Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 18.2 Eukaryotic Gene Expression Is Regulated at Many Stages
Eukaryotic cells precisely regulate gene expression at multiple levels, enabling complex multicellular organisms to differentiate various cell types from the same genome. Gene expression regulation includes chromatin structure modifications, transcription initiation control, and post-transcriptional processes.
Differential Gene Expression
- All cells of a multicellular organism contain the same genome but differ greatly in function.
- Differences in function are due to differential gene expression, where each cell type expresses only a subset of genes.
- Gene regulation is essential for cell differentiation, specialization, and development.
- Incorrect gene regulation can result in diseases, including cancer.
Regulation of Chromatin Structure
- Eukaryotic DNA is organized into chromatin, a complex of DNA and proteins.
- Histone modification (acetylation and methylation) affects chromatin structure and gene expression:
- Histone acetylation loosens chromatin, enhancing transcription.
- Histone methylation typically condenses chromatin, reducing transcription.
- DNA methylation involves adding methyl groups to DNA, usually reducing transcription:
- DNA methylation patterns are often maintained during cell divisions and are related to genomic imprinting.
- Epigenetic inheritance: heritable changes not involving DNA sequence alterations, often reversible and influenced by environmental factors.
Regulation of Transcription Initiation
- Most gene regulation occurs at the level of transcription initiation.
- Transcription factors are proteins that assist RNA polymerase in initiating transcription.
- Two categories of transcription factors:
- General transcription factors: necessary for transcription of all protein-coding genes, forming transcription initiation complexes.
- Specific transcription factors (activators and repressors): regulate the expression of particular genes.
- Enhancers:
- DNA regions located far from the promoter, containing distal control elements.
- Enhancers greatly increase gene expression through specific activators.
- DNA bending proteins enable enhancers to interact with promoters to initiate transcription.
- Combinations of specific control elements in enhancers determine cell-type-specific gene expression.
Coordinately Controlled Genes in Eukaryotes
- Unlike bacterial operons, eukaryotic coordinately controlled genes are not grouped together.
- Eukaryotic genes with related functions contain similar combinations of control elements.
- Coordinated gene expression occurs through specific transcription factors recognizing common control elements across dispersed genes.
- External signals, such as hormones, coordinate gene expression by activating transcription factors.
Post-Transcriptional Regulation
- Multiple regulatory mechanisms operate after transcription:
- Alternative RNA splicing creates different mRNA molecules (and thus proteins) from the same primary transcript.
- Translation initiation regulation: regulatory proteins can block translation by binding mRNA untranslated regions (UTRs).
- mRNA degradation: lifespan of mRNA in the cytoplasm affects protein production; longer-lived mRNAs produce more protein.
- Protein processing and degradation: proteins undergo processing (e.g., cleavage, phosphorylation) to become functional and are selectively degraded when no longer needed.
In a Nutshell
Eukaryotic gene expression involves complex and multi-layered regulatory mechanisms, including chromatin modifications, transcription initiation, alternative RNA splicing, translation control, and protein degradation. This intricate regulation enables multicellular organisms to develop diverse, specialized cell types from a common genome.