The default state for the lac operon is off, while the default state for the trp operon is on. Explain this
contradiction and the mechanism of control via the repressor proteins.
What will be an ideal response?
The lac operon's function is to allow a cell to utilize lactose as an energy source. It wouldn't make
sense to express these genes unless lactose were available in the environment. Because of this, the
presence of lactose serves to block the repressor from binding to the operator. This results in the lac
operon only being active when lactose is available.
In contrast, the trp operon's function is to allow the bacterial cell to synthesize tryptophan, which is an
amino acid used as a building block in protein synthesis. The cell must make it unless it is available in
the environment. For this reason, the trp operon is on unless there is excess tryptophan available (as
would be the case when it is in the environment) to bind to the trp repressor, activate it, and cause the
repressor to bind to the operator.
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Which of the following would describe the action of a chemical compound considered to be a mutagen?
A. Most are reactive and add bulky side groups to DNA bases. B. Most are able to bypass DNA repair mechanisms. C. Most are reactive and add bulky side groups to RNA bases. D. Most interfere with DNA replication. E. Most interfere with DNA transcription.
Calculate the pKa of lactic acid, given that when the concentration of lactic acid is 0.01 M and the concentration of lactate is 0.002 M, the pH is 2.9
A. 3.9 B. 3.6 C. 3.2 D. 2.5 E. 2.2
In capillary exchange, osmosis takes place ____
a. at the arterial end b. along the entire length of the capillary c. at the venous end d. in the middle part of the capillary e. osmosis does not take place in capillary exchange
What is the basic shape of a tRNA molecule?
A) A three-dimensional cloverleaf B) A straight strand C) A right-handed helix D) A globular ball E) A ?-pleated sheet