Comprehensive Notes on Gene Regulation and Immune System

Gene Expression in Prokaryotes and Eukaryotes

  • Prokaryotes are single-celled organisms; they regulate gene expression continuously because their DNA is always active.
  • Operons in prokaryotes:
    • Groups of genes regulated together, enabling simultaneous on/off control of related functions.

Eukaryotic Gene Regulation

  • Eukaryotes have more genes and cell specialization; gene expression can vary (on or off) depending on cell function.
  • Eukaryotic cells exhibit differential gene expression, allowing for specialization into different cell types such as skeletal muscle or osteocyte.

Overview of DNA Types in Eukaryotes

  • Only 1% of our DNA codes for proteins. The key types of genes include:

    1. Coding Genes

    • Single Copy Genes: One copy of a gene, follows Mendelian inheritance (e.g., Rh factor, attached/free earlobes).
    • Multigene Families: Groups of related genes or more than one copy of a single gene (e.g., genes coding for different types of RNA: rRNA, mRNA, tRNA, siRNA, microRNA).

    2. Noncoding DNA

    • Nonfunctional DNA: Portions of DNA that do not code for functional proteins. Includes:
      • Vestigial Genes: No longer serve a purpose.
      • Introns: Non-coding segments that are spliced out during RNA processing.
      • Short Tandem Repeats (STRs): Repeating sequences of DNA (e.g., GACA repeated multiple times).
      • Variable Number Tandem Repeats (VNTRs): Sequences that vary in number between individuals.
      • Transposons: Jumping genes that can move within the genome.

Chromosomes and Gene Examples

  • Chromosome 11 as an example:
    • Contains examples of short tandem repeats, hemoglobin genes, and single-copy genes such as the PKU gene.

Short Tandem Repeats (STRs)

  • STRs are short sequences (usually 2-5 base pairs) that repeat multiple times in the genome. Example: GACA repeated five times.
  • Unique to individuals, valuable in forensic science (e.g., CODIS).

Variable Number Tandem Repeats (VNTRs)

  • VNTRs are similar to STRs but their repeat numbers can vary.
  • Useful for distinguishing genetic variations among siblings and determining familial relationships.

Single Nucleotide Polymorphisms (SNPs)

  • SNPs are variations where a single base pair differs between individuals.
  • Important for familial linkage studies and understanding heredity (e.g., 23andMe).

Pseudogenes and Viral DNA

  • Pseudogenes: Genes that resemble functional genes but do not produce protein.
  • Viral DNA: Viral genetic materials that may remain in the human genome, possibly impacting immunological memory.

Transposons and Gene Regulation

  • Transposons can cause mutations by inserting themselves within or near functional genes.

Gene Regulation Mechanisms

  • Pre-transcriptional Control: Mechanisms that regulate gene expression before RNA synthesis occurs.

    • Heterochromatin features: DNA tightly wrapped around histones affecting accessibility.
    • Histone modifications:
    • Acetylation: Makes DNA more accessible by opening chromatin structure (e.g., promotes transcription).
    • Methylation: Closes down chromatin, preventing transcription, often related to epigenetic mechanisms.

  • Transcription Factors (TFs): Proteins that bind to specific DNA sequences to regulate gene transcription. TFs dictate which genes are expressed in specific cells based on cell type and function.

  • Example: Liver cells express genes for albumin but not crystalline.

Post-Transcriptional Control

  • Mechanisms that occur after RNA has been synthesized:
    • RNA Processing: Involves splicing out introns and joining exons, crucial for immune response (e.g., T cell and B cell generation).
    • mRNA Stability: Modifications like adding caps and tails determine mRNA's lifespan in the cytoplasm.
    • siRNAs: Small interfering RNAs that can degrade mRNA, preventing translation.
    • Ubiquitin-Proteasome System: Tags unwanted proteins for degradation (recycling amino acids).
    • MicroRNAs: Similar to siRNAs, regulate gene expression by blocking mRNA translation.

Cell Specialization and Communication

  • Stem cells differentiate into specialized cells through the regulation of gene expression.
  • Cell Communication Types:
    • Gap Junctions: Connect animal cells, facilitating direct communication.
    • Plasmodesmata: Connect plant cells for intercellular signaling.
    • Paracrine Signaling: Local signaling affecting nearby cells (e.g., SRY gene activation).
    • Synaptic Signaling: Neuron signaling using neurotransmitters.
    • Hormonal Signaling: Long-distance signaling via hormones influencing gene expression.

Hormonal Effects on Gene Expression

  • Water-Soluble Hormones (e.g., Insulin):
    • Bind to membrane receptors initiating phosphorylation cascades leading to gene expression.
  • Fat-Soluble Hormones (e.g., Steroid hormones):
    • Pass through cell membranes to activate transcription factors directly in the nucleus.

Mutation Types and Cancer Implication

  • Mutations: Changes in DNA that can affect RNA and proteins; can result in gain or loss of function.
    • Gain of Function: Gene product is overactive (e.g., mutated RAS protein leading to uncontrolled cell division).
    • Loss of Function: Gene product is nonfunctional (e.g., cystic fibrosis mutation).
  • Cancer Regulation: Genes like RAS (promotes cell cycle) and p53 (tumor suppressor) play crucial roles in cancer development.

Epigenetics and its Implications

  • Epigenetics involves reversible modifications of DNA that affect gene expression without altering the DNA sequence (e.g., methylation, acetylation).
  • Epigenetic regulation can influence phenotypes and are affected by environmental factors (e.g., agouti gene in mice).

Immune System Overview

  • Innate Immunity: Non-specific defenses present at birth (e.g., physical barriers, phagocytes).
  • Adaptive Immunity: Acquired immunity developed through exposure or vaccination, involving B cells and T cells.
    • B Cells: Produce antibodies; responsible for the humoral response.
    • T Cells: Cytotoxic cells; target infected or dysfunctional cells.
  • Immunological Memory: Allows for a faster response upon re-exposure to the same pathogen.

Final Thoughts

  • Overall, gene expression regulation is a complex interplay between various genetic, epigenetic, and environmental factors, significantly impacting cellular functions, immunity, and susceptibility to diseases.