7. Which of the following is involved in regulation of the tryp operon? a. repre

Question

7. Which of the following is involved in regulation of the tryp operon? a. repressor protein b. operator sequence (0) c. tryptophan d. answers A and B are both correct e. all of the above are correct ing to the operon model, for the expression of a repressible operon to occur, the a) End-prod b) Substrate must bind to the enzyme. c Substrate must bind to the repressor. d) Repressor must bind to the operator e) cAMP levels must be high. uct must not be in excess. 39) Which of the following statements about "'catabolite repression" islare TRUE? a) it is a mechanism b) it requires the cAMP activated protein (CAP or CRP) QlCAP or CRP activity is activated by high levels of gl by which the presence of glucose regulates the expression of many operons ucose in the environment d) answers A and B are both correct e) all of the above are correct 40. Which of the following types of mutations creates a stop codon? a. A missense mutation b. A nonsense mutation c. A frameshift mutation d. A silent mutation e. A spontaneous mutation

Explanation / Answer

37. E all are correct.

An operator sequence of DNA is recognized by a regulatory protein known as the trp repressor. When the repressor binds to the DNA of the operator, it keeps the operon from being transcribed by physically getting in the way of RNA polymerase, the transcription enzyme. The trp repressor protein is encoded by a gene called trpR.

38. a

For a repressible operon to function the end product should not be in excess as if end product is in excess the operon will not start.

39. e

All the above are correct.

The ability of glucose to inhibit the synthesis of certain enzymes, referred to as the glucose effect, was recognized in bacteria at an early date by Monod. He observed that when Escherichia coli encounters both glucose and lactose, for example, it metabolizes the glucose first and represses the use of lactose, resulting in biphasic growth. This phenomenon is due to the repressive effect of glucose on the synthesis of enzymes required for the metabolism of other sugars. Later, the term catabolite repression was introduced as a general name for the glucose effect because compounds closely related to glucose elicited varying degrees of repression of glucose-sensitive enzymes, and the catabolites derived from the repressing compound were assumed to cause the glucose effect. Studies on catabolite repression up to the early 1970s brought about the discovery of a positive control system of transcription by cyclic AMP (cAMP) and the cyclic AMP receptor protein (CRP) (also called the catabolite gene activator protein, (CAP)) and led to the concept that catabolite repression is caused by reduction of the level of intracellular cAMP . We now know that multiple and different mechanisms are operating, depending on the growth conditions and the target operons and that the cAMP-dependent mechanism is just one aspect of catabolite repression. Ironically, the preferential utilization of glucose over lactose, the prototype of catabolite repression, is not due to the reduction of the cAMP-CRP complex.

40. b

stop codon stops the synthesis of proteins thus it is called missense mutation.

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