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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

A taxonomic treatment of the Gentianaceae in Virginia

Hammond-Soltis, Georgia A. 01 January 1979 (has links)
No description available.
2

Biosystematic Studies of Three Sympatric Elephantopus Species (Compositae)

Sheffy, Mary V. 01 January 1967 (has links)
No description available.
3

Taxonomic Implications of Sporanglial Ultrastructure Within the Subfamily Melobesioideae Corallinales, Rhodophyta)

Griffin, Bethany Ann 01 January 1997 (has links)
No description available.
4

A Systematic Survey of the Tribe Cichorieae in Virginia

Nessler, Craig Lee 01 January 1972 (has links)
No description available.
5

Ultrastructural Features of Tetrasporgenesis Within the Corallinoideae and Taxonomic Implications for Coralline Red Algae (Corallinales, Rhodophyta)

Mays, Kristin Leigh 01 January 1997 (has links)
No description available.
6

Exploring and understanding signal-response relationships and response dynamics of microbial two-component signaling systems

Amin, Munia January 2013 (has links)
Two-component signaling systems are found in bacteria, fungi and plants. They mediate many of the physiological responses of these organisms to their environment and display several conserved biochemical and structural features. This thesis identifies a potential functional role for two commonly found architectures in two-component signaling system, the split kinases and phosphate sink, which suggests that by enabling switch-like behaviors they could underlie physiological decision making. I report that split histidine kinases, where autophosphorylation and phosphotransfer activities are segregated onto distinct proteins capable of complex formation, enable ultrasensitivity and bistability. By employing computer simulations and analytical approaches, I show that the specific biochemical features of split kinases “by design” enable higher nonlinearity in the system response compared to conventional two-component systems and those using bifunctional (but not split) kinases. I experimentally show that one of these requirements, namely segregation of the phosphatase activity only to the free form of one of the proteins making up the split kinase, is met in proteins isolated from Rhodobacter sphaeroides. While the split kinase I study from R. sphaeroides is specifically involved in chemotaxis, other split kinases are involved in diverse responses. Genomics studies suggest 2.3% of all chemotaxis kinases, and 2.8% of all kinases could be functioning as split kinases. Combining theoretical and experimental approaches, I show that the phosphate sink motif found in microbial and plant TCSs allows threshold behaviors. This motif involves a single histidine kinase that can phosphotransfer reversibly to two separate response regulators and examples are found in bacteria, yeast and plants. My results show that one of the response regulators can act as a “sink” or “buffer” that needs to be saturated before the system can generate significant responses. This sink, thereby allows the generation of a signal threshold that needs to be exceeded for there to be significant phosphoryl group flow to the other response regulator. Thus, this system can enable cells to display switch-like behavior to external signals. Using an analytical approach, I identify mathematical conditions on the system parameters that are necessary for threshold dynamics. I find these conditions to be satisfied in both of the natural systems where the system parameters have been measured. Further, by in vitro reconstitution of a sample system, I experimentally demonstrate threshold dynamics for a phosphate-sink containing two-component system. This study provides a link between these architectures of TCSs and signal-response relationship, thereby enabling experimentally testable hypotheses in these diverse two-component systems. These findings indicate split kinases and phosphate as a microbial alternative for enabling ultrasensitivity and bistability - known to be crucial for cellular decision making. By demonstrating ultrasensitivity, threshold dynamics and their mechanistic basis in a common class of two-component system, this study allows a better understanding of cellular signaling in a diverse range of organisms and will open the way to the design of novel threshold systems in synthetic biology. Thus, I believe that this study will have broad implications not only for microbiologists but also systems biologists who aim to decipher conserved dynamical features of cellular networks.
7

Analysis of fecal biomarkers to impact clinical care and public health

Matus García, Mariana Guadalupe January 2018 (has links)
Thesis: Ph. D., Massachusetts Institute of Technology, Computational and Systems Biology Program, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references. / DNA sequencing and metabolomics technologies have accelerated the discovery of novel biomarkers in clinical samples. In this thesis, I explore the potential of fecal biomarkers to impact clinical and public health practice through non-invasive assessments. First, I highlight the potential of the gut microbiome to provide novel diagnostic and therapeutic targets. By analyzing the gut microbiome and metabolome of mice exposed to a high salt diet, we identified Lactobacillus as a potential probiotic to counteract salt-sensitive conditions such as high blood pressure. Next, I present preliminary validation of wipe samples as a patient-friendly alternative to standard stool collection methods, in particular for the clinical management of Inflammatory Bowel Disease patients. By comparing paired stool and wipe samples, I show that wipe samples capture the same gut microbiome profiles as standard stool samples, and can also be used to quantify fecal calprotectin. Finally, I present the first ever analysis of the microbiome and metabolome of wastewater collected from a residential neighborhood. By testing samples collected hourly over one day, we identified thousands of bacteria and metabolites derived from human activity. Glucuronide compounds that directly reflect consumption of pharmaceutical products and drugs were identified for the first time in a wastewater epidemiology study. Our results highlight the potential of testing wastewater in geo-localized residential areas to produce high-quality data to inform public health practice. Together, these results show the potential of leveraging high-throughput technologies to create seamless readouts of human and population health. / by Mariana Guadalupe Matus García. / Ph. D.
8

Coordination of cellular force-generation during Drosophila ventral furrow formation

Xie, Shicong, Ph. D. Massachusetts Institute of Technology January 2016 (has links)
Thesis: Ph. D., Massachusetts Institute of Technology, Computational and Systems Biology Program, 2016. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 107-115). / Spatiotemporally coordinated cell behavior is observed during morphogenesis, in both embryonic development as well as tissue regeneration. An open question is how individual cells collectively generate force to achieve the correct tissue architecture. This thesis examines how the apical forces generated by Drosophila ventral furrow cells undergoing collective apical constriction are coordinated to fold the tissue. In Chapter 2, I investigate how discrete actomyosin contraction events are coordinated in time and between neighboring cells to yield tissue contraction and folding. I developed a computational pipeline to identify and classify contraction events from live images of ventral furrow formation. Using this framework, I found heterogeneity in contraction events, both in terms of contraction intensity as well as apical area behavior. I found that apical constricting cells transition in contractile behavior over time, from undergoing reversible contractions into a ratcheted state where contractions are irreversible. High expression of the transcription factor Twist is required for the transition into this irreversible, ratcheted state, which is associated with more neighboring contractions as well as cooperative interactions between neighbors. In Chapter 3, I examined how contractility is buffered against heterogeneity in cell apical area. I found that Cta-signaling is required to robustly maintain apical Factin cortex that can support contracting over larger apical distances. Without this buffering, apically larger cells progressively lose apical F-actin and exhibit delayed initiation of actomyosin contractions, leading to a lack of coordinated constriction. / by Shicong Xie. / Ph. D.
9

Binding affinity of a small molecule to an amorphous polymer in a solvent

Chunsrivirot, Surasak January 2011 (has links)
Thesis (Ph. D.)--Massachusetts Institute of Technology, Computational and Systems Biology Program, 2011. / Page 169 blank. Cataloged from PDF version of thesis. / Includes bibliographical references (p. 165-168). / Crystallization is a commonly used purification process in industrial practice. It usually begins with heterogeneous nucleation on a foreign surface. The complicated mechanism of heterogeneous nucleation is not well understood, but we hypothesize a possible correlation between binding affinity to a surface and nucleation enhancement. Amorphous polymers have been used in controlling crystallization. However, to our knowledge no attempt has been made to investigate the possibility of using binding affinity to help guide the selection of polymers promoting heterogeneous nucleation. This study investigated the possibility of using binding affinity of one molecule and many molecules to help guide the selection of these polymers. To measure the binding affinity of one molecule, we developed a two-step approach to compute the free energy of binding to a binding site, using a system of ethylene glycol, polyvinyl alcohol (PVA), and heavy water (D20). The first step of our approach uses Adsorption Locator to identify probable binding sites and molecular dynamics to screen for the best binding sites. The second step employs the Blue-Moon Ensemble method to compute the free energy of binding. We then applied our procedure to the systems of aspirin binding on the surfaces of four nonporous crosslinked polymers in ethanol-water 38 v%. These polymers are poly(4- acryloylmorpholine) (PAM), poly(2-carboxyethyl acrylate) (PCEA), poly(4-hydroxylbutyl acrylate) (PHBA), and polystyrene (PS), and they all are crosslinked with divinylbenzene (DVB). We developed an approach to construct these crosslinked polymers and built three independent surfaces for each polymer. We found the similarity between the trend of heterogeneous nucleation activity and that of the average free energies of binding to the best site of each polymer surface. To measure the binding affinity of many molecules, preferential interaction coefficient and the number of aspirin molecules associated with the area of the binding site was calculated. We found that there is also a similarity between the trend of heterogeneous nucleation activity and that of number of aspirin molecules associated with the area of the binding site (taken into account the effects of polar/apolar atom interactions between an aspirin and a polymer). These results suggest the possibility of using binding affinity, especially the free energy of binding to the best site and the number of nucleating molecule, to help guide the selection of polymers promoting heterogeneous nucleation. / by Surasak Chunsrivirot. / Ph.D.
10

Analysis of coordinated skipped exon pairs using single molecule sequencing technology

Adadey, Asa (Asa Owuraku) January 2014 (has links)
Thesis: S.M., Massachusetts Institute of Technology, Computational and Systems Biology Program, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 31-35). / Alternative splicing of mRNA transcripts is a significant step in the production of functioning protein. This process is a major source of molecular diversity, as numerous mRNA and protein products can arise from a single gene locus, and incorrect regulation has been implicated in numerous diseases. While many robust methods exist to study genome-wide single exon splicing patterns, no methodology has been established to accurately examine multiple events over a single isoform. Read sequencing technology has been the limiting factor; however, the recent development of real time, single molecule read sequencing provides an opportunity to characterize alternative splicing on the whole transcript level. We propose a computational approach to detect the splicing patterns of pairs of alternative exons in the same gene. Using a sequenced full-length cDNA library of human MCF-7 transcripts, we are able to evaluate 761 genes and identify three with evidence of non-random splicing of distinct nonadjacent alternative exons, all of which are frame-preserving and biased toward mutual inclusion. Characterizing their protein products reveals that the domain, secondary, and tertiary structures of the isoforms are not significantly affected. Low read coverage proves to be the greatest hindrance to a larger result set, but overall we provide a computational proof of concept for studying coordinated alternative splicing events on a transcriptomic scale. / by Asa Adadey. / S.M.

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