Genes encode proteins, and proteins dictate cell function.
Therefore, the thousands of genes expressed in a particular cell determine what that cell can do.
In addition, each step in the flow of information from the DNA to the RNA to the protein provides the cell with a potential checkpoint to self-regulate its functions by adjusting the amount and type of proteins it makes.
At a given time, the amount of a particular protein in a cell reflects the balance between that protein’s synthetic and degradable biochemical pathways. On the artificial side of this balance, remember that protein production begins in transcription (DNA to RNA) and continues with translation (RNA to protein).
Thus, the control of these processes plays a critical role in determining which proteins are present in a cell and in what quantities.
In addition, how a cell processes its RNA transcripts and newly created proteins also greatly influences protein levels.
How is gene expression regulated?
Concept: the quantities and types of mRNA molecules in a cell reflect the function of that cell. Thousands of transcripts occur every second.
Given this statistic, it is not surprising that the primary control point for gene expression is usually at the beginning of the protein production process, the start of transcription.
RNA transcription makes an efficient checkpoint because many proteins can be made from a single mRNA molecule.
The transcription processing provides an additional level of regulation for eukaryotes, and the presence of a nucleus makes it possible.
Gene expression in prokaryotes, the translation of a transcript begins before the transcription is completed due to the proximity of the ribosomes to the new mRNA molecules.
Gene expression in eukaryotes, however, transcripts are modified in the nucleus before they are exported to the cytoplasm for translation.
The eukaryotic transcripts are also more complex than the prokaryotic transcripts. For example, the primary transcripts synthesized by the RNA polymerase contain sequences that will not be part of the mature RNA.
These intermediate sequences are called introns and are removed before the mature mRNA leaves the nucleus. The remaining regions of the transcript, which include the areas that encode proteins, are called exons and are spliced to produce the mature mRNA.
Eukaryotic transcripts are also modified at their extremes, which affects their stability and translation.
Of course, there are many cases in which cells must respond quickly to changing environmental conditions. In these situations, the regulatory control point may come well after transcription.
For example, early development in most animals depends on translation control because very little transcription occurs during the first cell division after fertilization.
Therefore, eggs contain many mRNA transcripts of maternal origin as a ready-for-translation reserve after fertilization.
On the degrading side of the equilibrium, cells can rapidly adjust their protein levels through the enzymatic degradation of RNA transcripts and existing protein molecules.
Both actions result in decreased amounts of specific proteins. Often, this breakdown is linked to particular events in the cell. The cycle of eukaryotic cells provides an excellent example of how the degradation of proteins is linked to cellular events.
This cycle is divided into several phases, each characterized by different cyclin proteins that act as critical regulators for that phase.
Before a cell can progress from one phase of the cell cycle to the next, it must degrade the cyclin that characterizes that particular phase of the process. If a cyclin is not impaired, the cycle continues.
How do the different cells express the genes they need?
Only a fraction of the genes in a cell is expressed. The variety of gene expression profiles characteristic of different types of cells arises because these cells have different sets of transcription regulators.
Some of these regulators work to increase transcription, while others prevent or suppress it.
Typically, transcription begins when an RNA polymerase binds to a so-called promoter sequence in the DNA molecule.
This sequence is almost always found just upstream of the starting point for transcription, although it can be located downstream of the mRNA.
In recent years, researchers have discovered that other DNA sequences, known as enhancer sequences, also play an essential role in transcription by providing binding sites for regulatory proteins that affect the activity of RNA polymerase.
The binding of regulatory proteins to an enhancer sequence causes a change in the chromatin structure that promotes or inhibits the binding of the RNA polymerase and the transcription factor. A more open chromatin structure is associated with the transcription of active genes. In contrast, a more compact chromatin structure is associated with transcriptional activity.
Some regulatory proteins affect the transcription of multiple genes. This occurs because there are numerous copies of regulatory protein binding sites within the genome of a cell.
Consequently, regulatory proteins may have different functions for different genes, and this is a mechanism by which cells can coordinate the regulation of many genes simultaneously.
How is gene expression increased or decreased in response to environmental change?
In prokaryotes, regulatory proteins are often controlled by the availability of nutrients. This allows bacteria to quickly adjust their transcription patterns in response to environmental conditions.
In addition, the regulatory sites in the prokaryotic DNA are generally located near the areas of the transcription promoter, which plays a vital role in gene expression.
Imagine a bacterium with an excess of amino acids that signals the “ignition” of some genes and the “shutdown” of others to see an example of how this works.
In this particular example, the cells may want to “activate” the genes of the proteins that metabolize the amino acids and deactivate the genes of the proteins that synthesize amino acids.
Some of these amino acids would bind to positive regulatory proteins called activators. Activating proteins bind to regulatory sites in the DNA near promoter regions that act as on / off switches.
This binding facilitates the activity of RNA polymerase and the transcription of nearby genes. At the same time, however, other amino acids would bind to harmful regulatory proteins called repressors, which bind to regulatory sites in DNA that effectively block RNA polymerase binding.
The control of gene expression in eukaryotes is more complex than in prokaryotes. In general, a more significant number of regulatory proteins is involved, and the regulatory binding sites may be located quite far from the areas of the transcription promoter.
In addition, the eukaryotic gene expression is generally regulated by a combination of several regulatory proteins that act together, allowing greater flexibility in the control of gene expression.
As mentioned above, the enhancer sequences are DNA sequences linked by an activator protein. They can be located in thousands of base pairs of a promoter, either upstream or downstream of a gene.
It is believed that the binding of the activating protein causes the DNA to exit, connecting the activator protein to physical proximity to the RNA polymerase and the other proteins in the complex that promote the initiation of transcription.
Different cell types express distinct sets of transcriptional regulators. As multicellular organisms develop, other groups of cells within these organisms activate and deactivate specific combinations of regulators.
Such development patterns are responsible for the variety of cell types present in the mature organism.
To live, cells must be able to respond to changes in their environment. The regulation of the two main steps of protein production (transcription and translation) is fundamental for this adaptability.
Cells can control which genes are transcribed and which transcripts are translated; they can process transcripts and proteins biochemically to affect their activity.
The regulation of transcription and translation occurs in both prokaryotes and eukaryotes but is much more complex in eukaryotes.