Biology 210/212 Lecture 15 Study Notes

Lecture 15: Gene Regulation and Genomes

Post-Transcriptional Regulation

  • Post-transcriptional regulation occurs at the RNA level, involving various small RNAs.

    • Small RNAs participate in feedback loops that influence chromatin and gene expression.

Transcriptional Regulation in Eukaryotes

  • Eukaryotic transcription is characterized by complex regulatory mechanisms:

    • Longer regulatory regions are present in eukaryotic genes compared to prokaryotes.

    • A complex set of enhancers and repressors determines gene expression.

    • Example: In the Drosophila embryo, regulatory mechanisms dictate patterns of gene expression.

    • Chromosome packing significantly affects transcription processes.

Drosophila Example

  • Even-skipped (Eve) Gene Expression:

    • The Eve protein is expressed in narrow stripes within Drosophila embryos, a specific developmental regulator.

    • To analyze expression, embryos are stained with anti-Eve antibodies.

Analysis of Promoter Regulatory Regions

  • The even-skipped (eve) gene shows expression as seven stripes in the fly embryo when analyzed through staining.

  • Regulatory Modules:

    • Stripe 2 regulatory module, vital for localized expression, contains about 480 base pairs in length.

    • The module functions as a computational device integrating multiple inputs for gene expression.

Transcription Regulation Mechanisms

  • Differences between eukaryotic and prokaryotic transcription include:

    1. Eukaryotic RNA polymerases require additional proteins to function.

    2. Proteins can affect transcription even when bound far away from the promoter region.

    3. The actual packing and accessibility of DNA influence the targeting of transcription machinery.

Chromatin Structure and Gene Expression

  • Nucleosomes: DNA wraps around nucleosomes, impacting its accessibility for transcription.

  • Chromatin Types:

    • Euchromatin: Loosely packed and transcriptionally active.

    • Heterochromatin: Highly packed and transcriptionally inactive but can be either reversible (facultative) or permanently condensed (constitutive, as seen at telomeres).

  • Covalent modifications to histone tails (e.g., methylation and acetylation) determine chromatin packing states, thereby influencing gene expression.

Histone Modification Effects

  • Specifically, methylation alters chromatin states and plays a crucial role in developmental decisions.

  • The inhibition of methylase 1 (MET1) using “antisense” RNA leads to changes in methylation patterns across multiple genes in organisms like plants.

Chromosomal Spatial Arrangement

  • Chromosomes can loop to promote interactions between genes and their enhancers, facilitating transcription.

Human Genome Overview

  • The human genome comprises approximately:

    • 20,000 protein-coding genes

    • 5,000 non-coding genes

  • Cells may express between 5,000 and 15,000 protein-coding genes. Thus, there is a complex network of regulatory mechanisms ensuring proper gene expression.

Gene Regulatory Proteins

  • A single gene regulatory protein can modulate the expression of several different genes.

  • Combinations of gene regulatory proteins can generate various cell types throughout development.

  • Example: Introduction of specific neuronal transcription factors into liver cells can lead to their conversion into neurons.

Stable Gene Expression Patterns

  • Master transcription regulators can create stable patterns of gene expression that persist even in the absence of initial signaling, illustrating the significance of transient signals in initiating gene expression.

  • Example: The ey gene in fruit flies can induce the growth of eyes on legs, demonstrating genes controlling cell and tissue formation.

Plant Gene Regulation Examples

  • LEC (LEAFY COTYLEDON) genes can activate embryonic processes by switching on embryogenesis.

  • The regulatory function of LEC genes is evidenced by the difference in growth patterns between wild-type embryos and LEC mutant embryos.

Post-Transcriptional Control

  • Various regulatory processes occur after transcription, including:

    • Translational control via 5' UTRs of mRNA.

    • MicroRNAs (miRNAs): Influence mRNA stability and translation.

    • Approximately one-third of protein-coding genes are regulated by miRNAs in humans.

    • Mechanism involves the RNA-induced silencing complex (RISC) targeting RNA.

RNA Interference and Gene Silencing

  • RNA interference (RNAi) silences genes post-transcriptionally and involves processes initiated by short interfering RNAs (siRNAs).

  • The seminal discoveries in gene regulation led to notable Nobel prizes, recognizing the contributions of microRNAs, with Victor Ambros and Gary Ruvkun being acknowledged for their work in identifying these molecules.

Impact on Plants and Viruses

  • Studies highlight the phenomenon of gene silencing within the context of plant biology, emphasizing the importance of understanding gene regulation in plants to counter viral infections effectively.

Conclusion & Reflection

  • Plant cell biology has historically been overlooked, referred to as “plant blindness.”

  • The lecture concludes with a study question focusing on the differences in eukaryotic versus prokaryotic transcription mechanisms and the intricacies of gene regulation, particularly emphasizing the importance of promoter information and transcription factors in gene expression.