System of units of measurement

A system of units of measurement, also known as a system of units or system of measurement, is a collection of units of measurement and rules relating them to each other. Systems of measurement have historically been important, regulated and defined for the purposes of science and commerce. Instances in use include the International System of Units or (the modern form of the metric system), the British imperial system, and the United States customary system. History Units of measurement#History The French Revolution gave rise to the metric system, and this has spread around the world, replacing most customary units of measure. In most systems, length (distance), mass, and time are base quantities. Later science developments showed that an electromagnetic quantity such as electric charge or electric current could be added to extend the set of base quantities. Gaussian units have only length, mass, and time as base quantities, with no separate electr

Quantitative analysis of gene expression and transcriptional memory in mouse embryonic stem cells

Aleksandra Mandic

Quantitative studies of gene expression in have shown widespread variability in transcript and protein product abundance at the single-cell level. Such fluctuations in gene expression have been shown to lead to heterogeneous cellular responses and phenotypes, acting in a range of systems, from immunity, cancer to development. To better understand heterogeneity in cellular populations, we developed a novel method for the precise quantification of gene expression in single living cells. Furthermore, we focused on whether and to which extent transcriptional expression profiles are transmitted during cell division. In the first section, we described a new approach for quantitative measurements of gene expression, based on internal tagging of endogenous proteins with a reporter allowing luminescence and fluorescence time-lapse imaging. Using the currently brightest luminescent reporter, fluctuations of protein expression levels could be monitored in single living cells with high sensitivity and temporal resolution over extended time periods. Moreover, our system allowed measurement of single-cell protein degradation rates in the absence of protein synthesis inhibitors, as well as calculation of absolute synthesis rates over the cell cycle. Finally, the internal tag could be excised by inducible expression of Cre recombinase, which enabled estimation of endogenous mRNA half-lives by monitoring the decay of luminescent signal. Second, to monitor transcriptional activity and quantify memory across multiple endogenous genes in mouse embryonic stem cells, we generated cell lines in which a short-lived luciferase reporter allowed sensitive monitoring of transcriptional kinetics by luminescence imaging at high time resolution over long periods of time. By coupling live single-cell monitoring of transcriptional activity of 9 different promoters with mathematical modelling, we found that different levels of expression fluctuations shape transcriptional memory. Long fluctuations (1-10 generations) and short fluctuations (1-3 h), as well as their amplitudes, vary widely between across different genes and together shape transcriptional memory. Surprisingly, we have shown that there was a large level of gene-specific cell-to-cell synchronicity in transcriptional profiles. Overall, we uncovered that noise and timescales of slow and fast fluctuations, as well as their synchronisation over the cell cycle, defined how transcriptional dynamics were correlated within a lineage of related cells. In conclusion, this thesis described an innovative method that allows measurement of absolute protein expression levels, protein turnover, as well as mRNA half-lives for endogenously expressed genes. Our approach opens new avenues in quantitative studies of gene expression in single living cells. Furthermore, we revealed different levels of transcriptional fluctuations that shape gene specific transcriptional memory and its timescales in mouse embryonic stem cells.

EPFL2018

Capteur chimique à fibres optiques pour la mesure des ions chlore dans le béton à un stade précoce

Francine Laferrière

In the national and worldwide context, countless reinforced concrete structures are in an advanced state of deterioration. A principal cause of such degradation is chloride induced corrosion of reinforcement bars. This phenomenon is accentuated in countries where de-icing salts are used for road safety, as well as in maritime zones. To date, no non-destructive method reliably quantifies chloride content during the corrosion initiation phase. Measurement of such a parameter is important for development of a better understanding of the complexity of corrosion phenomena and more practically, for better management of existing structures. The principal objective of this study is to propose a method for non-destructive measurement in order to monitor continuously and in real time free chloride content in concrete pores. In this context, a chemical sensor that employs optical fibres was developed and tested. The sensor functions using the fluorescence of an indicator dye that is sensitive to chlorides. Through fluorescence spectroscopy, variations in the concentration of free chlorides are related to intensity fluctuations of fluorescence. In this type of sensor, the role reserved for optical fibres is limited to data transport, thereby enabling remote measurement. The use of optical fibres also provides an advantage compared with existing electric non-destructive detection systems due to superior electromagnetic stability. Theoretical and experimental studies calibrated and validated the sensor for implementation within mortar and concrete. Free chloride concentrations between 0 and 350 mM can be detected, this represents 0.05% and 0.6% of cement weight. The system stability is approximately 0.02% [Cl-]/year. Measurement accuracy is generally lower than ± 8%. Two experiments reproduced climatic variations in a controlled environment. The first test simulated a hot maritime climate and the second test simulated a cold continental climate. These tests confirmed that it is possible to determine with precision free chloride content. A constant temperature and relative humidity test showed a good concordance between results from the measurement system and an existing chloride transport model. Although measurements indicate faster chloride ingress compared with model predictions, test results and model predictions coincide after a few days. In conclusion, fluorescence spectroscopy with optical fibres offers an innovative means for early and non-destructive detection of free chloride content in concrete. Experiments and model predictions demonstrated complementary strengths and weaknesses. As a result, there is much potential for synergy in order to improve the science of corrosion process understanding and to plan appropriately for preventive action in practical situations.

EPFL2005