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.
The set of genes expressed in a cell determines the set of proteins and functional RNAs it contains, giving it its unique properties. Other chapters in Help Me Understand Genetics.
Genetics Home Reference has merged with MedlinePlus. Learn more. The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health. Can genes be turned on and off in cells? From Genetics Home Reference. The answers to such questions lie in the study of gene expression. Thus, this collection or articles begins by showing how a quiet, well-guarded string of DNA is expressed to make RNA , and how the messenger RNA is translated from nucleic acid coding to protein coding to form a protein.
Along the way, the article set also examines the nature of the genetic code , how the elements of code were predicted, and how the actual codons were determined. Next, we turn to the regulation of genes. Genes can't control an organism on their own; rather, they must interact with and respond to the organism's environment. Some genes are constitutive, or always "on," regardless of environmental conditions. Such genes are among the most important elements of a cell's genome, and they control the ability of DNA to replicate, express itself, and repair itself.
These genes also control protein synthesis and much of an organism's central metabolism. In contrast, regulated genes are needed only occasionally — but how do these genes get turned "on" and "off"? What specific molecules control when they are expressed?
It turns out that the regulation of such genes differs between prokaryotes and eukaryotes. For prokaryotes, most regulatory proteins are negative and therefore turn genes off. Here, the cells rely on protein—small molecule binding, in which a ligand or small molecule signals the state of the cell and whether gene expression is needed. The repressor or activator protein binds near its regulatory target: the gene.
Some regulatory proteins must have a ligand attached to them to be able to bind, whereas others are unable to bind when attached to a ligand. In prokaryotes, most regulatory proteins are specific to one gene, although there are a few proteins that act more widely. For instance, some repressors bind near the start of mRNA production for an entire operon, or cluster of coregulated genes.
Furthermore, some repressors have a fine-tuning system known as attenuation, which uses mRNA structure to stop both transcription and translation depending on the concentration of an operon's end-product enzymes. In eukaryotes, there is no exact equivalent of attenuation , because transcription occurs in the nucleus and translation occurs in the cytoplasm, making this sort of coordinated effect impossible.
Yet another layer of prokaryotic regulation affects the structure of RNA polymerase , which turns on large groups of genes. Here, the sigma factor of RNA polymerase changes several times to produce heat- and desiccation-resistant spores.
Here, the articles on prokaryotic regulation delve into each of these topics, leading to primary literature in many cases. For eukaryotes, cell-cell differences are determined by expression of different sets of genes. For instance, an undifferentiated fertilized egg looks and acts quite different from a skin cell, a neuron, or a muscle cell because of differences in the genes each cell expresses. A cancer cell acts different from a normal cell for the same reason: It expresses different genes.
Using microarray analysis , scientists can use such differences to assist in diagnosis and selection of appropriate cancer treatment. Interestingly, in eukaryotes, the default state of gene expression is "off" rather than "on," as in prokaryotes. Why is this the case? The secret lies in chromatin, or the complex of DNA and histone proteins found within the cellular nucleus. The histones are among the most evolutionarily conserved proteins known; they are vital for the well-being of eukaryotes and brook little change.
When a specific gene is tightly bound with histone, that gene is "off. This is where the histone code comes into play.
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