Introduction to gene expression
Gene expression is the process by which the information encoded in the genotype is used to create a functional gene product (i.e. a protein or RNA) from genes encoded within DNA.
Control of gene expression , or gene regulation is essential for all living organisms. Even single-celled bacteria need to control when and where their genes are expressed.
In multicellular animals and plants, gene expression is even more complicated.
Plants and animals need to be able to control gene expression in a single cell. They also need to be able to control gene expression across a whole host of cell types.
Gene regulation allows the cell to turn on or off (express) genes at different times and in different cell types.
Thanks to the expression of different genes, a neuronal cell and an adipose cell (fat cell) carry exactly the same genome, but can have vastly different forms and functions. The two different cells express different genes, which allow each cell to specialise.
Gene regulation also allows organisms to respond to changes in conditions. E. coli is able to regulate the production of enzymes that break down lactose depending on its environment.
Gene regulation also allows the cell to prevent the waste of resources on producing unwanted proteins.
Gene expression can be controlled at every level of protein synthesis, including transcription, post-transcriptional modification and translation.
Gene amplification or duplication refers to circumstances in which there are several copies of the same gene situated on one chromosome or spread across several.
The eukaryotic genome contains long repetitive sections that code for gene products that are required in large quantities.
These genes are expressed when large amounts of the gene product are needed, such as the production of histones after DNA replication and the production of rRNA during early cell development.
Gene amplification is often the result of mutations. Most duplicate genes serve no functional purpose and are lost.
There are two well-known cases where gene amplification is essential:
Sometimes cells need to produce large amounts of proteins, such as after cell division, when they need to grow. This means the cell needs lots of ribosomes.
The genome contains many copies of the genes coding for rRNA. These genes are expressed to ensure that enough ribosomes are present to produce all of the proteins that the cell needs.
HistonesWhenever a cell divides a large number of histones need to be produced in order to package the newly replicated DNA.
In these cases, simply increasing the rate of transcription is not enough. Instead, many copies of identical genes are present on the genome and all these copies are transcribed simultaneously. This produces vast amounts of the gene product.
The control of eukaryotic gene expression is more complicated than the control of prokaryotic gene expression.
There are several reasons for this:
- The eukaryotic genome is larger than the prokaryotic genome and therefore involves the transcription of a larger number of genes.
- The eukaryotic genome contains long stretches of non-coding DNA between genes, meaning that the genes are more spread out. This means each gene must be expressed individually.
- The eukaryotic genome contains non-coding DNA within genes, which needs to be removes by a process called splicing before the genes are expressed.
- Eukaryotes are often multicellular, so different genes need to be expressed in different cell types.
Cell compartmentalisation: gene expression takes place in different parts of the cell. Transcription takes place in the nucleus, while translation takes place in the cytoplasm.
Control mechanisms are required to aid the transition between locations.
The regulation of gene expression can still occur after mRNA has left the nucleus, and once a polypeptide has been synthesised.
The product of a gene usually needs modification before it is functional.
Post-transcriptional modification: The transcribed mRNA sequence can be altered to influence the rate of translation and to alter the function of the protein produced.
Sections of mRNA can be removed to change the proteins that will be produced.
Post-transcriptional modification is a more common form of gene regulation in eukaryotes than in prokaryotes.
Translation: Proteins and some RNA molecules can bind to specific mRNA sequences during translation. This stops translation by preventing tRNA and ribosomes from binding to the mRNA.
Post-translational modification: once a polypeptide has been transcribed, the protein can be altered or activated by changing its structure.
Similarly, the rate of protein degradation can be controlled. This enables the cell to respond to changes in its environment, and to only use the most functional new proteins.