Notes on Cell Differentiation and Gene Expression

Cell Differentiation and Gene Expression

  • Cell Differentiation Achieved Through Dissimilar Gene Expression

    • All cells in an organism originate from a single cell.
    • Despite having an identical genome, cells differ significantly in structure and function.
    • Neurons and hepatocytes possess the same genome but exhibit very different phenotypes and functions.

  • Universal Genetic Information

    • Yes, all cells in an organism contain the same and complete genetic information (the genome).
    • Different cells express various genes pertinent to their specialized functions.
    • Example: Varying mRNA levels for:
    • Beta actin (expressed in all cell types)
    • Tyrosine aminotransferase (specific to liver cells)

  • Gene to Protein Regulation

    • Gene expression or protein expression is regulated at multiple levels.
    • Mechanisms exist that selectively express subsets of genes in different cell types.

Transcription Control

  • How is gene transcription controlled?

    • Transcription is regulated by a regulatory region in the DNA located near the transcription initiation site.
    • Regulators of Gene Transcription Elements:
    1. Cis-regulatory sequences:
      • Short stretches of DNA (5-10 nucleotides) involved in regulation.
    2. Binding proteins:
      • Known as transcription regulators or transcription factors;
      • Recognize and bind to specific DNA sequences (can act as enhancers or repressors).

  • Structural Features of DNA Bases

    • Each DNA base pair has hydrogen bond donors, acceptors, and hydrophobic patches exposed on its surface.
    • Gene regulatory proteins contain structural motifs recognizing specific DNA sequences.
    • Binding Interaction:
    • A transcription regulator forms 10 to 20 contacts with the DNA during binding.

DNA Binding Protein Motifs

  • Transcription Regulators
    • DNA-binding proteins recognize specific DNA sequences and regulate gene expression.
    • Common DNA Binding Protein Motifs:
    • Helix-turn-helix:
      • Two alpha-helixes connected at a specific angle.
    • Homeodomain:
      • Transcription factors regulating developmental processes (around 60 proteins identified in the human genome).
    • Leucine Zipper:
      • Contains long alpha-helixes with leucine residues; binds DNA as dimers, allowing for homo- and hetero-dimerization.
    • Beta-sheet Binding Proteins:
      • Specific DNA sequences recognized by different amino acid sequences within the beta-sheet.
    • Zinc Finger Proteins (Cys-Cys-His-His family):
      • Diverse group of proteins utilizing one or more zinc atoms to form loops resembling fingers.
    • Helix-loop-helix (HLH):
      • Protein dimers of two alpha-helices that bind DNA as a dimer.
    • Helix-turn-Helix factors typically function as dimers in transcription regulation.