Gene Expression: Prokaryotes and Eukaryotes

Controlling Expression

Gene expression is the process through which genetic information is used to produce proteins.  Expression of a particular gene is a two-step process that involves the production of a messenger RNA (mRNA) through transcription.  The mRNA that is created undergoes the next step in the process called translation to produce a functional protein.  For translation to occur, mRNA must leave the nucleus to enter the cytoplasm where it is surrounded by ribosomes.  It is here that ribonucleotides are read in groups of three, with each group referred to as a codon.  The translation of one codon results in the formation of one amino acid.  Translation of the mRNA produces a chain of precisely arranged amino acids that are folded in complex patterns to produce active proteins.

Expression of a single gene (mRNA) or a set of particular genes may equip the individual with better survival capabilities.  However, insufficient or over expression may lead to disease.  Multiple approaches are utilized to regulate gene expression.  Gene silencers, transcription enhancing factors, and varying rates of mRNA degradation are only a few of these therapeutic strategies.  Other approaches include exposure to hormones and controlling the timing of gene expression effects regulates protein synthesis and function of abnormal pathways.

Gene Expression in Prokaryotes and Eukaryotes

At transcription, the first product is termed a precursor mRNA (pre-mRNA), also called the primary transcript.  The pre-mRNA contains coding exons as well as non-coding introns.  Only exons are capable of producing proteins.  The pre-mRNA is prepared for protein synthesis by removing all the introns through a process called splicing.  The resulting mature mRNA is a strand of nucleic acids that resembles the pre-mRNA without the introns.

Scientists have observed that prokaryotic cells exhibit constantly switched-on genes.  These genes need to be explicitly switched off so the organism can respond to its environment.  However, in eukaryotic cells, gene expression needs to be switched on.  A single trigger point may activate multiple genes involved in the same pathway.  Gene regulation in eukaryotes helps cell differentiation and specialization which in turn eventually equips the organism with the ability to adapt to external conditions.

Eukaryotic organisms can contain approximately 30,000 genes, which includes ~1000 microRNA genes (part of the RNAi mechanism).  Protein expression is required for essential cellular functions in all cell types.  However, some genes are expressed at a higher level in some cells while the responsibility of other genes may play a more active role in different cell types.  For instance, the type and expression level of a group of proteins translated in brain cells may not be produced by pancreatic cells, although both types of cells contain the same genomic information in the nucleus.  This phenomenon, referred to as differential gene expression, occurs due to transcription and translation regulating factors that switch the genes on and off.

There is a vast array of research methods and experimental techniques used to measure gene expression as an end-point analysis in in vitro and in vivo systems.  Transfection, quantitative polymerase chain reaction (qPCR), serial analysis of gene expression (SAGE), and DNA microarray procedures are commonly used for determining the extent of RNA transcription and changes in expression levels.  Western blotting is typically used to quantify the amount of protein levels.