D2.2 Gene Expression (HL)
Exploring the regulation of transcription and post-transcriptional modifications.
Gene Expression in Eukaryotes
Application: Gene expression is precisely regulated to ensure cells differentiate and adapt to their environments.
Unlike prokaryotes, eukaryotic gene expression involves complex regulatory mechanisms including epigenetics, intricate promoter regions, and RNA splicing.
Promoters and Transcription Factors
The promoter is a non-coding sequence of DNA located upstream of the coding region.
- Provides a binding site for RNA polymerase and general transcription factors.
- Contains specific consensus sequences (e.g., TATA box).
- Binding of transcription factors is required to form the pre-initiation complex.
Skill: Analyze the impact of promoter mutations on transcription efficiency.
Epigenetics
Epigenetics refers to heritable changes in gene expression that do not involve changes to the underlying DNA sequence.
Mechanisms include:
- DNA Methylation: Addition of methyl groups ($-CH_3$) to cytosine bases, typically repressing gene transcription.
- Histone Modification: Acetylation, methylation, and phosphorylation of histone tails. Acetylation neutralizes the positive charge on histones, reducing their affinity for DNA and promoting a transcriptionally active euchromatin state.
Vocabulary Interactive
Loosely packed chromatin, associated with active transcription.
Densely packed chromatin, associated with transcriptional repression.
Post-Transcriptional Modification: Splicing
In eukaryotes, the primary transcript (pre-mRNA) must be processed before translation.
- Introns: Non-coding sequences that are removed.
- Exons: Coding sequences that are spliced together.
- Alternative Splicing: A single gene can code for multiple proteins by selectively including or excluding specific exons. This greatly expands the proteome relative to the genome.
Splicing is catalyzed by the spliceosome, a complex of snRNAs and proteins.
Math in Gene Expression
Consider the probability of transcription factor binding based on concentration $C$ and dissociation constant $K_d$:
$$ P_{bound} = \frac{[C]}{[C] + K_d} $$
Check Your Understanding
Why is alternative splicing biologically significant?
It allows a single gene to produce multiple distinct protein isoforms, increasing the diversity of the proteome without increasing the size of the genome.
How does histone acetylation affect transcription?
Acetylation removes positive charges from histone tails, decreasing their interaction with the negatively charged DNA phosphate backbone. This relaxes the chromatin structure, allowing transcription factors and RNA polymerase to access the promoter.
