Control of Genetic Expression: Transcription Levels, Lac Operon, and More

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Diya Shah

06/01/2023

Biology

Control of genetic expression

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Biology

The control of gene expression (a-level only)

Gene expression is controlled by a number of features (a-level only)

Control of Genetic Expression: Transcription Levels, Lac Operon, and More

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Gene expression control in prokaryotes and eukaryotes involves complex mechanisms at transcriptional and post-transcriptional levels. Transcription factors play a crucial role in regulating gene expression by binding to specific DNA sequences. In prokaryotes, operons like the lac operon control gene expression, while eukaryotes have more complex regulatory systems involving multiple factors. Post-transcriptional modifications, such as RNA splicing and polyadenylation, further refine gene expression control.

• Transcription factors are proteins that bind to DNA and regulate gene expression
• Operons in prokaryotes, like the lac operon in E. coli, control gene expression efficiently
• Eukaryotic gene regulation involves complex mechanisms at multiple levels
• Post-transcriptional modifications, including RNA splicing and polyadenylation, play important roles
• Post-translational modifications, such as phosphorylation, can activate or deactivate enzymes

...

06/01/2023

320

Control of genetic expression
→ All cells carry the same genes but not all genes in
every cell are switched on
→ cells show different gene e

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Operons and the Lac Operon

Operons are sections of DNA found in prokaryotes that contain a cluster of structural genes transcribed together. These genes typically code for useful proteins such as enzymes. An operon consists of several key elements:

Structural genes
Promoter $control element$
Regulator $activator or repressor$
Operator

The lac operon, found in E. coli, is a classic example of an operon system. It controls the expression of genes necessary for lactose metabolism when glucose is unavailable.

Example: The lac operon in E. coli contains three structural genes: lac Z, lac Y, and lac A, which produce proteins that help bacteria digest lactose.

When lactose is absent, the lac repressor $produced by the regulator gene lacI$ binds to the operator site, blocking transcription. When lactose is present, it binds to the repressor, changing its shape and allowing RNA polymerase to begin transcription of the structural genes.

Highlight: The lac operon demonstrates how prokaryotes can efficiently regulate gene expression in response to environmental changes.

Post-transcriptional modifications also play a role in gene expression control. For example, when mRNA is first made $pre-mRNA$ , it contains both exons and introns. Through a process called splicing, introns are removed, and exons are joined together to produce mature mRNA.

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Post-Transcriptional and Post-Translational Modifications

Post-transcriptional modifications are crucial in refining gene expression control. These modifications occur after the initial transcription of DNA into RNA and include:

RNA splicing: Removal of introns and joining of exons
Addition of 5' cap: Occurs as new RNA leaves the RNA polymerase
Addition of 3' polyadenylation: Adds a chain of adenine molecules to the 3' end

Definition: RNA splicing is the process of removing introns and joining exons to produce mature mRNA.

The 5' cap and 3' poly $A$ tail serve important functions:

They help prevent endonuclease activity on the RNA
Encourage translation
Prevent RNA degradation

Post-translational modifications also play a significant role in gene expression control. One of the most important post-translational adaptations is phosphorylation.

Highlight: Phosphorylation activates or deactivates nearly half of all enzymes, making it a crucial mechanism for regulating protein function.

For example, cAMP activates an enzyme called PKA $Protein Kinase A$ , which in turn activates other enzymes by adding a phosphate group. This process is involved in various cellular functions, including:

Enzymes in glycogenolysis
Enzymes promoting cardiac muscle contraction

Example: The activation of PKA by cAMP demonstrates how post-translational modifications can create a cascade of effects, amplifying the initial signal and leading to significant changes in cellular function.

These various levels of control – transcriptional, post-transcriptional, and post-translational – allow for precise regulation of gene expression, enabling cells to respond efficiently to their environment and maintain proper function.

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Subjects

Biology

The control of gene expression (a-level only)

Gene expression is controlled by a number of features (a-level only)

Biology

•

320

•

6 Jan 2023

•

3 pages

Control of Genetic Expression: Transcription Levels, Lac Operon, and More

Diya Shah

@diyashah583

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Operons and the Lac Operon

Structural genes
Promoter $control element$
Regulator $activator or repressor$
Operator

The lac operon, found in E. coli, is a classic example of an operon system. It controls the expression of genes necessary for lactose metabolism when glucose is unavailable.

Example: The lac operon in E. coli contains three structural genes: lac Z, lac Y, and lac A, which produce proteins that help bacteria digest lactose.

Highlight: The lac operon demonstrates how prokaryotes can efficiently regulate gene expression in response to environmental changes.

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Post-Transcriptional and Post-Translational Modifications

Post-transcriptional modifications are crucial in refining gene expression control. These modifications occur after the initial transcription of DNA into RNA and include:

RNA splicing: Removal of introns and joining of exons
Addition of 5' cap: Occurs as new RNA leaves the RNA polymerase
Addition of 3' polyadenylation: Adds a chain of adenine molecules to the 3' end

Definition: RNA splicing is the process of removing introns and joining exons to produce mature mRNA.

The 5' cap and 3' poly $A$ tail serve important functions:

They help prevent endonuclease activity on the RNA
Encourage translation
Prevent RNA degradation

Post-translational modifications also play a significant role in gene expression control. One of the most important post-translational adaptations is phosphorylation.

Highlight: Phosphorylation activates or deactivates nearly half of all enzymes, making it a crucial mechanism for regulating protein function.

Enzymes in glycogenolysis
Enzymes promoting cardiac muscle contraction

Example: The activation of PKA by cAMP demonstrates how post-translational modifications can create a cascade of effects, amplifying the initial signal and leading to significant changes in cellular function.

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Join milions of students

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Control of Genetic Expression at Transcription Levels

Regulation of gene expression is a fundamental process in both prokaryotes and eukaryotes. All cells carry the same genes, but not all genes are expressed in every cell. This differential gene expression is controlled at both transcriptional and translational levels.

Transcription factors play a crucial role in controlling gene expression. These proteins bind to DNA and can switch genes on or off by altering the rate of transcription. An increase in transcription leads to increased mRNA production, which in turn results in increased protein production.

Vocabulary: Transcription factors are proteins made by regulatory genes that bind to specific DNA sequences to control gene expression.

The structure of genes includes both coding regions $exons$ and non-coding regions $introns$ . At the start of each gene, there is a promoter region, which contains a sequence called the TATA box.

Definition: The TATA box is a binding site near the start of the promoter sequence where transcription factors can attach.

In eukaryotes, transcription factors bind to specific DNA sites near the start of their target genes. In prokaryotes, gene expression control often involves transcription factors binding to operons.

Highlight: The difference in gene expression control between eukaryotes and prokaryotes reflects their different cellular organizations and needs.

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