Control of gene expression
Control of gene expression is a fundamental biological process that determines whether a gene is switched on or off and to what extent it is expressed. This regulation ensures that only the necessary proteins are made when and where they are needed.
Definition of Gene Expression
Gene expression is the transcription of a gene into messenger RNA (mRNA) and the subsequent translation of that mRNA into a protein (polypeptide). The sequence of amino acids in a polypeptide forms the primary structure of a protein, and this sequence is determined by the order of bases in a gene.
Purpose of Control
All cells in an organism generally contain the same DNA (genome), but they do not express all of their genes simultaneously. This selective gene expression is crucial for cell specialisation (also known as differentiation). By expressing different genes, cells produce different proteins, which in turn modify the cell's structure and control its processes, leading to its specialized function.
Gene control also prevents the waste of energy and materials by ensuring that proteins, such as enzymes, are only produced when required.
Mechanisms of Gene Expression Control There are several mechanisms through which gene expression is controlled:
Transcription Factors
Transcription factors are protein molecules that bind to DNA and affect whether or not a gene is transcribed.
In eukaryotes, transcription factors move from the cytoplasm into the nucleus and bind to specific DNA sites called promoters. Promoters are DNA sequences located near the start of their target genes.
Some transcription factors act as activators, stimulating or increasing the rate of transcription. They can help RNA polymerase bind to the start of the target gene.
Others act as repressors, inhibiting or decreasing the rate of transcription, for example, by preventing RNA polymerase from binding.
Differential gene expression through transcription factors ensures cell specialisation.
Hormonal Control
Hormones, such as oestrogen and gibberellin, can influence gene expression by interacting with transcription factors.
Oestrogen, a steroid hormone, can bind to an oestrogen receptor (a transcription factor) to form a complex that then moves into the nucleus and binds to specific DNA sites near the start of a target gene, activating transcription.
Gibberellin (a plant growth regulator) controls seed germination and stem elongation. It does this by causing the breakdown of DELLA proteins, which are repressors that normally inhibit factors promoting transcription. When DELLA proteins are deactivated or destroyed, transcription of genes like those for amylase can proceed, leading to responses like starch hydrolysis in germinating seeds.
Epigenetic Control
Epigenetic control involves heritable changes in gene function without altering the DNA base sequence itself. These changes are caused by environmental factors like diet, pollution, and stress.
Epigenetic marks are chemical groups attached to DNA or histone proteins. They influence how easily the DNA can be accessed and transcribed.
Increased methylation of DNA: A methyl group (-CH3) attaches to DNA, typically at CpG sites (cytosine and guanine next to each other). Increased methylation alters DNA structure, preventing transcriptional machinery from interacting with and transcribing the gene, effectively "switching it off".
Decreased acetylation of histones: Histones are proteins around which DNA wraps to form chromatin. When histones are acetylated, chromatin is less condensed, allowing transcription to occur. Removal of acetyl groups by histone deacetylase (HDAC) enzymes causes chromatin to become highly condensed, repressing gene transcription. Abnormal methylation of tumour suppressor genes and oncogenes can contribute to cancer.
RNA Interference (RNAi)
RNA interference (RNAi) involves small, double-stranded RNA molecules (siRNA and miRNA) that stop mRNA from target genes from being translated into proteins.
siRNA and associated proteins bind to target mRNA, leading to the mRNA being cut up into fragments, thus blocking translation. miRNA works similarly by binding to target mRNA and blocking translation.
Prokaryotic Gene Control: The Lac Operon Example
The lac operon is a well-studied example in bacteria like E. coli. It ensures that enzymes needed for lactose metabolism (like β-galactosidase, permease, and transacetylase) are only produced when lactose is present and glucose is not.
The lac operon consists of structural genes (coding for useful proteins like enzymes), control elements (promoter and operator), and a regulatory gene (coding for a repressor or activator).
In the absence of lactose: The regulatory gene produces a repressor protein. This repressor binds to the operator region, preventing RNA polymerase from binding to the promoter and thus stopping transcription of the structural genes.
In the presence of lactose: Lactose binds to the repressor protein, changing its shape and preventing it from binding to the operator. This allows RNA polymerase to bind to the promoter, and the structural genes are transcribed, leading to the production of the enzymes needed for lactose digestion.
β-galactosidase is an inducible enzyme, meaning its production occurs only when its substrate (lactose) is present. Conversely, repressible enzymes are generally produced continuously unless an effector molecule stops their production.
Eukaryotic vs. Prokaryotic Control
Eukaryotes do not have operons like prokaryotes. Their gene regulation mechanisms are more complex, often involving many more transcription factors. Eukaryotic genes also contain introns (non-coding sequences) and exons (coding sequences), and pre-mRNA undergoes splicing to remove introns before translation. Prokaryotic DNA does not have introns.
Impact of Mutations on Gene Expression
Mutations, which are changes in the DNA base sequence, can affect gene expression and the resulting phenotype. For example, a mutation in a gene controlling cell division can lead to uncontrolled cell growth and cancer. Mutations in tumour suppressor genes or proto-oncogenes can cause cancer by affecting the proteins that regulate cell division.Control of gene expression is a fundamental biological process that determines whether a gene is switched on or off and to what extent it is expressed. This regulation ensures that only the necessary proteins are made when and where they are needed.
Here's a summary of the key aspects of gene expression control:
Definition of Gene Expression
Gene expression is the transcription of a gene into messenger RNA (mRNA) and the subsequent translation of that mRNA into a protein (polypeptide). The sequence of amino acids in a polypeptide forms the primary structure of a protein, and this sequence is determined by the order of bases in a gene.
Purpose of Control
All cells in an organism generally contain the same DNA (genome), but they do not express all of their genes simultaneously. This selective gene expression is crucial for cell specialisation (also known as differentiation). By expressing different genes, cells produce different proteins, which in turn modify the cell's structure and control its processes, leading to its specialized function.
Gene control also prevents the waste of energy and materials by ensuring that proteins, such as enzymes, are only produced when required.
Mechanisms of Gene Expression Control There are several mechanisms through which gene expression is controlled:
Transcription Factors
Transcription factors are protein molecules that bind to DNA and affect whether or not a gene is transcribed.
In eukaryotes, transcription factors move from the cytoplasm into the nucleus and bind to specific DNA sites called promoters. Promoters are DNA sequences located near the start of their target genes.
Some transcription factors act as activators, stimulating or increasing the rate of transcription. They can help RNA polymerase bind to the start of the target gene.
Others act as repressors, inhibiting or decreasing the rate of transcription, for example, by preventing RNA polymerase from binding.
Differential gene expression through transcription factors ensures cell specialisation.
Hormonal Control
Hormones, such as oestrogen and gibberellin, can influence gene expression by interacting with transcription factors.
Oestrogen, a steroid hormone, can bind to an oestrogen receptor (a transcription factor) to form a complex that then moves into the nucleus and binds to specific DNA sites near the start of a target gene, activating transcription.
Gibberellin (a plant growth regulator) controls seed germination and stem elongation. It does this by causing the breakdown of DELLA proteins, which are repressors that normally inhibit factors promoting transcription. When DELLA proteins are deactivated or destroyed, transcription of genes like those for amylase can proceed, leading to responses like starch hydrolysis in germinating seeds.
Epigenetic Control
Epigenetic control involves heritable changes in gene function without altering the DNA base sequence itself. These changes are caused by environmental factors like diet, pollution, and stress.
Epigenetic marks are chemical groups attached to DNA or histone proteins. They influence how easily the DNA can be accessed and transcribed.
Increased methylation of DNA: A methyl group (-CH3) attaches to DNA, typically at CpG sites (cytosine and guanine next to each other). Increased methylation alters DNA structure, preventing transcriptional machinery from interacting with and transcribing the gene, effectively "switching it off".
Decreased acetylation of histones: Histones are proteins around which DNA wraps to form chromatin. When histones are acetylated, chromatin is less condensed, allowing transcription to occur. Removal of acetyl groups by histone deacetylase (HDAC) enzymes causes chromatin to become highly condensed, repressing gene transcription. Abnormal methylation of tumour suppressor genes and oncogenes can contribute to cancer.
RNA Interference (RNAi)
RNA interference (RNAi) involves small, double-stranded RNA molecules (siRNA and miRNA) that stop mRNA from target genes from being translated into proteins.
siRNA and associated proteins bind to target mRNA, leading to the mRNA being cut up into fragments, thus blocking translation. miRNA works similarly by binding to target mRNA and blocking translation.
Prokaryotic Gene Control: The Lac Operon Example
The lac operon is a well-studied example in bacteria like E. coli. It ensures that enzymes needed for lactose metabolism (like β-galactosidase, permease, and transacetylase) are only produced when lactose is present and glucose is not.
The lac operon consists of structural genes (coding for useful proteins like enzymes), control elements (promoter and operator), and a regulatory gene (coding for a repressor or activator).
In the absence of lactose: The regulatory gene produces a repressor protein. This repressor binds to the operator region, preventing RNA polymerase from binding to the promoter and thus stopping transcription of the structural genes.
In the presence of lactose: Lactose binds to the repressor protein, changing its shape and preventing it from binding to the operator. This allows RNA polymerase to bind to the promoter, and the structural genes are transcribed, leading to the production of the enzymes needed for lactose digestion.
β-galactosidase is an inducible enzyme, meaning its production occurs only when its substrate (lactose) is present. Conversely, repressible enzymes are generally produced continuously unless an effector molecule stops their production.
Eukaryotic vs. Prokaryotic Control
Eukaryotes do not have operons like prokaryotes. Their gene regulation mechanisms are more complex, often involving many more transcription factors. Eukaryotic genes also contain introns (non-coding sequences) and exons (coding sequences), and pre-mRNA undergoes splicing to remove introns before translation. Prokaryotic DNA does not have introns.
Impact of Mutations on Gene Expression
Mutations, which are changes in the DNA base sequence, can affect gene expression and the resulting phenotype. For example, a mutation in a gene controlling cell division can lead to uncontrolled cell growth and cancer. Mutations in tumour suppressor genes or proto-oncogenes can cause cancer by affecting the proteins that regulate cell division.
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