UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
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Item A Daily Diary Analysis of Preschool Depressive Behaviors: Same Day and Prospective Associations Across 14 Days(2018) Leppert, Katherine A; Dougherty, Lea R; Psychology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Depressive disorders can be observed in early childhood and are associated with significant concurrent and prospective impairment. Although young children demonstrate similar depressive behaviors as older children and adults, certain depressive behaviors, such as sadness and irritability, are more common in early childhood whereas other depressive behaviors, such as loss of pleasure and suicidal ideation, are much less common. However, little is known about day-to-day variations in common depressive behaviors and factors impacting those variations in early childhood. The current study examined the day-to-day variability and co-occurrence of two common depressive behaviors in young children, sadness and irritability, and predictors of their day-to-day change. Participants included 291 parents of preschool-aged children (ages 3-5). Parents completed a baseline questionnaire assessing demographics, current emotional/behavioral problems, and functional impairment, and then completed an electronic daily diary for 14 days assessing the frequency of daily depressive behaviors, sleep quality, and parent-child relationship functioning. Results indicated that irritability and sadness frequently co-occurred during the same day and were concurrently and prospectively associated with parent-child relationship functioning but not sleep quality. Moreover, we observed between-person stability, but within-person variability, in children’s sadness and irritability across 14 days, and that this variability was moderated by several factors. With few exceptions, we observed greater between-person stability in sadness and irritability for older preschoolers, for males, and for children with overall better functioning (e.g., absence of less common depressive symptoms, fewer baseline psychiatric symptoms, lower baseline impairment, better parent-child relationship functioning and sleep quality). Importantly, our findings highlight stability of children’s sadness and irritability relative to peers and variability relative to their own mean sadness and irritability, as well as provide evidence regarding factors predicting the day-to-day stability or variability of these common depressive behaviors. Understanding daily variation in child depressive behaviors and factors predicting variation can identify at-risk children and provide targets for prevention and intervention, which is particularly crucial given that preschool depression predicts later depression and other psychiatric disorders.Item INVESTIGATION OF LAMIN A PROCESSING AND REGULATION(2017) Wu, Di; Cao, Kan; Cell Biology & Molecular Genetics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Lamin A is a major component of the lamina, which creates a dynamic network underneath the nuclear envelope. Mutations in the lamin A gene (LMNA) cause severe genetic disorders. One of the most striking cases is Hutchinson-Gilford progeria syndrome (HGPS). It is caused by a lamin A mutant protein named progerin. Due to the abnormal retaining of a permanent C-terminal farnesyl tail, progerin gradually accumulates on the nuclear membrane, resulting in abnormal nuclear morphology during interphase and perturbing a diversity of signaling and transcriptional events. To better understand lamin A gene’s function and regulation, I studied lamin A from three aspects in my dissertation, including its post-translational processing, post-transcriptional degradation, and transcriptional regulation. For post-translational processing, I examined the potential effects of cytoplasmic progerin based on a previous observation that membrane-associated progerin forms visible cytoplasmic aggregates in mitosis. After removal of the nuclear localization signal, I find that both LAΔNLS and PGΔNLS mutants are farnesylated in the cytosol and associated with a sub-domain of the ER via their farnesyl tails. While the farnesylation on LAΔNLS can be gradually removed by Zempste24, PGΔNLS remains permanently farnesylated and aggregated in the cytosol. Moreover, both ΔNLS mutants dominantly affect emerin’s nuclear localization. Previously, the accumulation of progerin has led to the speculation that progerin is more stable than the wild type lamin A. However, the low solubility of lamin proteins renders traditional immunoprecipitation-dependent methods ineffective for comparing the relative stabilities of mutant and wild type lamins. Therefore, to investigate the post-translational degradation of lamin A, I employed a novel platform based on viral 2A peptide-mediated co-translational cleavage to infer differences in lamin stability. My results support the notion that progerin is more stable than lamin A. In addition, treatment of FTI reduces progerin relative stability to the level of wild type lamin A. Last but not the least, I investigated the function of LMNA first intron in order to better understand the transcription regulation of lamin A. My results show that a highly conserved region within LMNA first intron is essential for the expression repression of lamin A in HL60 cells. This process is fulfilled by the interaction between this conserved region and transcription factor Sp1. Taken together, my results reveal new insights into biogenesis, protein interaction and transcription regulation of lamin A.Item The Control of a Mathematical Analog of a Tentacle(2015) Xu, Xin; Levine, William S; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A 2-dimensional dynamic analog of squid tentacles was presented. The tentacle analog consists of a multi-cell structure, which can be easily replicated to a large scale. Each cell of the model is a quadrilateral with unit masses at the corners. Each side of the quadrilateral is a spring-damper system in parallel. The spring constants are the controls for the system. The dynamics are subject to the constraint that the area of each quadrilateral must remain constant. The system dynamics was analyzed, and various equilibrium points were found with different controls. Then these equilibrium points were further determined experimentally, demonstrated to be asymptotically stable. A simulation built in MATLAB was used to find the convergence rates under different controls and damping coefficients. Finally, a control scheme was developed and used to drive the system to several configurations observed in real tentacle.Item The Stability of the Schwarzschild Metric(1968) Vishveshwara, C. V.; Misner, Charles W.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)The stability of the Schwarzschild exterior metric against small perturbations is investigated. The exterior extending from the Schwarzschild radius r =2m to spatial infinity is visualized as having been produced by a spherically symmetric mass distribution that collapsed into the Schwarzschild horizon in the remote past. As a preamble to the stability analysis, the phenomenon of spherically symmetric gravitational collapse is discussed under the conditions of zero pressure, absence of rotation and adiabatic flow. This is followed by a brief study of the Kruskal coordinates in which the apparent singularity at r = 2m is no longer present; the process of spherical collapse and the consequent production of the Schwarzschild empty space geometry down to the Schwarzschild horizon are depicted on the Kruskal diagram. The perturbations superposed on the Schwarzschild background metric are the same as those given by Regge and Wheeler consisting of odd and even parity classes, and with the time dependence exp(-ikt), where k is the frequency. An analysis of the Einstein field equations computed to first order in the perturbations away from the Schwarzschild background metric shows that when the frequency is made purely imaginary, the solutions that vanish at large values of r, conforming to the requirement of asymptotic flatness, will diverge near the Schwarzschild surface in the Kruskal coordinates even at the initial instant t = 0. Since the background metric itself is finite at this surface, the above behaviour of the perturbation clearly contradicts the basic assumption that the perturbations are small compared to .the background metric. Thus perturbations with imaginary frequencies that grow exponentially with time are physically unacceptable and hence the metric is stable. In the case of the odd perturbations, the above proof of stability is made rigorous by showing that the radial functions for real values of k form a complete set, by superposition of which any well behaved initial perturbation can be represented so that the time development of such a perturbation is non-divergent, since each of the component modes is purely oscillatory in time. A similar rigorous extension of the proof of stability has not been possible in the case of the even perturbations because the frequency (or k2) does not appear linearly in the differential equation. A study of stationary perturbations (k = 0) shows that the only nontrivial stationary perturbation that can exist is that due to the rotation of the source which is given by the odd perturbation with the angular momentum £ = 1. Finally, complex frequencies are introduced under the boundary conditions of only outgoing waves at infinity and purely incoming waves at the Schwarzschild surface. The physical significance of this situation is discussed and its connection with phenomena such as radiation damping and resonance scattering, and with the idea of causality is pointed out.Item An Analysis of the Stability, Aggregation Propensity, and Negative Cooperativity of the Escherichia coli Chaperonin GroEL(2013) Wehri, Sarah; Lorimer, George H; Biochemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Since the discovery of chaperonin GroEL and co-chaperonin GroES, there has been a deluge of literature investigating many aspects of the system. Substrate proteins are protected from aggregation through a cycle of capture, encapsulation, and release made possible through rigid body motions of the GroE system driven by a combination of allosteric controls influenced by nucleotide, potassium and denatured protein termed substrate protein (SP). This dissertation first explores the sequential transition of GroEL that maintains the rings operating in an alternating fashion. To do this, an intra-subunit, inter-domain mutant, GroELD83AT state. Steady state ATPase assays, stopped-flow fluorescence, and gel filtration chromatography were all used to demonstrate that the trans ring must access the T state before ligands can be discharged from the cis ring. The dual-heptameric ring structure of GroEL and the post-translational assembly of the protein make creating mutants with a mutation within a single subunit of a ring almost impossible, however the ability to do so opens the opportunity for a myriad of experiments that explore the allosteric transitions of GroEL. Two potential recombination methods, acetone treatment and heat treatment, were investigated. Förster resonance energy transfer (FRET) and electrospray ionization mass spectrometry (ESI-MS) were used to study recombination facilitated by such treatments. Recombination using the acetone method resulted in a one-in-one-out subunit exchange, however aggregation complicated the exchange. Heat treatment resulted in exchange of rings. Finally, dynamic light scattering (DLS) was used to investigate stability and aggregation on the chaperonin. It was observed that the chaperonin is stable for over 30 days while incubated continuously at 37°C in sterile buffered solution, however interesting aggregation kinetics are observed upon addition of acetone, the solvent used to strip SP from GroEL during the standard lab purification procedure. GroEL partitions into 10nm and 100nm species that are extremely stable before the appearance of macromolecular aggregates and precipitation is observed.Item Exploring what stabilizes teachers' attention and responsiveness to the substance of students' scientific thinking in the classroom(2013) Richards, Jennifer; Elby, Andrew; Hammer, David; Curriculum and Instruction; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Teachers' attention and responsiveness to the substance of students' disciplinary thinking is critical for promoting students' disciplinary engagement and learning, yet such attention is rare and fleeting in American classrooms. In this dissertation, I aim to learn more from teachers who do attend and respond to students' scientific ideas while teaching. I explore the classroom practices of three focal teachers in a professional development program who consistently place students' ideas at the core of their instruction with an eye toward the following research question: What might stabilize teachers' attention and responsiveness to the substance of students' scientific thinking during sustained classroom episodes? Examining three episodes from each teacher, I identify aspects within these episodes that are salient to the teachers and plausibly interrelated with their attention and responsiveness to student thinking. My primary data chapters include analyses of specific pairs of episodes that speak to my broader research question as well as other relevant topics in the literature on attending and responding to student thinking. The first data chapter makes the case that professional development efforts aimed at supporting responsiveness to student thinking primarily help teachers within planned discussions or progressions, but struggle to help teachers adapt their ongoing instruction in response to unexpected directions from students. I examine two episodes in which the discussions that emerged were not preplanned but rather emergent from students' contributions, with an eye toward what initiated and sustained teachers' responsiveness. The second data chapter contributes to discussions on what constitutes favorable change in attending and responding to the substance of student thinking, emphasizing the importance of disciplinary-specific considerations. Finally, I draw on the entire data set in noting specific commonalities within and across teachers, suggesting two complementary professional development approaches: 1) remaining open to and aware of what hooks and sustains individual teachers and their classroom practice, and 2) emphasizing aspects that cut across teachers, which might serve as meaningful foci for professional development efforts aimed at promoting an instructional focus on students' ideas.Item A Cross-Layer Study of the Scheduling Problem(2009) Pantelidou, Anna; Ephremides, Anthony; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis is inspired by the need to study and understand the interdependence between the transmission powers and rates in an interference network, and how these two relate to the outcome of scheduled transmissions. A commonly used criterion that relates these two parameters is the Signal to Interference plus Noise Ratio (SINR). Under this criterion a transmission is successful if the SINR exceeds a threshold. The fact that this threshold is an increasing function of the transmission rate gives rise to a fundamental trade-off regarding the amount of time-sharing that must be permitted for optimal performance in accessing the wireless channel. In particular, it is not immediate whether more concurrent activations at lower rates would yield a better performance than less concurrent activations at higher rates. Naturally, the balance depends on the performance objective under consideration. Analyzing this fundamental trade-off under a variety of performance objectives has been the main steering impetus of this thesis. We start by considering single-hop, static networks comprising of a set of always-backlogged sources, each multicasting traffic to its corresponding destinations. We study the problem of joint scheduling and rate control under two performance objectives, namely sum throughput maximization and proportional fairness. Under total throughput maximization, we observe that the optimal policy always activates the multicast source that sustains the highest rate. Under proportional fairness, we explicitly characterize the optimal policy under the assumption that the rate control and scheduling decisions are restricted to activating a single source at any given time or all of them simultaneously. In the sequel, we extend our results in four ways, namely we (i) turn our focus on time-varying wireless networks, (ii) assume policies that have access to only a, perhaps inaccurate, estimate of the current channel state, (iii) consider a broader class of utility functions, and finally (iv) permit all possible rate control and scheduling actions. We introduce an online, gradient-based algorithm under a fading environment that selects the transmission rates at every decision instant by having access to only an estimate of the current channel state so that the total user utility is maximized. In the event that more than one rate allocation is optimal, the introduced algorithm selects the one that minimizes the transmission power sum. We show that this algorithm is optimal among all algorithms that do not have access to a better estimate of the current channel state. Next, we turn our attention to the minimum-length scheduling problem, i.e., instead of a system with saturated sources, we assume that each network source has a finite amount of data traffic to deliver to its corresponding destination in minimum time. We consider both networks with time-invariant as well as time-varying channels under unicast traffic. In the time-invariant (or static) network case we map the problem of finding a schedule of minimum length to finding a shortest path on a Directed Acyclic Graph (DAG). In the time-varying network case, we map the corresponding problem to a stochastic shortest path and we provide an optimal solution through stochastic control methods. Finally, instead of considering a system where sources are always backlogged or have a finite amount of data traffic, we focus on bursty traffic. Our objective is to characterize the stable throughput region of a multi-hop network with a set of commodities of anycast traffic. We introduce a joint scheduling and routing policy, having access to only an estimate of the channel state and further characterize the stable throughput region of the network. We also show that the introduced policy is optimal with respect to maximizing the stable throughput region of the network within a broad class of stationary, non-stationary, and anticipative policies.Item LOSS OF CONTACT AND TIME DELAY DYNAMICS OF MILLING PROCESSES(2006-04-19) Long, Xinhua; Balachandran, Balakumar; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Considering the feed motion, the dynamics of constant spindle speed (CSS) milling processes is described by a set of delay differential equations with periodic coefficients and a variable time delay associated with each cutting tooth. This model, which has been developed for the first time as a part of this dissertation, is used to study the dynamics and stability of the system. The semi-discretization scheme, a numerical scheme with an analytical basis, is refined to examine the stability of periodic solutions of this system. This scheme can be used to predict not only the stability but also the chatter frequencies for a wide variety of milling operations ranging from full-immersion to partial-immersion operations. From the results obtained thus far, it can be stated that feed-rate effects can be neglected during full-immersion and high-immersion operations, where the influence of loss of contact nonlinearities is not pronounced. However, for low-immersion milling operations, where loss of contact effects have a strong influence on system stability behavior, high feed-rate effects are pronounced and this effects can't be ignored. Along with investigations into the dynamics of CSS milling processes, in this dissertation, a better delay approximation has been used in the modeling of variable spindle speed (VSS) milling processes, and the benefits of VSS milling operations are discussed by comparing the stability charts of VSS milling operations with those obtained for CSS milling operations. The nonsmooth characteristics of milling processes are pointed out by presenting the simulated results for cutting forces. Work conducted with a nonsmooth mechanical system, a simplified system related to the milling process, is presented and the numerical results and experimental results obtained show evidence for grazing and other bifurcations in this nonsmooth system.