Procaryotic cells have two mechanisms that control transcription of genes – promoter and operator. Promoter is a spot on the DNA where RNA polymerase attaches to begin transcription. An operator is located distally from promoter, but before a certain gene or a group of genes (operon). A lac operon is a group of three genes that are always transcribed together which code proteins needed for the metabolism of lactose. When lactose is absent from the outer environment, a repressor protein is bound to the operator and blocks the transcription. However, if lactose is present, it binds on a specific spot on the repressor protein, which changes its structure, so it falls off the operon site allowing RNA polymerase to transcribe lac operon.

Eukaryotic cells have three mechanisms that control transcription of genes – transcription factors, cell specialization, and RNA interference. Transcription factors are able to bind on the spot of DNA molecule right before gene starts and attract RNA polymerase. They can also help in unwrapping chromatin so that certain genes would be exposed. These two mechanisms promote the transcription process. However, transcription factors may also block particular genes.
Cell specialization means that certain genes are expressed in some types of cells and not the others.
RNA interference is a process in which a miRNA bind to its complementary region on the mRNA and damages it, which stops the transcription.

Master control genes are the ones included in the development and differentiation of cells and tissues. Because of extraordinary similarity among these genes in various organisms we can conclude about their shared past.

An operator is located distally from promoter, but before a certain gene or a group of genes (operon). A lac operon is a group of three genes that are always transcribed together which code proteins needed for the metabolism of lactose. When lactose is absent from the outer environment, a repressor protein is bound to the operator and blocks the transcription. However, if lactose is present, it binds on a specific spot on the repressor protein, which changes its structure, so it falls off the operon site allowing RNA polymerase to transcribe lac operon. This way of transcription control preserves the energy of E. coli, so proteins that these bacteria need in presence of lactose aren’t synthesized when this molecule is absent.

Homeotic genes are included in the development and differentiation of cells and tissues. A subdivision of homeotic genes is known as Hox genes. They define a body plan for a developing embryo. In a fruit fly, different hox genes are expressed in the specific areas of their body. For example, genes that induce legs formation are expressed in the thorax region of the fruit fly. The fruit fly is an insect whose genome contains about 14 000 genes. Therefore, each gene doesn’t match to the specific body part, but each Hox gene is expressed in the certain segment of the body.