761. We thankPhyllis Tayler for her technical help in this investigation.     

The pseudoautosomal boundary in man is defined by an Alu repeat sequence inserted on the Y chromosome

The Y chromosome, which in man determines the male sex, is composed of two functionally distinct regions. The pseudoautosomal region is shared between the X and Y chromosome and is probably required for the correct segregation of the sex chromosomes during male meiosis. The second region includes the sex-determining gene(s), the presence of which is necessary for the development of testes. The two regions have contrasting genetic properties: the pseudoautosomal region recombines between the X and Y chromosome; the Y-specific region must avoid recombination otherwise the chromosomal basis of sex-determination breaks down. The pseudoautosomal region is bounded at the distal end by the telomere and at the proximal end by X- and Y-specific DNA. We have found that the proximal boundary was formed by the insertion of an Alu sequence on the Y chromosome early in the primate lineage. Proximal to the Alu insertion there is a small region where similarity between the X and Y chromosomes is reduced and which is no longer subject to recombination.     

Population structure of the human pseudoautosomal boundary

The mammalian sex chromosomes are composed of two genetically distinct segments: the pseudoautosomal region, where recombination occurs between the X and Y chromosomes, and the sex chromosome-specific parts. Between these two segments the human sex chromosomes differ by the insertion of an Alu element on the Y chromosome. We have surveyed the sequence variation in the boundary region using the polymerase chain reaction. Fifty seven Y and sixty X chromosomes from ten different human populations were analysed. The X chromosomes were found to be polymorphic at five positions in a 300-base-pair region. By contrast, all Y chromosomes were identical except for one distal polymorphism shared with the X chromosome.