How does S–Cdk help guarantee that replication occurs only once during each cell cycle?
(a) It blocks the rise of Cdc6 concentrations early in G1.
(b) It phosphorylates and inactivates DNA helicase.
(c) It phosphorylates the Cdc6 protein, marking it for destruction.
(d) It promotes the assembly of a prereplicative complex.
(c) The concentration of Cdc6 rises early in G1, independent of S–Cdk [choice (a)]. Cdc6 guides the assembly of the prereplicative complex at an origin [choice (d)]. By phosphorylating Cdc6, S-Cdk targets the protein for destruction, so that once an origin fires and replicates, Cdc6 will not be around to re-initiate replication. S-Cdk phosphorylates and activates DNA helicases[choice (b)].
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The breakdown of glucose to pyruvate by a cell is an example of a(n) ________ reaction.
A) anabolic B) aerobic C) catabolic D) amphibolic E) synthesis
All of the following involve active transport across membranes except
A) the movement of mineral nutrients from the apoplast to the symplast. B) the movement of sugar from mesophyll cells into sieve-tube elements. C) the movement of sugar from one sieve-tube element to the next. D) the movement of K+ across guard cell membranes during stomatal opening. E) the movement of mineral nutrients into cells of the root cortex.
Enzymes can have both active and regulatory sites. What is the purpose of these sites?
A.) the binding of CTP at a regulatory site on the protein causes decreased production of carbamoyl aspartate B.) the binding of CTP at a regulatory site on the protein causes increased production of carbamyl aspartate C.) the binding of CTP at the active site on the protein causes decreased production of carbamoyl aspartate D.) the binding of CTP at the active site on the protein causes increased production of carbamoyl aspartate
A common source of antibiotics is
A. Acetobacter. B. Aspergillus oryzae and Lactobacillus. C. Lactoccocus cremoris. D. Streptococcus thermophilus. E. Streptomyces.