Chemical synthesis of DNA sequences provides a powerful tool for modifying genes and for studying gene function, structure and expression. Here, we report a simple, high-fidelity and cost-effective PCR-based two-step DNA synthesis (PTDS) method for synthesis of long segments of DNA. The method involves two steps. (i) Synthesis of individual fragments of the DNA of interest: ten to twelve 60mer oligonucleotides with 20 bp overlap are mixed and a PCR reaction is carried out with high-fidelity DNA polymerase Pfu to produce DNA fragments that are approximately 500 bp in length. (ii) Synthesis of the entire sequence of the DNA of interest: five to ten PCR products from the first step are combined and used as the template for a second PCR reaction using high-fidelity DNA polymerase pyrobest, with the two outermost oligonucleotides as primers. Compared with the previously published methods, the PTDS method is rapid (5-7 days) and suitable for synthesizing long segments of DNA (5-6 kb) with high G + C contents, repetitive sequences or complex secondary structures. Thus, the PTDS method provides an alternative tool for synthesizing and assembling long genes with complex structures. Using the newly developed PTDS method, we have successfully obtained several genes of interest with sizes ranging from 1.0 to 5.4 kb.
Fine particulate matter (PM) air pollution outbreaks have recently occurred frequently in China. However, evidence of the associations between short-term exposure to PM and cardiovascular morbidity is still limited in China. This study aimed to evaluate the associations between PM and hospital emergency room visits (ERVs) for cardiovascular diseases in urban areas in Beijing. Daily counts of cardiovascular ERVs were collected from ten large general hospitals from Jan 1 to Dec 31, 2013. Air pollution data were obtained from the Beijing Environmental Protection Bureau including 17 monitoring stations. A generalized additive Poisson model was used to examine the associations between PM and cardiovascular ERVs after controlling for seasonality, day of the week, public holidays, influenza outbreaks, and weather conditions. In total, there were 56,221 cardiovascular ERVs during the study period. The daily mean PM concentration was 102.1 μg/m, ranging from 6.7 μg/m to 508.5 μg/m. Per 10 μg/m increase in PM was associated with a 0.14% (95% confidence interval [CI]: 0.01%-0.27%) increase in cardiovascular ERVs at lag. Cumulative delayed estimates were greatest at lag (0.30%, 95% CI: 0.09%-0.52%). The estimates of percentage change in daily ERVs per 10 μg/m increase in PM were 0.56% (95%CI: 0.16%-0.95%) for ischemic heart disease (IHD) at lag, 0.81% (95%CI: 0.05%-1.57%) for heart rhythm disturbances (HRD) at lag and 1.21% (95%CI: 0.27%-2.15%) for heart failure (HF) at lag, respectively. The effects of PM on IHD ERVs during high temperature days (>11.01 °C) were significantly higher than that on low temperature days (≤11.01 °C) at lag, lag, lag and lag (P < 0.05). The study suggests that PM has acute impacts on cardiovascular ERVs in Beijing, especially on IHD, HRD and HF. The effects of PM on IHD ERVs vary by temperature.
The mechanism of the ternary breakup of the very heavy system 197 Au + 197 Au at an energy of 15A MeV has been studied by using the improved quantum molecular dynamics model. The calculation results reproduce the characteristic features in ternary breakup events explored in a series of experiments; i.e., the masses of three fragments are comparable in size and the very fast, nearly collinear breakup of the colliding system is dominant in the ternary breakup events. Further, the evolution of the time scales of different ternary reaction modes and the behavior of mass distributions of three fragments with impact parameters are studied. The time evolution of the configurations of the composite reaction systems is also studied. We find that for most of the ternary breakup events with the features found in the experiments, the configuration of the composite system has two-preformed-neck shape. The study shows that those ternary breakup events having the characteristic features found in the experiments happen at relatively small impact parameter reactions, but not at peripheral reactions. The ternary breakup reaction at peripheral reactions belongs to binary breakup with a neck emission.
Wet flue gas desulfurization (WFGD) system is the core equipment for removing SO2 from coal-fired power plants, and it also has an important synergistic effect on the removal of selenium. However, the removal efficiency of Se across WFGD systems is not as expected, and it varies greatly in different coal-fired units (12.5–96%). In this study, a mathematical model was established to quantitatively describe the selenium migration behavior in WFGD spray towers, including the conversion of gaseous selenium to particulate selenium and the capture of gaseous SeO2 and particles by droplets. The calculation results show that the behavior of selenium in the spray tower can be divided into three stages: preparation, condensation, and removal. The condensation stage significantly affected the selenium distribution and its total removal efficiency. Furthermore, five factors which may affect the selenium behavior were investigated. Among them, the inlet particle size distribution and the droplet temperature had great impacts on the outlet selenium concentration, which may be the reason for the unstable selenium removal efficiencies. This study can help in understanding the migration process of selenium in WFGD spray towers and provide some guidance for the development of specific selenium control technologies.
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