Bacterial genes are groups into operons. Operons consist of a promoter, an operator and the genes. The promoter is where the RNA polymerase attaches, the operator allows the RNA polymerase to access or not access the genes. Also involved is a repressor protein that can bind to the operator. When the repressor is bound the operon is considered "off" because the RNA polymerase can not reach the genes.
There are two major types of operons: repressible and inducible. A repressible operons job is to make molecules and they are typically "on." The operon may make a molecule (corepressor) that can bind to the repressor protein and turn it off. An inducible operons job is to break down molecules. They are typically "off" until the molecules they need to break down is present. An inducer may bind to the repressor protein and turn it on.
There are multiple types of gene regulation that occur in eukaryotic cells. One example is DNA methylation. This is when a methyl group attaches the the bases on DNA which impedes the attachment of transcription factors. Histone methylation attaches a methyl group to the histone protein which causes it to pack tighter and prevent transcription. Histone acetylation is when an acetyl group is attached to the histone molecules so that the histone becomes less tightly packed and transcription can occur.
The transcription complex can also control if transcription takes place. General transcription factors are attached to the promotor by RNA polymerase. This needs to be attached before transcription can occur. Specific transcription factors are attached in specific cells, genes or at specific times. They attach away from the gene on enhancers. The DNA will then fold so the DNA portion with the enhancer can become part of the general transcription factors.
RNA interference can occur when RNA molecules (micro RNA or small interfering RNA) bind to the mRNA in the cytoplasm and either degrade it or stop translation.
Source: Holtzclaw, Fred, and Theresa Holtzclaw. AP Test Prep Series. San Francisco: Pearson Education Inc., 2013. Print. PACK, P. (2013). AP BIOLOGY. (4TH ED., PP. 73-81). BOSTON: HOUGHTON MIFFLIN HARCOURT.