GRE Subject Test: Biology › Understanding Operons and Promoters
Which of the following accurately describes the promoter?
A sequence of DNA used to signal the beginning point of transcription
The attachment point for a ribosome before translation
The binding site for DNA polymerase on DNA
The protein that attaches to DNA in order to create mRNA
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.
Which conditions would result in the largest levels of lac operon transcription?
High lactose and low glucose
High lactose and high glucose
Low lactose and high glucose
Low lactose and low glucose
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.
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?
It binds to the lac repressor, causing it to detach from the operator
It binds to the promoter, signaling the polymerase to attach
It attaches to the operator, blocking polymerase from attaching
It stimulates the transcription of the lac repressor gene
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.