mRNA, and eventually proteins, are produced when and where they are needed in an organism.
Some genes, such as those for housekeeping proteins in processes like glycolosis,
are expressed all of the time.
These genes undergo what is known as constitutive transcription.
Other proteins are only produced at specific times in certain cells.
In these cells, genes undergo a process
called regulated transcription.
Transcription is regulated by proteins called transcription factors.
These factors are proteins that are produced in the cytoplasm
and eventually migrate into the nucleus
where they interact with DNA and activate transcription.
These transcription factors only interact with specific genes,
those genes whose transcription they control.
Most eukaryotic species have over 1000 transcription factors.
Regulated transcription begins when a signal is received by the cell.
This signal, often a protein,
begins a signal transduction cascade
that lets the cell know certain proteins are now needed.
The most common activation event for a protein in a signal pathway
is the addition of a phosphate group.
That activated protein in turn interacts with another protein.
This interaction leads to the phosphorylation of the next protein in the pathway.
There is often a series of proteins that are activated by phosphorylation.
At the end of this cascade,
the final protein will be activated and then enter the nucleus.
It must make this journey because transcription only occurs inside the nucleus.
After entering through a nuclear pore,
the protein interacts with the specific transcription factor responsible for activating
mRNA production.
As with the earlier proteins,
the transcription factor is modified through phosphorylation.
The activated transcription factor will next bind to an enhancer region.
The enhancer is a region of DNA
upstream of the transcription start site
that binds a transcription factor.
The transcription factor then binds to the DNA
and moves to interact with the rest of the transcription protein complex,
located at the transcription start site.
When the formation of this complex is complete,
transcription of the gene will begin.
It is this sequence of events
that ensures genes required for a specific tissue
or only at a specific time
are expressed appropriately.