GRE Subject Test: Biology : Understanding Operons and Promoters

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

Example Question #1 : Understanding Operons And Promoters

Which of the following accurately describes the promoter?

Possible Answers:

The attachment point for a ribosome before translation

The binding site for DNA polymerase on DNA

A sequence of DNA used to signal the beginning point of transcription

The protein that attaches to DNA in order to create mRNA

Correct answer:

A sequence of DNA used to signal the beginning point of transcription

Explanation:

The promoter is a specific segment of DNA that signals the starting point of transcription. RNA polymerase attaches to the promoter and proceeds to create the mRNA primary transcript.

DNA polymerase binds to the RNA primer to begin DNA replication. Ribosomes bind to the 5' cap on eukaryotic mRNA.

Example Question #2 : Understanding Operons And Promoters

Which conditions would result in the largest levels of lac operon transcription?

Possible Answers:

High lactose and low glucose

Low lactose and low glucose

Low lactose and high glucose

High lactose and high glucose

Correct answer:

High lactose and low glucose

Explanation:

The important thing to remember about the lac operon is that it is transcribed when glucose is absent from the cell, but lactose is present and can be utilized. As a result, the operon's transcription would be high if there are both high levels of lactose available, and very little amounts of glucose.

Example Question #3 : Understanding Operons And Promoters

The lac operon is typically found in prokaryotes in order to utilize lactose in the event that glucose is absent. How does the presence of lactose affect the lac operon?

Possible Answers:

It attaches to the operator, blocking polymerase from attaching

It binds to the promoter, signaling the polymerase to attach

It binds to the lac repressor, causing it to detach from the operator

It stimulates the transcription of the lac repressor gene

Correct answer:

It binds to the lac repressor, causing it to detach from the operator

Explanation:

The lac operon is set up in a way so that the lac repressor is able to be transcribed, regardless of glucose and lactose levels. The lac repressor will then attach to the operator, which inhibits transcription. If lactose is present, it will bind to the lac repressor, and make it detach from the operator.

This process allows the operon to be transcribed in the event that glucose is absent. If glucose is absent, but lactose is not present, then the repressor will remain in place and transcription will not take place.

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