54 research outputs found

    MEMS 411: Mini Test Frame

    Get PDF
    A lightweight, cheap, and portable compression test frame designed for testing students\u27 structural designs for the Machine Elements class. Measures load and displacement applied on the specimen to determine the specimen with the highest strength

    Real-space imaging of several molecular layers of C60 in the rotational glass phase

    Get PDF
    C60 is a model system to study molecule–surface interactions and phase transitions due to its high symmetry and strong covalent π bonding within the molecule versus weak van-der-Waals coupling between neighboring molecules. In the solid, at room temperature, the molecule rotates and behaves as a sphere. However, the pentagonal and hexagonal atomic arrangement imposes deviations from the spherical symmetry that become important at low temperatures. The orientation of the C60 can be viewed to represent classic spins. For geometrical reasons the preferred orientation of neighboring C60 cannot be satisfied for all of the neighboring molecules, making C60 a model for disordered spin systems with frustration. We study several molecular layers of C60 islands on highly oriented pyrolytic graphite using scanning tunneling microscopy at liquid nitrogen temperatures. By imaging several layers we obtain a limited access to the three-dimensional rotational structure of the molecules in an island. We find one rotationally disordered layer between two partially rotationally ordered layers with hexagonal patterns. This exotic pattern shows an example of the local distribution of order and disorder in geometrically frustrated systems. Scanning tunneling spectroscopy data confirms the weak interactions of neighboring molecules

    Real-space imaging of several molecular layers of C60_{60} in the rotational glass phase

    Get PDF
    C60_{60} is a model system to study molecule–surface interactions and phase transitions due to its high symmetry and strong covalent π bonding within the molecule versus weak van-der-Waals coupling between neighboring molecules. In the solid, at room temperature, the molecule rotates and behaves as a sphere. However, the pentagonal and hexagonal atomic arrangement imposes deviations from the spherical symmetry that become important at low temperatures. The orientation of the C60_{60} can be viewed to represent classic spins. For geometrical reasons the preferred orientation of neighboring C60_{60} cannot be satisfied for all of the neighboring molecules, making C60_{60} a model for disordered spin systems with frustration. We study several molecular layers of C60_{60} islands on highly oriented pyrolytic graphite using scanning tunneling microscopy at liquid nitrogen temperatures. By imaging several layers we obtain a limited access to the three-dimensional rotational structure of the molecules in an island. We find one rotationally disordered layer between two partially rotationally ordered layers with hexagonal patterns. This exotic pattern shows an example of the local distribution of order and disorder in geometrically frustrated systems. Scanning tunneling spectroscopy data confirms the weak interactions of neighboring molecules

    A cell atlas of human thymic development defines T cell repertoire formation.

    Get PDF
    The thymus provides a nurturing environment for the differentiation and selection of T cells, a process orchestrated by their interaction with multiple thymic cell types. We used single-cell RNA sequencing to create a cell census of the human thymus across the life span and to reconstruct T cell differentiation trajectories and T cell receptor (TCR) recombination kinetics. Using this approach, we identified and located in situ CD8αα+ T cell populations, thymic fibroblast subtypes, and activated dendritic cell states. In addition, we reveal a bias in TCR recombination and selection, which is attributed to genomic position and the kinetics of lineage commitment. Taken together, our data provide a comprehensive atlas of the human thymus across the life span with new insights into human T cell development

    GC-Rich Sequence Elements Recruit PRC2 in Mammalian ES Cells

    Get PDF
    Polycomb proteins are epigenetic regulators that localize to developmental loci in the early embryo where they mediate lineage-specific gene repression. In Drosophila, these repressors are recruited to sequence elements by DNA binding proteins associated with Polycomb repressive complex 2 (PRC2). However, the sequences that recruit PRC2 in mammalian cells have remained obscure. To address this, we integrated a series of engineered bacterial artificial chromosomes into embryonic stem (ES) cells and examined their chromatin. We found that a 44 kb region corresponding to the Zfpm2 locus initiates de novo recruitment of PRC2. We then pinpointed a CpG island within this locus as both necessary and sufficient for PRC2 recruitment. Based on this causal demonstration and prior genomic analyses, we hypothesized that large GC-rich elements depleted of activating transcription factor motifs mediate PRC2 recruitment in mammals. We validated this model in two ways. First, we showed that a constitutively active CpG island is able to recruit PRC2 after excision of a cluster of activating motifs. Second, we showed that two 1 kb sequence intervals from the Escherichia coli genome with GC-contents comparable to a mammalian CpG island are both capable of recruiting PRC2 when integrated into the ES cell genome. Our findings demonstrate a causal role for GC-rich sequences in PRC2 recruitment and implicate a specific subset of CpG islands depleted of activating motifs as instrumental for the initial localization of this key regulator in mammalian genomes.Burroughs Wellcome FundCharles E. Culpeper FoundationMassachusetts General HospitalBroad Institute of MIT and Harvar

    Tracking the international spread of SARS-CoV-2 lineages B.1.1.7 and B.1.351/501Y-V2

    Get PDF
    Publisher Copyright: © 2021 O'Toole Á et al.Late in 2020, two genetically-distinct clusters of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) with mutations of biological concern were reported, one in the United Kingdom and one in South Africa. Using a combination of data from routine surveillance, genomic sequencing and international travel we track the international dispersal of lineages B.1.1.7 and B.1.351 (variant 501Y-V2). We account for potential biases in genomic surveillance efforts by including passenger volumes from location of where the lineage was first reported, London and South Africa respectively. Using the software tool grinch (global report investigating novel coronavirus haplotypes), we track the international spread of lineages of concern with automated daily reports, Further, we have built a custom tracking website (cov-lineages.org/global_report.html) which hosts this daily report and will continue to include novel SARS-CoV-2 lineages of concern as they are detected.Peer reviewe
    • 

    corecore