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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 this to happen, there must be some mechanism that controls when a gene is expressed to make RNA and protein, how much 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 themselves, such as B. the conversion of 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 in order for it to 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. Even unicellular organisms, both eukaryotic and prokaryotic, turn genes on and off in response to the demands of their environment so they can respond to particular conditions.
Controlling gene expression is extremely complex. Malfunction of this process is 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. Consequently, the main way to control what type and how much protein is expressed in a prokaryotic cell is by regulating the transcription of DNA into RNA. All further steps take place automatically. When more protein is needed, more transcription takes place. Therefore, the control of gene expression in prokaryotic cells lies almost exclusively at the level of transcription.
The first example of such a control was discovered withE.coliin the 1950s and 1960s by French researchers and is calledtiredOperon. ÖtiredOperon is a stretch of DNA with three contiguous genes that encode proteins involved in the absorption and metabolism of lactose, a food sourceE.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 can 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 is a region called the operator. In the absence of lactose, a protein known as a repressor binds to the operator and, except in rare cases, prevents RNA polymerase from binding to the promoter. Therefore, very little of the protein products of the three genes are 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.
In contrast, eukaryotic cells have intracellular organelles and are much more complex. Remember that in eukaryotic cells, DNA is contained in the cell nucleus, where it is transcribed into mRNA. The newly synthesized mRNA is then transported from the nucleus to the cytoplasm, where ribosomes translate the mRNA into protein. The transcription and translation processes 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 take place at all stages of the process (Figure \(\PageIndex{1}\)). Regulation can occur when DNA is unwound and released from nucleosomes to bind transcription factors (epigenetic level), when RNA is transcribed (transcriptional level), when RNA is processed after transcription and exported to the cytoplasm (transcriptional level). 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 is missing | contains core |
| RNA transcription and protein translation occur almost simultaneously |
|
| Gene expression is mainly regulated at the transcriptional level | Gene expression is regulated at many levels (epigenetic, transcriptional, post-transcriptional, translational and post-translational) |
EVOLUTION IN ACTION: Alternative RNA splicing
Genes exhibiting alternative RNA splicing were first observed in the 1970s. Alternative RNA splicing is a mechanism that makes it possible to produce different protein products 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 mostly it is controlled and acts as a mechanism of gene regulation, whereby the frequency of different splicing alternatives is controlled by the cell to control the production of different protein products in different cells. or in different cells, 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 might alternative splicing evolve? Introns have a beginning and ending recognition sequence, and it is easy to imagine that the splicing mechanism is unable to identify the end of one intron and find the end of the next intron, thereby removing two introns and the exon in between. Indeed, there are mechanisms to prevent such exon skipping, but mutations are likely to cause their failure. Such "mistakes" would likely result in a dysfunctional protein. In fact, the cause of many genetic diseases is alternative splicing rather than mutations in a sequence. However, alternative splicing would produce a protein variant without losing the original protein, opening up opportunities to adapt the new variant to new functions. Gene duplication has similarly played an important role in the evolution of new functions - by providing genes that can evolve without eliminating the original functional protein.
Summary
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 that encodes them 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 controlling 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 occurs in the nucleus and is separate from translation, which occurs in the cytoplasm. Gene expression in prokaryotes is only regulated at the transcriptional level, whereas in eukaryotic cells gene expression is regulated at the epigenetic, transcriptional, post-transcriptional, translational, and post-translational levels.
glossary
- alternatives RNA-Spleißen
- a mechanism of post-transcriptional gene regulation in eukaryotes in which multiple protein products are produced by a single gene through alternative splicing combinations of the RNA transcript
- epigenetic
- Description of non-genetic regulatory factors such as B. Changes in histone and DNA protein modifications that control access to genes on chromosomes
- genetic expression
- Processes that control whether a gene is expressed
- posttranskriptional
- Control of gene expression after the RNA molecule has been created but before it is translated into protein
- posttranslational
- Control of gene expression after formation of a protein
employees and tasks
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).