Control of Gene Expression
RNA and Protein Synthesis Discussion
Overview: The focus of this discussion is on the synthesis and function of RNAs and proteins, specifically highlighting examples using the actin gene and tyrosine aminotransferase.
Actin Gene and Protein
Importance of Actin:
Actin protein is critical for various cellular functions, particularly in maintaining the structure of the cytoskeleton.
All eukaryotic cells possess a cytoskeleton which requires actin for integrity and movement.
Gene Structure:
The actin gene is representative of a typical eukaryotic gene structure that includes
Exons (coding regions) and
Introns (non-coding regions).
Expression Across Cell Types:
High levels of actin messenger RNA (mRNA) are present in diverse cell types:
Lung cells
Skin cells
Embryonic stem cells
Muscle cells
The reason for ubiquitous expression is that all these cells require a cytoskeleton.
Tyrosine Aminotransferase Gene
Specificity to Liver Cells:
In contrast to actin, the tyrosine aminotransferase gene is only expressed in liver cells.
Its expression indicates it is a differentiated gene, contributing to the distinct functionalities of liver cells compared to neurons.
Housekeeping Genes vs. Differentiated Genes:
Housekeeping Genes (like actin):
Commonly expressed across many cell types.
Essential for basic cellular functions.
Differentiated Genes (like tyrosine aminotransferase):
Specific to certain cell types, contributing to unique functions.
Gene Expression Control
Transcriptional Control:
The act of controlling whether a gene is transcribed into RNA.
The mechanism involves whether the cell decides to make the mRNA or not.
This control can be understood as:
"Will I make this RNA?"
It emphasizes that regulation can occur at various steps in gene expression, but the most common regulatory step is at the transcription phase.
The rationale for this high level of control is energy efficiency: It is wasteful to synthesize RNA or proteins if they are not needed for cellular functions.
External Signals Influencing Gene Expression
Types of External Signals:
Nutrient availability can affect gene expression.
Hormones can induce differential gene expression—some genes may be activated while others are suppressed.
Environmental factors such as temperature can alter when certain genes are expressed (e.g., hibernation-related genes).
Multi-step Regulation of Gene Expression
Process Steps:
DNA to RNA: Transcription of the gene into mRNA.
RNA Processing: RNA undergoes splicing, capping, and polyadenylation.
Nuclear Export: mRNA moves from the nucleus to the cytoplasm.
Translation: Ribosomes translate mRNA into a protein.
Post-translational Modifications: Proteins may undergo modifications before becoming active.
Mechanisms of Control at Each Step:
RNA may be synthesized but not processed.
Processed RNA may not leave the nucleus.
mRNA may be targeted for degradation prematurely.
Proteins may be synthesized but remain inactive due to lack of necessary modifications.
These various steps indicate numerous opportunities for gene regulation and control.
Focus on Transcriptional Control Mechanisms
Key Components of Transcription Control:
Promoter: Region upstream of the gene that initiates transcription, often containing a TATA box.
TATA Box: A specific DNA sequence crucial for the binding of transcription machinery, especially RNA polymerase II for mRNA synthesis.
General Transcription Factors: Proteins that bind to the promoter along with RNA polymerase to initiate transcription.
Cis Regulatory Sequences:
These sequences can either enhance or repress transcription.
They are located on the same chromosome as the gene they regulate, hence the name cis regulatory sequences.
These sequences can be found far upstream or downstream of the gene, or even within introns, adding complexity to regulatory mechanisms.
Conclusion
Future Focus: Future discussions will explore cis regulatory sequences in greater detail to understand their roles in transcription regulation and overall gene expression control.