All living cells within an organism contain the same genome, but why does each cell function differently from others? It is because the level of gene expression in different cells is controlled. Your skin cells are different from your hair cells, and your muscle cells are different from your cardiac cells because of the control of gene expression.
By controlling what genes are expressed, we can regulate the metabolic activities of different cells. This is because genes encode for proteins which, in turn, determine the function of a cell. As a result, cells become specialized to perform specific functions. This is why regulating gene expression is highly important because, without it, we wouldn't have specialized cells.
But first off, we need to have cells that have the ability to specialize into a large range of cells. These are called stem cells.
Stem cells
Stem cells are defined as unspecialized cells that divide indefinitely by mitosis and differentiate into many cell types. There are three classes:
Totipotent stem cells
Pluripotent stem cells
Multipotent stem cells
Totipotent stem cells are present during the zygote stage and have the ability to differentiate into all cell types, including extra-embryonic cells (like the placenta). As you make your way down the list above, the range of differentiation decreases. When these cells become specialized, they can no longer become any other type of cell.
The fact that specialized cells no longer divide continuously is a good thing! It means they can efficiently perform their metabolic activities.
Control of Gene Expression
The regulation of gene expression begins before transcription, the first stage of protein synthesis. Regulatory proteins called transcription factors dictate which genes are expressed (turned 'on') and which genes are not expressed (turned 'off'). The diagram below illustrates the binding of a transcription factor to DNA. This process is tightly controlled so that specific messenger RNA (mRNA) molecules are produced, and therefore only specific proteins are produced.
Fig. 1 - The binding of a transcription factor to DNA
Another type of gene expression regulation that occurs at the transcriptional level includes the alteration of the DNA-histone complex. This type is especially fascinating because the base sequence of DNA is not changed, and yet it still has the ability to direct which genes are expressed. This is called epigenetics.
Epigenetics
Epigenetics is the study of DNA and histone modifications to control gene expression. These modifications are heritable and are not caused by any changes to the base sequence of DNA. Modifications include DNA methylation and histone acetylation, illustrated in the diagram below. This has the effect of either condensing or loosening the DNA-histone complex. The pattern of modifications is called the epigenome.
Fig. 2 - DNA methylation and histone acetylation
Gene expression is tightly regulated, and for good reason. If the wrong genes are expressed or silenced, genetic diseases can arise. Cancer is a disease that is characterized by the uncontrollable proliferation of cells, and in some cases, the cause has an epigenetic origin.
Translational control of Gene Expression
Before translation occurs, additional changes to the mRNA molecule can occur. This can include mRNA splicing which is the removal of introns (non-coding DNA) from the molecule.
Even after translation, the polypeptide can be modified even further, such as the addition of chemical groups. A great example of this is the addition of a phosphate group to a polypeptide, catalysed by protein kinases. This addition can alter the folding of a protein and therefore change the protein function.
mRNA splicing occurs only in eukaryotic cells as their genome includes both introns and exons. Prokaryotic genomes contain only introns so mRNA splicing is unnecessary.
Genome projects and technology
The sequencing of a genome maps out of the complete set of genetic information contained within an organism. For example, the Human Genome Project (1990-2003) was a collaborative effort to determine all the genes in our cells. But this was no easy endeavour. The project used technologies like whole-genome shotgun (WGS) sequencing, and from this came other advances. These include DNA hybridization, which is used to locate specific alleles of a gene and genetic fingerprinting. These technological advances have wide applications in disease treatment and forensic science.
Control of Gene Expression - Key takeaways
- All living cells contain the same genome, but specialized cells express specific genes.
- Transcriptional control of gene expression includes transcription factors and epigenetic modifications.
- Epigenetics is the study of DNA and histone modifications to control gene expression. This does not include changes to the base sequence of DNA.
- Translational control of gene expression includes the addition of chemical groups to polypeptides, such as phosphate groups.
- DNA technologies, such as DNA hybridization, emerged from genetic studies and are now widely used in medical and forensic applications.
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