Identification of DSB-1, a protein required for initiation of meiotic recombination in Caenorhabditis elegans, illuminates a crossover assurance checkpoint.

Meiotic recombination, an essential aspect of sexual reproduction, is initiated by programmed DNA double-strand breaks (DSBs). DSBs are catalyzed by the widely-conserved Spo11 enzyme; however, the activity of Spo11 is regulated by additional factors that are poorly conserved through evolution. To expand our understanding of meiotic regulation, we have characterized a novel gene, dsb-1, that is specifically required for meiotic DSB formation in the nematode Caenorhabditis elegans. DSB-1 localizes to chromosomes during early meiotic prophase, coincident with the timing of DSB formation. DSB-1 also promotes normal protein levels and chromosome localization of DSB-2, a paralogous protein that plays a related role in initiating recombination. Mutations that disrupt crossover formation result in prolonged DSB-1 association with chromosomes, suggesting that nuclei may remain in a DSB-permissive state. Extended DSB-1 localization is seen even in mutants with defects in early recombination steps, including spo-11, suggesting that the absence of crossover precursors triggers the extension. Strikingly, failure to form a crossover precursor on a single chromosome pair is sufficient to extend the localization of DSB-1 on all chromosomes in the same nucleus. Based on these observations we propose a model for crossover assurance that acts through DSB-1 to maintain a DSB-permissive state until all chromosome pairs acquire crossover precursors. This work identifies a novel component of the DSB machinery in C. elegans, and sheds light on an important pathway that regulates DSB formation for crossover assurance.

Spectrophotometric titration of bimetallic metal cation binding in polyamido(amine) dendrimer templates.

Spectrophotometric titration and a binding isotherm were used to accurately assess the loading capacity of generation four polyamido(amine) (PAMAM) dendrimer templates with terminal alcohol groups (G4-OH). Preparation of bimetallic G4-OH dendrimer-encapsulated metal nanoclusters (DENs) necessitates knowledge of the precise metal-ion binding capacity. The binding of metal ions such as Pt(2+) and Pd(2+) has proven difficult to assess via UV-vis spectroscopy because the absorbance shifts associated with metal-ion binding within the dendrimer template are masked by the absorbance of the PAMAM dendrimer itself. In contrast, the binding of Cu(2+) to G4-OH PAMAM dendrimer results in a strong, distinct absorption band at 300 nm, making UV-vis spectrophotometric titration with copper straightforward. Here we use copper binding as a means to assess the number of binding sites remaining within the PAMAM G4-OH dendrimer after the complexation of a specified molar excess of Pd(2+) or Pt(2+). In addition, we use a binding isotherm to mathematically estimate the loading capacity of the dendrimer in each case. The loading capacities for M(2+) in the G4-OH dendrimer were found to be ∼16 for copper alone, ∼21 for copper combined with palladium, and ∼25 for copper combined with platinum.

Benefit-Cost Analysis of Undergraduate Education Programs: An Example Analysis of the Freshman Research Initiative.

Institutions and administrators regularly have to make difficult choices about how best to invest resources to serve students. Yet economic evaluation, or the systematic analysis of the relationship between costs and outcomes of a program or policy, is relatively uncommon in higher education. This type of evaluation can be an important tool for decision makers considering questions of resource allocation. Our purpose with this essay is to describe methods for conducting one type of economic evaluation, a benefit-cost analysis (BCA), using an example of an existing undergraduate education program, the Freshman Research Initiative (FRI) at the University of Texas Austin. Our aim is twofold: to demonstrate how to apply BCA methodologies to evaluate an education program and to conduct an economic evaluation of FRI in particular. We explain the steps of BCA, including assessment of costs and benefits, estimation of the benefit-cost ratio, and analysis of uncertainty. We conclude that the university's investment in FRI generates a positive return for students in the form of increased future earning potential.

Effects of Discovery, Iteration, and Collaboration in Laboratory Courses on Undergraduates' Research Career Intentions Fully Mediated by Student Ownership.

Course-based undergraduate research experiences (CUREs) provide a promising avenue to attract a larger and more diverse group of students into research careers. CUREs are thought to be distinctive in offering students opportunities to make discoveries, collaborate, engage in iterative work, and develop a sense of ownership of their lab course work. Yet how these elements affect students' intentions to pursue research-related careers remain unexplored. To address this knowledge gap, we collected data on three design features thought to be distinctive of CUREs (discovery, iteration, collaboration) and on students' levels of ownership and career intentions from ∼800 undergraduates who had completed CURE or inquiry courses, including courses from the Freshman Research Initiative (FRI), which has a demonstrated positive effect on student retention in college and in science, technology, engineering, and mathematics. We used structural equation modeling to test relationships among the design features and student ownership and career intentions. We found that discovery, iteration, and collaboration had small but significant effects on students' intentions; these effects were fully mediated by student ownership. Students in FRI courses reported significantly higher levels of discovery, iteration, and ownership than students in other CUREs. FRI research courses alone had a significant effect on students' career intentions.

Early Engagement in Course-Based Research Increases Graduation Rates and Completion of Science, Engineering, and Mathematics Degrees.

National efforts to transform undergraduate biology education call for research experiences to be an integral component of learning for all students. Course-based undergraduate research experiences, or CUREs, have been championed for engaging students in research at a scale that is not possible through apprenticeships in faculty research laboratories. Yet there are few if any studies that examine the long-term effects of participating in CUREs on desired student outcomes, such as graduating from college and completing a science, technology, engineering, and mathematics (STEM) major. One CURE program, the Freshman Research Initiative (FRI), has engaged thousands of first-year undergraduates over the past decade. Using propensity score-matching to control for student-level differences, we tested the effect of participating in FRI on students' probability of graduating with a STEM degree, probability of graduating within 6 yr, and grade point average (GPA) at graduation. Students who completed all three semesters of FRI were significantly more likely than their non-FRI peers to earn a STEM degree and graduate within 6 yr. FRI had no significant effect on students' GPAs at graduation. The effects were similar for diverse students. These results provide the most robust and best-controlled evidence to date to support calls for early involvement of undergraduates in research.

A Systematic Investigation of p-Nitrophenol Reduction by Bimetallic Dendrimer Encapsulated Nanoparticles.

We demonstrate that the reduction of p-nitrophenol to p-aminophenol by NaBH4 is catalyzed by both monometallic and bimetallic nanoparticles (NPs). We also demonstrate a straightforward and precise method for the synthesis of bimetallic nanoparticles using poly(amido)amine dendrimers. The resulting dendrimer encapsulated nanoparticles (DENs) are monodisperse, and the size distribution does not vary with different elemental combinations. Random alloys of Pt/Cu, Pd/Cu, Pd/Au, Pt/Au, and Au/Cu DENs were synthesized and evaluated as catalysts for p-nitrophenol reduction. These combinations are chosen in order to selectively tune the binding energy of the p-nitrophenol adsorbate to the nanoparticle surface. Following the Brønsted-Evans-Polanyi (BEP) relation, we show that the binding energy can reasonably predict the reaction rates of p-nitrophenol reduction. We demonstrate that the measured reaction rate constants of the bimetallic DENs is not always a simple average of the properties of the constituent metals. In particular, DENs containing metals with similar lattice constants produce a binding energy close to the average of the two constituents, whereas DENs containing metals with a lattice mismatch show a bimodal distribution of binding energies. Overall, in this work we present a uniform method for synthesizing pure and bimetallic DENs and demonstrate that their catalytic properties are dependent on the adsorbate's binding energy.

A role for cortactin in Listeria monocytogenes invasion of NIH 3T3 cells, but not in its intracellular motility.

Cortactin is an F-actin binding protein that binds to the Arp2/3 complex, stimulates its actin nucleation activity, and inhibits actin filament debranching. Using RNA interference directed against cortactin, we explored the importance of cortactin for several processes involving dynamic actin assembly. Silencing cortactin expression was efficiently achieved in HeLa and NIH 3T3 cells, with less than 5% of cortactin expression in siRNA-treated cells. Surprisingly, endocytosis in HeLa and NIH 3T3 cells, and cell migration rates, were not altered by RNAi-mediated cortactin silencing. Listeria utilizes actin-based motility to move within and spread among mammalian host cells; its actin-clouds and tails recruit cortactin. We explored the role of cortactin during the Listeria life cycle in cortactin "knockdown" NIH 3T3 cells. Interestingly, cortactin siRNA-treated cells showed a significant reduction in the efficiency of the bacteria invasion in NIH 3T3 cells. However, cortactin depletion did not interfere with assembly of Listeria actin clouds or actin tails, or Listeria intracellular motility or speed. Therefore, our findings suggest that cortactin plays a role in Listeria internalization, but not in the formation of actin clouds and tails, or in bacteria intracellular motility.

Critical conformational changes in the Arp2/3 complex are induced by nucleotide and nucleation promoting factor.

Actin nucleation and branching by the Arp2/3 complex is tightly regulated by activating factors. However, the mechanism of Arp2/3 complex activation remains unclear. We used fluorescence resonance energy transfer (FRET) to probe the conformational dynamics of the Arp2/3 complex accompanying its activation. We demonstrate that nucleotide binding promotes a substantial conformational change in the complex, with distinct conformations depending on the bound nucleotide. Nucleotide binding to each Arp is critical for activity and is coupled to nucleation promoting factor (NPF) binding. The binding of Wiskott-Aldrich syndrome protein (WASP) family NPFs induces further conformational reorganization of the Arp2/3 complex, and the ability to promote this conformational reorganization correlates with activation efficiency. Using an Arp2/3 complex that is fused to the actin binding domain of WASP, we confirm that the NPF-induced conformational change is critical for activation, and that the actin and Arp2/3 binding activities of WASP are separable, but are independently essential for activity.