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A. Transcriptional Regulation

A diagram of a circle containing a double helix representing the L-C-T gene. The helix is shown being transcribed.
A diagram of a circle with a double-stranded red helix with irregularly spaced green squares superimposed on it. The helix is labeled L-C-T gene. A downward pointing arrow labeled Transcription appears below the helix. The arrow is marked with the letter A. It points to a single strand of the helix in red with the same sequence of green squares superimposed on it.

Not transcribing a gene into RNA in the first place is the most complete and efficient way to turn off gene expression. It is especially suitable if a gene needs to be turned off for a long time.

Preventing gene transcription is one of the main ways of regulating gene expression. But if a cell needs to change its gene expression rapidly, it may not be efficient to shut down expression at the transcriptional level.