Why regulate gene expression

Therefore, in prokaryotic cells, the control of gene expression is mostly at the transcriptional level. Eukaryotic cells, in contrast, have intracellular organelles that add to their complexity. The newly synthesized RNA is then transported out of the nucleus into the cytoplasm, where ribosomes translate the RNA into protein.

The processes of transcription and translation are physically separated by the nuclear membrane; transcription occurs only within the nucleus, and translation occurs only outside the nucleus in the cytoplasm. The regulation of gene expression can occur at all stages of the process Figure 1. Regulation may occur when the DNA is uncoiled and loosened from nucleosomes to bind transcription factors epigenetic level , when the RNA is transcribed transcriptional level , when the RNA is processed and exported to the cytoplasm after it is transcribed post-transcriptional level , when the RNA is translated into protein translational level , or after the protein has been made post-translational level.

Figure 1. Prokaryotic transcription and translation occur simultaneously in the cytoplasm, and regulation occurs at the transcriptional level. Eukaryotic gene expression is regulated during transcription and RNA processing, which take place in the nucleus, and during protein translation, which takes place in the cytoplasm. Further regulation may occur through post-translational modifications of proteins.

The differences in the regulation of gene expression between prokaryotes and eukaryotes are summarized in Table 1. The regulation of gene expression is discussed in detail in subsequent modules.

Prokaryotic cells can only regulate gene expression by controlling the amount of transcription. As eukaryotic cells evolved, the complexity of the control of gene expression increased. For example, with the evolution of eukaryotic cells came compartmentalization of important cellular components and cellular processes. A nuclear region that contains the DNA was formed. Transcription and translation were physically separated into two different cellular compartments.

It therefore became possible to control gene expression by regulating transcription in the nucleus, and also by controlling the RNA levels and protein translation present outside the nucleus.

Some cellular processes arose from the need of the organism to defend itself. Cellular processes such as gene silencing developed to protect the cell from viral or parasitic infections. If the cell could quickly shut off gene expression for a short period of time, it would be able to survive an infection when other organisms could not. Therefore, the organism evolved a new process that helped it survive, and it was able to pass this new development to offspring.

Answer the question s below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times. Use this quiz to check your understanding and decide whether to 1 study the previous section further or 2 move on to the next section. Skip to main content. Module Gene Expression. Search for:. Regulation of Gene Expression Define the term regulation as it applies to genes For a cell to function properly, necessary proteins must be synthesized at the proper time.

Learning Objectives Discuss why every cell does not express all of its genes Compare prokaryotic and eukaryotic gene regulation. Figure 2. Growth factor prompting cell division. Different cells in a multicellular organism may express very different sets of genes, even though they contain the same DNA. Enhancers are binding sites for activators. When an enhancer is far away from a gene, the DNA folds such that the enhancer is brought into proximity with the promoter, allowing interaction between the activators and the transcription initiation complex Figure Like prokaryotic cells, eukaryotic cells also have mechanisms to prevent transcription.

Transcriptional repressors can bind to promoter or enhancer regions and block transcription. Both activators and repressors respond to external stimuli to determine which genes need to be expressed. Post-transcriptional regulation occurs after the mRNA is transcribed but before translation begins. This regulation can occur at the level of mRNA processing, transport from the nucleus to the cytoplasm, or binding to ribosomes. Recall from chapter 5 that in eukaryotic cells the RNA primary transcript often contains introns, which are removed prior to translation.

Alternative RNA splicing is a mechanism that allows different combinations of introns, and sometimes exons, to be removed from the primary transcript Figure This allows different protein products to be produced from one gene. Alternative splicing can act as a mechanism of gene regulation. Differential splicing is used to produce different protein products in different cells or at different times within the same cell.

Alternative splicing is now understood to be a common mechanism of gene regulation in eukaryotes; up to 70 percent of genes in humans are expressed as multiple proteins through alternative splicing.

How could alternative splicing evolve? Introns have a beginning and ending recognition sequence; it is easy to imagine the failure of the splicing mechanism to identify the end of an intron and instead find the end of the next intron, thus removing two introns and the intervening exon.

In fact, there are mechanisms in place to prevent such intron skipping, but mutations are likely to lead to their failure. Indeed, the cause of many genetic diseases is alternative splicing rather than mutations in a sequence. However, alternative splicing would create a protein variant without the loss of the original protein, opening up possibilities for adaptation of the new variant to new functions.

Gene duplication has played an important role in the evolution of new functions in a similar way by providing genes that may evolve without eliminating the original, functional protein. The longer an mRNA exists in the cytoplasm, the more time it has to be translated, and the more protein is made. Many factors contribute to mRNA stability, including the length of its poly-A tail. After an mRNA has been transported to the cytoplasm, it is translated into proteins.

Control of this process is largely dependent on the mRNA molecule. As previously discussed, the stability of the mRNA will have a large impact on its translation into a protein. Translation can also be regulated at the level of binding of the mRNA to the ribosome. Once the mRNA bound to the ribosome, the speed and level of translation can still be controlled. An example of translational control occurs in proteins that are destined to end up in an organelle called the endoplasmic reticulum ER.

The first few amino acids of these proteins are a tag called a signal sequence. As soon as these amino acids are translated, a signal recognition particle SRP binds to the signal sequence and stops translation while the mRNA-ribosome complex is shuttled to the ER. Once they arrive, the SRP is removed and translation resumes.

The final level of control of gene expression in eukaryotes is post-translational regulation. This type of control involves modifying the protein after it is made, in such as way as to affect its activity. One example of post-translational regulation is enzyme inhibition. When an enzyme is no longer needed, it is inhibited by a competitive or allosteric inhibitor, which prevents it from binding to its substrate. The inhibition is reversible, so that the enzyme can be reactivated later.

This is more efficient than degrading the enzyme when it is not needed and then making more when it is needed again. Sometimes these modifications can regulate where a protein is found in the cell—for example, in the nucleus, the cytoplasm, or attached to the plasma membrane.

The addition of an ubiquitin group to a protein marks that protein for degradation. Tagged proteins are moved to a proteasome , an organelle that degrades proteins Figure One way to control gene expression, therefore, is to alter the longevity of the protein. Skip to content Figure The control of which genes are expressed dictates whether a cell is a an eye cell or b a liver cell.

It is the differential gene expression patterns that arise in different cells that give rise to c a complete organism. By the end of this section, you will be able to: Discuss why every cell does not express all of its genes. Describe some major differences between prokaryotic and eukaryotic gene regulation.

By the end of this section, you will be able to: Describe the steps involved in prokaryotic gene regulation. Explain the roles of activators, inducers, and repressors in gene regulation. Explain the process of transcriptional gene regulation in eukaryotic cells.

Explain the process of post-transcriptional gene regulation in eukaryotic cells. Explain the process of translational gene regulation in eukaryotic cells. Alternative RNA splicing Recall from chapter 5 that in eukaryotic cells the RNA primary transcript often contains introns, which are removed prior to translation. Figure Pre-mRNA can be alternatively spliced to create different proteins. Evolution of Alternative Splicing How could alternative splicing evolve?

Previous: Chapter The Central Dogma: Genes to Traits. Next: Chapter Mendelian Genetics. Share This Book Share on Twitter. Eukaryotic organisms. Lack nucleus.

Contain nucleus. DNA is found in the cytoplasm. DNA is in the nucleus. DNA, Genetics, and Evolution. Contents All Modules. Control of Gene Expression By gene expression we mean the transcription of a gene into mRNA and its subsequent translation into protein. The structural genes contain the code for the proteins products that are to be produced. Regulation of protein production is largely achieved by modulating access of RNA polymerase to the structural gene being transcribed.

The promoter gene doesn't encode anything; it is simply a DNA sequence that is initial binding site for RNA polymerase. The operator gene is also non-coding; it is just a DNA sequence that is the binding site for the repressor. The regulator gene codes for synthesis of a repressor molecule that binds to the operator and blocks RNA polymerase from transcribing the structural genes. There is also a regulator gene, which codes for the synthesis of a repressor molecule hat binds to the operator Example of Inducible Transcription: The bacterium E.

However, the enzymes are usually present in very low concentrations, because their transcription is inhibited by a repressor protein produced by a regulator gene see the top portion of the figure below. The repressor protein binds to the operator site and inhibits transcription.

However, if lactose is present in the environment, it can bind to the repressor protein and inactivate it, effectively removing the blockade and enabling transcription of the messenger RNA needed for synthesis of these genes lower portion of the figure below. Example of Repressible Transcription: E.