Question

Only the proteins that the bacteria Escherichia coli (E. coli) needs are synthesized at any given time. For example, when lactose is available in the environment, E. coli will express the lac genes that allow it to break down lactose to obtain energy. When lactose is not present, these genes are not expressed.

A microscopic image of E. coli.

Which prevents the lac genes in the DNA of E. coli from being expressed most of the time?
the lac repressor
the lac operator
the lac promoter
the lac locator

Answer 1

the lac repressor

Explanation: In the absence of lactose the lac repressor protein binds the operator sequence and blocks RNA polymerase, preventing transcription of the lac genes. When lactose (or allolactose) is present it binds the repressor and releases it, allowing expression.