APBio Ch 13

Gene Expression Overview

  • Different genes are activated or deactivated at specific times and in various cell types.

  • Heterochromatin: coiled (inactive), Euchromatin: unwound (active).

  • Prokaryotic regulation mainly involves operons.

Prokaryotic Regulation

  • Operons: Clustered genes functioning as a unit.

    • Structural genes: Code for metabolic enzymes.

    • Regulator gene: Forms a repressor (affects transcription).

    • Promoter: RNA polymerase binding site.

    • Operator: Active repressor binds preventing transcription.

  • Bacteria transcribe/translate only needed DNA immediately after transcription.

Operon Types

  • trp Operon (Repressible):

    • Inactive repressor allows transcription until tryptophan levels rise (corepressor). —> End-Product InhibitionQuiz

  • lac Operon (Inducible):

    • Initially off; activated by lactose when glucose is absent (inactivates repressor).

    • cAMP levels influence RNA polymerase affinity for the promoter.

Eukaryotic Regulation

  • Five levels of control for gene expression, primarily positive regulation.

  1. Chromatin Structure (Nucleus):

    • Methylation tightens DNA; acetylation loosens. Determines whether DNA is accessible.

  2. Transcriptional Control (Nucleus):

    • Transcription factors help RNA polymerase bind; TATA box importance.

  3. Posttranscriptional Control (Nucleus):

    • Intron removal by snRNPs; addition of cap/tail to RNA.

  4. Translational Control (Cytoplasm):

    • Regulates mRNA-ribosome interactions.

  5. Posttranslational Control (Cytoplasm):

    • Protein processing and degradation of misfolded proteins.

Gene Mutations

  • Permanent changes in DNA sequence; significant in germ cells (passed on) vs. somatic cells.

    • Types:

    • Point mutation: change in one nucleotide.

    • Frameshift mutation: addition or deletion altering the reading frame.

  • Mutagens and carcinogens can induce mutations, potentially leading to cancer.

Epigenetics

  • Heritable gene expression changes without DNA sequence alteration (e.g., DNA methylation).