Regulation of Gene Expression and Epigenetics: Study Notes

Regulation of Gene Expression

Transcription Factors

Transcription factors are proteins that influence the ability of RNA polymerase to transcribe genes. They play a central role in regulating gene expression by binding to specific DNA sequences.

• Definition: Proteins that bind to DNA and regulate transcription by promoting or inhibiting RNA polymerase activity.

• Types: General transcription factors (required for all genes) and regulatory transcription factors (specific to certain genes).

• Example: TFIID is a general transcription factor that binds to the TATA box in promoters.

DNA Sites Bound by Regulatory Transcription Factors

Regulatory transcription factors bind to specific DNA sequences to control gene expression.

• Trans-acting factors: Proteins that regulate genes distant from their own coding sequence.

• Cis-acting factors: DNA sequences (such as enhancers or silencers) that regulate genes on the same DNA molecule.

• Promoters: DNA sequences where transcription factors and RNA polymerase bind to initiate transcription.

• Conserved sequences: Some regulatory sites have similar sequences in all cells, indicating their importance.

Homodimers and DNA Binding

Many transcription factors function as dimers, which can be homodimers or heterodimers.

• Homodimer: A protein complex formed by two identical subunits, often with a helix-loop-helix DNA binding motif.

• Function: Homodimers typically bind DNA to regulate gene expression.

Response Elements

Response elements are specific DNA sequences within genes that bind transcription factors in response to cellular signals.

• Location: May be found within the gene itself or in regulatory regions such as promoters or enhancers.

TFIID vs. Mediator in Transcription Regulation

TFIID and mediator are protein complexes that regulate transcription initiation.

• TFIID: Binds to the TATA box and is essential for the formation of the transcription initiation complex.

• Mediator: Acts as a bridge between transcription factors and RNA polymerase II, facilitating transcription.

• Regulatory Effects: Transcription factors bound to enhancers stimulate binding of both TFIID and mediator, while those bound to silencers inhibit their binding.

Transcription Factors and Response Elements

Transcription factors bind to their specific response elements to regulate gene expression.

• Specificity: Each transcription factor recognizes and binds its own response element.

Glucocorticoid Receptor and Dimerization

The glucocorticoid receptor is a transcription factor that regulates gene expression in response to glucocorticoid hormones.

• Dimerization: The receptor dimerizes to bind DNA and regulate transcription.

• Function: Dimerization is necessary for effective DNA binding and gene regulation.

GREs (Glucocorticoid Response Elements)

GREs are DNA sequences that bind the glucocorticoid receptor to regulate gene expression.

• Location: Found near promoters of many genes.

• Activation: GREs are activated in response to increased glucocorticoid levels.

• Binding: GREs are bound by dimerized glucocorticoid receptors.

Extracellular Signaling Molecules

Extracellular signals can regulate gene expression by affecting transcription.

• Mechanism: Signals are usually transmitted via plasma membrane receptors, which then pass along the signal to affect transcription.

CREB Activation and Transcription

CREB (cAMP response element-binding protein) is a transcription factor activated by phosphorylation.

• Activation: CREB must be phosphorylated to activate transcription.

• Function: Phosphorylated CREB binds to cAMP response elements in DNA to regulate gene expression.

Gene Amplification

Gene amplification is a process that increases the number of copies of a gene in the genome.

• Difference from Other Alterations: Gene amplification involves loss of DNA, not just addition or chemical modification.

• Chromatin Structure: Amplification may also involve changes in chromatin structure.

DNA Configuration and Transcription

The physical configuration of DNA affects its transcriptional activity.

• Open Configuration: DNA in an open configuration is more likely to be transcribed than DNA in a closed configuration.

DNase-I Susceptibility

DNase-I is an enzyme that digests DNA, and its activity depends on DNA structure.

• Susceptibility: 30nm fiber DNA is less susceptible to DNase-I digestion than 10nm fiber DNA, which is more open and accessible.

CpG Islands

CpG islands are regions of DNA with a high frequency of CG dinucleotides, often found near gene promoters.

• Definition: A CpG island consists of about 2000 copies of the CG dinucleotide.

• Function: CpG islands are important in gene regulation and are often unmethylated in housekeeping genes.

Housekeeping Genes

Housekeeping genes are essential for basic cellular function and are expressed in all cell types.

• Expression: Constitutively expressed and typically have unmethylated CpG islands.

• Function: Encode proteins required for general cellular processes.

DNA Methylation

DNA methylation is an epigenetic modification that affects gene expression.

• Effect on Transcription: Methylation of CpG islands can prevent binding of regulatory transcription factors and activation of enhancers.

• Heritability: Methylation patterns are heritable and can be passed to daughter cells.

Interference of DNA Methylation with Transcription Factor Binding

DNA methylation can interfere with transcription factor binding in several ways.

• Competition: Methyl-CpG binding proteins compete with transcription factors for binding to methylated DNA.

• Modification: Methylation can change the shape of the response element, affecting transcription factor binding.

• Dimerization: Methylation may prevent dimerization of DNA binding proteins.

Single RNA Molecule and Protein Coding

A single RNA molecule can encode several different proteins through alternative splicing and other mechanisms.

• Alternative Splicing: Allows for the production of multiple protein isoforms from a single gene.

Post-Transcriptional Regulation

Alternative Splicing

Alternative splicing is a process by which different combinations of exons are joined to produce multiple mRNA variants from a single gene.

• Control: Involves repressors, enhancers, and splicing factors that modulate spliceosome function.

• Function: Increases protein diversity and allows for tissue-specific gene expression.

RNA Editing

RNA editing is a post-transcriptional process that alters nucleotide sequences in RNA molecules.

• Guide RNA: Directs the editing process, which can change uracil to another base or insert/delete nucleotides.

• Frequency: Occurs in widely varied species and can have minor or major effects on RNA function.

Poly-A Tail and mRNA Stability

The poly-A tail is a stretch of adenine nucleotides added to the 3' end of eukaryotic mRNA, affecting its stability and translation.

• Length: Longer poly-A tails generally increase mRNA stability.

mRNA Stability

mRNA stability is influenced by several factors, including destabilizing elements, poly-A tail length, and antisense RNA.

• Enhancement: Stability can be enhanced by the absence of destabilizing elements, a long poly-A tail, and lack of antisense RNA.

Epigenetics and Gene Regulation

Alternative Forms of Regulation

Genes can have different functions or similar functions under different forms of regulation, such as alternative splicing or epigenetic modification.

• Function: Regulation can result in completely different or similar functions depending on the context.

RNA Virus Resistance

Cellular resistance to viruses can depend on the type of viral genome (DNA or RNA) and the cell's ability to recognize and degrade viral RNA.

• Specificity: Cells resistant to double-stranded RNA viruses tend to be resistant to all double-stranded RNA viruses.

Receptor Tyrosine Kinases

Receptor tyrosine kinases are transmembrane proteins that transmit extracellular signals to the cell interior, primarily affecting posttranslational modification.

• Function: Activation leads to phosphorylation of target proteins, influencing cell signaling pathways.

Posttranslational Modification and Gene Expression

Posttranslational modifications such as phosphorylation or acetylation can alter protein function and gene expression.

• Histones: Modification of histones affects chromatin structure and gene accessibility.

• TFIID: Phosphorylation can affect the ability of TFIID to initiate transcription.

Key Table: Effects of DNA Methylation on Transcription Factor Binding

Mechanism	Effect on Transcription Factor Binding
Methyl-CpG binding protein competition	Competes with transcription factors for binding to methylated DNA
Modification of response element shape	Alters the shape, reducing transcription factor binding
Prevention of dimerization	Inhibits formation of transcription factor dimers

Key Equations and Concepts

• Gene Amplification: $\text{Gene Copy Number}_{\text{after amplification}} = n \times \text{Gene Copy Number}_{\text{before amplification}}$

• Transcription Factor Binding: $\text{TF} + \text{DNA}_{\text{response element}} \rightarrow \text{TF-DNA complex}$

• DNA Methylation: $\text{DNA}_{\text{CpG island}} + \text{Methyl group} \rightarrow \text{Methylated DNA}$

Additional info: These notes expand on the original question set by providing definitions, context, and examples for each concept, making them suitable for exam preparation in a college genetics course.