Gene Expression: Overview of Control

Cells can regulate gene expression at every step along the way, from DNA to the final protein, as shown in Figure 1. Genetic information in DNA is first copied to form an RNA molecule, in a process known as transcription. The RNA used to make proteins is called messenger RNA (mRNA) because it carries information from the DNA to the ribosome, where protein synthesis occurs.

The mRNA serves as a template to guide protein synthesis. Scientists refer to protein synthesis as "translation" because ribosomes translate an mRNA sequence into a protein sequence. Prokaryotic cells use the mRNA directly as a template for protein synthesis.

Eukaryotic cells, however, must modify the precursor mRNA in several ways before it can be used to guide protein synthesis. The two ends are chemically altered, and sections of the RNA that do not encode protein sequences, called introns, are spliced out. Together, these modifications are called "mRNA processing." After processing, the mature mRNA moves from the nucleus into the cytoplasm, where it binds to the ribosome and serves as a template for synthesis of a protein.

The most important stage for the regulation of most genes is when transcription begins. This is because it costs the cell less energy to regulate transcription than to regulate the steps after transcription. The second point where regulation occurs is during RNA processing. Cells can regulate the rate of processing. In addition, the final mRNA product can be altered through alternative splicing, as shown in Figure 2. Alternative splicing can regulate the types of proteins produced from a single gene.

Cells can also regulate mRNA transport out of the nucleus. Once the mRNA has moved into the cytoplasm, the abundance of mRNA can be regulated by RNA degradation. Cells can regulate translation, controlling the number of proteins each mRNA produces. Finally, even after a cell has generated a protein, it can regulate the abundance and activity of that protein. For instance, cells regulate the activity of many proteins by post-translational modifications such as phosphorylation. Cells can also regulate the abundance of most proteins by degrading them.

Figure 2. Alternative mRNA splicing leads to isoforms, or related proteins.