The promoter of the araBAD operon from E. coli is activated in the presence of arabinose and the absence of glucose and directs expression of genes encoding enzymes required for arabinose metabolism. 

Unlike the cases of lac and gal genes, where a repressor and an activator work together, here two activators work together: AraC and CAP. 

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PR segment – Conduction through AV node

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PR interval - Atrioventricular conduction

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QT interval includes the QRS complex, the ST segment, and the T wave.

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ST segment - plateau portion of the ventricular action potential.

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The U wave is an inconstant finding that may be due to ventricular myocytes with long action potentials.

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#Unit 13. Methods in Biology

Heart rate is measured by counting the number of QRS complexes or R waves because they are most prominent per minute.