Of the approximately 21,000 protein-coding genes in human cells, only a fraction of these are expressed in any one cell type, or at any particular time in development, or at any one stage in the cell cycle. The ability for a gene to be transcribed depends on the presence of numerous proteins.
The workhorse protein in the transcription process is, of course, RNA polymerase—specifically, RNA polymerase II for protein-coding genes. RNA polymerase II creates a complementary RNA strand from the code in a DNA template strand. However, RNA polymerase II is unable to bind to DNA and begin transcription on its own. It requires other proteins, called transcription factors, to come into play. In the accompanying animation, we examine the events of transcription initiation in eukaryotes, highlighting the various transcription factors, regulatory proteins, and DNA regions that play important roles in the process.
In the accompanying animation, we examined the formation of a transcription initiation complex in eukaryotes. Protein-coding genes in eukaryotes require the formation of this complex in order for transcription to begin. In addition, many genes also require the binding of regulator and activator proteins. The presence of such proteins in a cell thus determines whether a gene is transcribed.
For example, in the immature red blood cells of bone marrow, which make a large amount of β-globin, the transcription of globin genes is stimulated by the binding of 7 regulators and 6 activators. But in white blood cells in the same bone marrow, these 13 proteins are not made, and therefore they cannot bind to their sites adjacent to the β-globin genes; consequently, these genes are hardly transcribed at all.
Textbook Reference: Concept 11.2 Eukaryotic Genes Are Regulated by Transcription Factors