External signals may be chemical substances or in some cases energy in the form of light. Transcription factor proteins are affected directly or indirectly by these signals. The three dimensional shape of the protein or its chemical properties are changed by the presence of the signal. The Transcription factor may loose its ability to bind to the DNA of the Promoter region and TURN OFF the gene. The transcription factor may gain the ability to bind to the DNA of the promoter region and TURN ON the gene.
Hormone molecules pass through the cell membrane.
Depending on the gene involved the transcription factor gains or looses the ability to bind to the DNA of the promoter region of the gene. Hormone molecules can not pass through the cell membrane. Hormone molecules interact with receptor proteins embedded in the surface of the cell membrane. The receptor proteins are changed by the hormone and transmit a chemical signal inside the cell. The chemical signal interacts through a series of intermediate chemical steps with transition factors.