The expression of the tryptophan operon is controlled by a(n)
A) repressor that is active when it is alone.
B) repressor that is inactive when it binds to lactose.
C) repressor that is active when it binds to tryptophan.
D) activator that turns the operon on by binding to DNA.
Answer: C
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Which of the following offers the best description of neural transmission across nearly all mammalian synaptic clefts?
A. Neural impulses involve the flow of K+ and Na+ across the cleft. B. Neural impulses travel across the gap as electrical currents. C. Neural impulses cause the release of chemicals that diffuse across the cleft. D. Neural impulses travel across the cleft in both directions. E. The calcium within the axons and dendrites of nerves adjacent to a synapse acts as the neurotransmitter.
Which organ system is the principle regulatory system of the body, functioning to monitor and respond to changes in the environment?
1.A
2.B
3.C
4.D
Which term refers to two or more different tissues arranged to carry out a specific function?
a. organ system b. tissue c. organism d. organ
The principal radiation interactions within the human body are assumed to be _____.
a) indirect effects b) direct effects c) DNA damage d) cell destruction