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For a cell to function properly, the necessary proteins must be synthesized at the right time. All organisms and cells control or regulate the transcription and translation of their DNA into proteins. The process of activating a gene to produce RNA and protein is called gene expression. Whether in a simple unicellular organism or a complex multicellular organism, each cell controls when and how its genes are expressed. For that to happen, there must be a mechanism to control when a gene is expressed to make RNA and protein, how much of the protein is made, and when it's time to stop making that protein because it's no longer needed.
Cells in multicellular organisms are specialized; cells in different tissues look very different and perform different functions. For example, a muscle cell is very different from a liver cell, which is very different from a skin cell. These differences are a consequence of the expression of different sets of genes in each of these cells. All cells have certain basic functions that they must perform for themselves, such as converting energy from sugar molecules into energy from ATP. Each cell also has many genes that are not expressed, and it expresses many that are not expressed by other cells, so that it can carry out its specialized functions. In addition, cells turn certain genes on or off at different times in response to changes in the environment or at different times during the organism's development. Single-celled organisms, both eukaryotic and prokaryotic, also turn genes on and off in response to the demands of their environment to respond to specific conditions.
The control of gene expression is extremely complex. Errors in this process are harmful to the cell and can lead to the development of many diseases, including cancer.
Prokaryotic versus eukaryotic gene expression
To understand how gene expression is regulated, we must first understand how a gene becomes a functional protein in a cell. The process occurs in prokaryotic and eukaryotic cells, just in slightly different ways.
Because prokaryotic organisms lack a cell nucleus, transcription and translation processes occur almost simultaneously. When the protein is no longer needed, transcription stops. As a result, the main method of controlling what type and how much protein is expressed in a prokaryotic cell is by regulating the transcription of DNA into RNA. All subsequent steps are done automatically. When more protein is needed, more transcription takes place. Therefore, the control of gene expression in prokaryotic cells is almost exclusively at the level of transcription.
The first example of such a control was discovered usingE.coliin the 1950s and 1960s by French researchers and is calledtiredoperon. Otiredoperon is a stretch of DNA with three adjacent genes that encode proteins that participate in the absorption and metabolism of lactose, a food source forE.coli. When lactose is not present in the bacterial environment, thetiredgenes are transcribed in small amounts. When lactose is present, the genes are transcribed and the bacteria are able to use lactose as a food source. The operon also contains a promoter sequence to which RNA polymerase binds to initiate transcription; between the promoter and the three genes there is a region called the operator. When lactose is not present, a protein known as the repressor binds to the operator and prevents RNA polymerase from binding to the promoter, except in rare cases. Thus, very little of the protein products of the three genes is produced. When lactose is present, an end product of lactose metabolism binds to the repressor protein and prevents it from binding to the operator. This allows RNA polymerase to bind to the promoter and freely transcribe the three genes, allowing the organism to metabolize lactose.
Eukaryotic cells, on the other hand, have intracellular organelles and are much more complex. Remember that in eukaryotic cells the DNA is contained in the nucleus of the cell and is transcribed into mRNA there. The newly synthesized mRNA is then transported out of the nucleus into the cytoplasm, where ribosomes translate the mRNA into protein. The transcription and translation processes are physically separated by the nuclear membrane; transcription takes place only inside the nucleus and translation takes place only outside the nucleus in the cytoplasm. Regulation of gene expression can occur at all stages of the process (Figure \(\PageIndex{1}\)). Regulation can occur when DNA is unwound and separated from nucleosomes to bind transcription factors (epigenetic level), when RNA is transcribed (transcriptional level), when RNA is processed and exported to the cytoplasm after transcription (post-transcriptional level), transcriptional), when RNA is translated into protein (translational level) or after protein production (post-translational level).
Differences in the regulation of gene expression between prokaryotes and eukaryotes are summarized in Table \(\PageIndex{1}\).
| prokaryotic organisms | eukaryotic organisms |
|---|---|
| core missing | contains core |
| RNA transcription and protein translation occur almost simultaneously |
|
| Gene expression is regulated primarily at the transcriptional level | Gene expression is regulated at multiple levels (epigenetic, transcriptional, post-transcriptional, translational and post-translational) |
EVOLUTION IN ACTION: Alternative RNA Splicing
In the 1970s, genes that exhibited alternative RNA splicing were first observed. Alternative RNA splicing is a mechanism that allows different protein products to be produced from a gene when different combinations of introns (and sometimes exons) are removed from the transcript (Figure \(\PageIndex{2}\)). This alternative splicing can be random, but most of the time it is controlled and acts as a gene regulation mechanism, where the frequency of different splicing alternatives is controlled by the cell as a way to control the production of different protein products in different cells, or at different stages of development. Alternative splicing is now understood as a common mechanism of gene regulation in eukaryotes; by one estimate, 70% of genes in humans are expressed as multiple proteins through alternative splicing.
How can alternative splicing evolve? Introns have an initial and final recognition sequence, and it is easy to imagine that the splicing mechanism could not identify the end of one intron and find the end of the next intron, thereby removing two introns and the intervening exon. Indeed, there are mechanisms to prevent such exon skipping, but it is likely that mutations lead to their failure. Such "mistakes" would likely produce a non-functional protein. In fact, the cause of many genetic diseases is alternative splicing rather than mutations in a sequence. Alternative splicing, however, would create a protein variant without losing the original protein, opening possibilities for adapting the new variant to new functions. Gene duplication has played an important role in the evolution of new functions in a similar way - providing genes that can evolve without eliminating the original functional protein.
Resume
Although all somatic cells in an organism contain the same DNA, not all cells in that organism express the same proteins. Prokaryotic organisms express all of the DNA they encode in each cell, but not necessarily all at the same time. Proteins are only expressed when they are needed. Eukaryotic organisms express a subset of the DNA encoded in a given cell. In each cell type, the type and amount of protein is regulated by the control of gene expression. To express a protein, DNA is first transcribed into RNA, which is then translated into proteins. In prokaryotic cells, these processes occur almost simultaneously. In eukaryotic cells, transcription takes place in the nucleus and is separate from translation, which takes place in the cytoplasm. Gene expression in prokaryotes is regulated only at the transcriptional level, whereas gene expression in eukaryotic cells is regulated at the epigenetic, transcriptional, post-transcriptional, translational and post-translational levels.
Dictionary
- alternative RNA splicing
- a mechanism of post-transcriptional gene regulation in eukaryotes in which multiple protein products are produced from a single gene through alternative splicing combinations of the RNA transcript
- epigenetic
- describes non-genetic regulatory factors, such as changes in histone and DNA protein modifications, which control the accessibility of genes on chromosomes
- genetic expression
- processes that control whether a gene is expressed
- post-transcriptional
- control of gene expression after the RNA molecule has been created, but before it has been translated into protein
- post-translation
- control of gene expression after creation of a protein
Contributors and Attributions
Samantha Fowler (Clayton State University), Rebecca Roush (Sandhills Community College), James Wise (Hampton University). Conteúdo original da OpenStax (CC BY 4.0; acesso gratuito emhttps://cnx.org/contents/b3c1e1d2-83...4-e119a8aafbdd).
FAQs
How are genes regulated? ›
Gene regulation can occur at any point during gene expression, but most commonly occurs at the level of transcription (when the information in a gene's DNA is passed to mRNA). Signals from the environment or from other cells activate proteins called transcription factors.
What is gene regulation answers? ›Gene regulation is the process of controlling which genes in a cell's DNA are expressed (used to make a functional product such as a protein). Different cells in a multicellular organism may express very different sets of genes, even though they contain the same DNA.
What is a gene regulation quizlet? ›Gene Regulation. Refers to the ability of cells to control the expression of their genes. Cell Differentation. The process by which cells become specialized into particular types.
What are the 5 levels of regulation in eukaryotic gene expression? ›What are the five levels of genetic regulatory control in eukaryotes? Gene expression is controlled at the epigenetic, transcriptional, post-transcriptional, translational, and post-translational levels in eukaryotic cells.
How is the expression of genes regulated or controlled quizlet? ›Gene expression can be regulated at the level of RNA processing through alternative splicing, RNA editing, or transport of mRNA out of the nucleus.
Why does gene regulation matter? ›Gene regulation is essential because it ensures that cells express only the genes necessary to grow, develop, and properly function. Cells regulate their genes based on both internal and external factors, such as nutrient levels or DNA damage.
What are regulatory genes in biology? ›regulatory gene. noun. variants or regulator gene. : a gene that regulates the expression of one or more structural genes by controlling the production of a protein (as a genetic repressor) which regulates their rate of transcription.
What are the three types of gene regulation? ›All three domains of life use positive regulation (turning on gene expression), negative regulation (turning off gene expression), and co-regulation (turning multiple genes on or off together) to control gene expression, but there are some differences in the specifics of how these jobs are carried out between ...
What is a regulation quizlet? ›Regulations. A law, rule or other order prescribed by authority, especially to regulate conduct. Legislators.
What is genome regulation? ›Genome regulation encompasses all facets of gene expression, from the biochemical modifications of DNA, to the physical arrangement of chromosomes and the activity of the transcription machinery. The genome regulation programs that cells engage control which proteins are produced, and to what level.
What is gene regulation in cell cycle? ›
Cell cycle-dependent gene transcription is tightly controlled by the retinoblastoma (RB):E2F and DREAM complexes, which repress all cell cycle genes during quiescence. Cyclin-dependent kinase (CDK) phosphorylation of RB and DREAM allows for the expression of two gene sets.
What are the levels of regulation? ›There are three primary levels of regulation: registration, statutory certification, and licensure.
What is a regulatory element in DNA? ›Regulatory elements are found at transcriptional and post-transcriptional levels and further enable molecular networks at those levels. For example, at the post-transcriptional level, the biochemical signals controlling mRNA stability, translation and subcellular localization are processed by regulatory elements.
What are 4 different ways that gene expression can be regulated? ›Adding further complexity is that the control of gene expression can occur at multiple steps: accessibility of a gene to activating transcription factors, transcription initiation, transcript elongation, splicing of the pre-mRNA, as well as post-transcriptional regulation.
How is gene expression regulated simple? ›Regulation of Gene Expression
Gene expression is the process by which the instructions present in our DNA are converted into a functional product, such as a protein. This process is a tightly coordinated process which allows a cell to respond to its changing environment.
Gene expression in eukaryotic cells is regulated by repressors as well as by transcriptional activators.
What genes control gene expression? ›Epigenetics controls gene expression without changing the DNA base sequence. Epigenetic modifications concern nucleosome positioning, histone posttranslational modifications, DNA methylation, and noncoding RNAs.
What affects gene regulation? ›Similarly, drugs, chemicals, temperature, and light are among the external environmental factors that can determine which genes are turned on and off, thereby influencing the way an organism develops and functions.
What is the most common form of gene regulation? ›Regulation of transcription is the most common form of gene control. The action of transcription factors allows for unique expression of each gene in different cell types and during development.
Which is a regulatory gene? ›Regulator gene: the gene that codes for a site-specific protein (e.g., transposase) that can move the transposon sequence which is flanked by appropriate inverted and/or direct repeat DNA sequences.
What is the summary of gene regulation? ›
Gene regulation is the process used to control the timing, location and amount in which genes are expressed. The process can be complicated and is carried out by a variety of mechanisms, including through regulatory proteins and chemical modification of DNA.
What do regulatory genes produce? ›genetic regulation
operon is controlled by a regulator gene, which produces a small protein molecule called a repressor.
All three domains of life use positive regulation (turning on gene expression), negative regulation (turning off gene expression), and co-regulation (turning multiple genes on or off together) to control gene expression, but there are some differences in the specifics of how these jobs are carried out between ...
What are three ways gene expression is regulated? ›Answer d. Control of gene expression in eukaryotic cells occurs at epigenetic, transcriptional, post-transcriptional, translational, and post-translational levels.
What are three ways gene regulation can be controlled in a cell? ›Thus a cell can control the proteins it makes by (1) controlling when and how often a given gene is transcribed (transcriptional control), (2) controlling how the RNA transcript is spliced or otherwise processed (RNA processing control), (3) selecting which completed mRNAs in the cell nucleus are exported to the ...
At what stages are genes regulated? ›Gene expression in prokaryotes is regulated only at the transcriptional level, whereas in eukaryotic cells, gene expression is regulated at the epigenetic, transcriptional, post-transcriptional, translational, and post-translational levels.
What is the most common method of gene regulation in both? ›transcriptional - level control. The most common form of gene expression regulation in both bacteria and eukaryotes is the transcriptional - level control.
What does regulatory gene mean? ›Regulatory genes are those genes that code for proteins or factors that control the expression of structural genes. Location. In prokaryotes, the structural genes of related functionality are usually present adjacent to each other and regulated by a single promoter and operator.
What factors regulate gene expression? ›Transcription factors are essential for the regulation of gene expression. Under the effect of transcription factors, the various cells of the body can function differently though they have the same genome.
What are the two types of regulatory genes? ›Regulatory genes can also be described as positive or negative regulators, based on the environmental conditions that surround the cell. Positive regulators are regulatory elements that permit RNA polymerase binding to the promoter region, thus allowing transcription to occur.
Why must gene expression be regulated? ›
Gene expression is regulated to ensure that the correct proteins are made when and where they are needed. Regulation may occur at any point in the expression of a gene, from the start of the transcription phase of protein synthesis to the processing of a protein after synthesis occurs.
What is the first level of gene regulation? ›The first level, the DNA code, forms an interaction platform by providing protein binding sites for transcription factors that, together with non-coding RNAs and histone modifications, form the next layer of gene regulation.
Where does gene regulation occur primarily? ›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.
What are the 5 environmental factors that can regulate genes? ›Environmental factors such as diet, temperature, oxygen levels, humidity, light cycles, and the presence of mutagens can all impact which of an animal's genes are expressed, which ultimately affects the animal's phenotype.