Simone Lawson
Gene expression is the process by which information from a gene is used to generate a functional gene product, most commonly a protein. Transcription and translation are the means by which cells read out, or express, the genetic instructions in their genes. DNA is the genetic blueprint of most organisms on Earth, containing instructions for development, growth, and function. Constitutive genes are always on, regardless of environmental conditions, and are transcribed continually. The optimal constitutive expression level depends on how the costs and benefits increase with the expression level. The regulation of gene expression is vital to allow a cell to produce the gene products it needs when it needs them, giving cells the flexibility to adapt to variable environments, external signals, and other stimuli.
| Characteristics | Values |
|---|---|
| Definition | Constitutive genes are always "on", regardless of environmental conditions. |
| Gene Expression | DNA ensures constitutive genes are expressed more frequently through transcription and translation, creating multiple identical RNA copies from the same gene. |
| Transcription Factors | Proteins that act as transcription factors can turn specific genes "on" or "off" by binding to DNA. Activator transcription factors boost transcription, while repressors decrease it. |
| RNA Splicing | Alternative splicing of RNA can give rise to different proteins from the same gene. Constitutive genes may produce alternatively spliced mRNAs continuously. |
| Gene Regulation | Gene regulation is essential for maintaining homeostasis and ensuring cells have the necessary gene products. Constitutive genes are continually transcribed, unlike facultative genes, which are only transcribed when needed. |
| Optimal Expression | The optimal constitutive expression level depends on the costs and benefits associated with the expression. In some cases, intermediate expression maximizes growth, while in others, the gene is either fully expressed or fully repressed. |
| Noise | Gene expression inherently produces "noise" or stochastic variation in gene products. Constitutive genes exhibit minimal intrinsic stochasticity, which is crucial for reliable cellular functions. |
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What You'll Learn
Transcription factors
The activity of transcription factors is modulated by intracellular signals, which cause post-translational modifications to proteins. These changes influence the ability of transcription factors to bind to promoter DNA and recruit RNA polymerase. The nuclear membrane in eukaryotes further regulates transcription factors by controlling the duration of their presence in the nucleus.
Transcription factor binding sites are short DNA sequences (5-20 base pairs long) specifically bound by one or more transcription factors. The identification of these binding sites and their functions is an ongoing area of research in computational biology.
Overall, transcription factors play a crucial role in gene expression by regulating the transcription of specific genes and ensuring that the appropriate genes are expressed in the correct cells and at the right time.
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Gene regulation
Genes can be classified as constitutive or facultative. Constitutive genes are those that are always "on", regardless of environmental conditions, and are transcribed continually. On the other hand, facultative genes are only transcribed when needed.
The regulation of gene expression is achieved through the modulation of various steps in the gene expression process, including transcription, RNA splicing, translation, and post-translational modification of a protein. Transcription factors, which are proteins, play a crucial role in gene regulation by acting as activators or repressors of gene expression. They do this by binding to nearby DNA, making it easier or harder for RNA polymerase to bind to the DNA promoter and initiate transcription. The presence of certain transcription factors and their binding to DNA can be influenced by intracellular signals and environmental stimuli, resulting in the regulation of gene expression.
The optimal constitutive expression level depends on the costs and benefits associated with the expression level. In a changing environment, the optimal constitutive expression level differs from the average demand for the gene product. Constitutive expression can provide higher fitness than responsive expression, even when regulatory machinery is not a factor. However, environmental and intracellular noise favour the responsive strategy, reducing the fitness of the constitutive strategy.
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Environmental factors
Temperature
Temperature is an important environmental factor that can significantly influence gene expression. For example, the sex of some reptile and turtle species is determined by the temperature at which their eggs are incubated. At 25°C, all Emys obicularis turtles are born male, while at 30°C, all are born female. Similarly, the C gene in Himalayan rabbits, responsible for pigment development, is regulated by temperature. The gene is inactive above 35°C, and most active between 15°C and 25°C. This results in rabbits with distinctive coat colouring, with white fur on their warmer body parts and black fur on their cooler extremities.
Oxygen Levels
Oxygen levels have been found to affect gene expression, as seen in the case of supplemental oxygen administration causing blindness in premature infants. Increasing oxygen levels can convert the breathing pattern of premature infants to a "normal" rhythm, but there is a causal relationship between oxygen administration and retinopathy of prematurity (ROP).
Light
Light is another external environmental factor that can impact gene expression. Himalayan rabbits, for instance, carry the C gene, which is involved in pigment development in their fur, skin, and eyes. The expression of this gene is influenced by the temperature and light exposure, resulting in variations in their coat colouring.
Chemicals and Drugs
The presence of chemicals and drugs in the environment can also influence gene expression. For example, thalidomide is a drug that has been associated with anticancer properties, and its impact on gene expression has been studied in this context.
Diet and Humidity
In summary, environmental factors play a significant role in gene expression, interacting with an organism's genes to shape its development, health, and observable traits. The dynamic interplay between genes and the environment presents both challenges and opportunities in understanding and manipulating gene expression for optimal health and disease prevention.
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Protein synthesis
DNA ensures that constitutive genes are expressed more frequently through the process of gene expression. Gene expression is the process by which information from a gene is used to synthesise a functional gene product, which can be a protein or a non-coding RNA molecule. This process allows the cell to produce the gene products it needs when it needs them, giving it the flexibility to adapt to a variable environment, external signals, damage, and other stimuli.
The synthesis of proteins involves two major steps: transcription and translation. During transcription, the DNA sequence of the gene is copied to make an RNA molecule, specifically a messenger RNA (mRNA) molecule. This mRNA molecule acts as a messenger, carrying the information from the DNA out of the nucleus into the cytoplasm. Each 3-letter combination of mRNA bases specifies one of the twenty amino acids that are the building blocks of proteins.
The plasticity of RNA splicing allows the cell to regulate the pattern of RNA splicing, resulting in different proteins being produced from the same gene. This process is known as alternative splicing and can be constitutive, with alternatively spliced mRNAs being produced continuously by cells. However, in most cases, the splicing patterns are regulated by the cell, resulting in different forms of the protein being produced at different times and in different tissues.
Transcription factors, which are proteins, play a crucial role in regulating gene expression. They can act as activators or repressors, increasing or decreasing gene transcription, respectively. Transcription factors bind to DNA and influence the ability of RNA polymerase to bind to the DNA promoter, thereby controlling the expression of specific genes.
The regulation of gene expression is essential for maintaining the internal balance of the body, known as homeostasis. Genes involved in metabolism, hormone regulation, and energy balance are tightly regulated to ensure optimal body function. Dysregulation of gene expression can lead to various diseases, including cancer.
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Gene expression noise
Gene expression is the process by which information from a gene is used to generate a functional gene product, most commonly a protein. This process is used by all known life forms to generate the macromolecular machinery for life. Gene expression is the most fundamental level at which the genotype gives rise to the phenotype, i.e. observable trait.
Genetically identical cells and organisms exhibit remarkable diversity even when they have identical histories of environmental exposure. This diversity is referred to as gene expression noise. Gene expression noise is the measured level of variation in gene expression among cells within a supposedly identical population.
There are four potential sources of variation in gene expression:
- The inherent stochasticity of biochemical processes that are dependent on infrequent molecular events involving small numbers of molecules.
- Variation in gene expression owing to differences in the internal states of a population of cells, either from predictable processes such as cell cycle progression or from a random process such as partitioning of mitochondria during cell division.
- Subtle environmental differences, such as morphogen gradients in multicellular development.
- Ongoing genetic mutation, either random or directed.
Transcription factors, as the key components of gene regulatory networks, play a central role in the generation of expression noise. Transcription factors are proteins that help turn specific genes "on" or "off" by binding to nearby DNA. They act like a control panel for gene expression. Some transcription factors are activators, boosting a gene’s transcription, while others are repressors, decreasing transcription.
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Frequently asked questions
Constitutive genes are genes that are transcribed continually, as opposed to facultative genes, which are only transcribed when needed. They are always "on" regardless of environmental conditions.
DNA ensures constitutive genes are expressed more frequently through the process of gene expression. This involves the transcription of the DNA sequence of the gene into an RNA molecule, which is then decoded to build a protein. The activity of transcription factors, which are proteins that can act as activators or repressors of gene expression, is modulated by intracellular signals and the binding of other proteins. This results in the regulation of gene expression, allowing for the control of the timing, location, and amount of a gene product.
Constitutive genes that are expressed more frequently can provide higher fitness and maximize growth in a changing environment. They also give cells the flexibility to adapt to variable environments, external signals, and other stimuli. Additionally, the regulation of gene expression is essential for maintaining the internal balance of the body, known as homeostasis.
