Study Notes on Genetics, Operons, and Horizontal Gene Transfer

Grading and Exam Information

  • Exams are nearly graded, but not finished in time for today's session.
  • Grades will be released this afternoon, after one more exam is graded.

Topic Overview: Probiotics and Genetics

  • The session will transition to discussing probiotics after genetics is covered.
  • Initial focus is on genetics, specifically operons, as a foundation for understanding probiotics.

Operons

Definition

  • An operon is a functioning unit of genomic DNA containing a cluster of genes under the control of a single promoter.

Types of Operons

Repressible Operons
  • Can be switched off by the presence of a specific metabolite.
  • Feedback Inhibition: The end product acts as a co-repressor that binds to the repressor protein, which can then bind to the operator and stop transcription.
  • Example: TRP (Tryptophan) Operon
    • Operon is active when tryptophan levels are low and inactive when tryptophan levels are high.
    • The repressor protein is synthesized in an inactive form and is activated by binding to tryptophan, leading to transcription cessation when tryptophan is abundant.
Inducible Operons
  • Need to be activated or induced; typically initiated by a small molecule (inducer) that binds to the repressor, preventing it from blocking transcription.
  • Example: Lac (Lactose) Operon
    • Contains conditions needing to be met for transcription, primarily the presence of lactose (which is converted to allolactose that acts as an inducer) and absence of glucose.
  • Two conditions required for transcription to occur:
    1. Presence of lactose (or its derivative) where the operator is free to allow transcription.
    2. Absence of glucose, which can be detected by the presence of cyclic AMP (cAMP) that binds CAP (Catabolite Activator Protein) to the CAP binding site to facilitate transcription.
Differences between Repressible and Inducible Operons
  • Repressible Operons: Generally involved in anabolic processes such as amino acid synthesis.
  • Inducible Operons: Usually involved in catabolic processes, such as lactose metabolism.

Active Studying Techniques

  • Drawing operons to visually comprehend their structures and the different conditions influencing them can enhance understanding.
  • This method is encouraged as more effective than rereading notes, fostering deeper learning.

Horizontal Gene Transfer

Importance

  • Major mechanism for generating diversity in bacteria, alongside mutations.

Types of Horizontal Gene Transfer

1. Conjugation
  • Direct transfer of DNA between bacteria through specialized structures called sex pili (found in Gram-negative bacteria).
  • Involves F+ cells (donor, has F factor) transferring plasmids to F- cells (recipient, no F factor).
  • Occasionally results in the transfer of chromosomal DNA when F factor integrates into the host genome, creating HFR (High-Frequency Recombinant) cells.
2. Transformation
  • Uptake of naked DNA by bacteria from their environment.
  • Not all bacteria are naturally competent (able to take up DNA); E. coli, for example, is not, and requires laboratory techniques to facilitate DNA uptake.
  • Historically demonstrated by Griffith's experiment involving Staphylococcus pneumoniae, showcasing how dead cells can transfer genetic information to living cells, resulting in transformed phenotypes.
3. Transduction
  • Involves bacteriophages (viruses that infect bacteria) transferring genetic material between cells.
  • Can be generalized (random bacterial DNA is packaged) or specialized (certain bacterial gene sequences are specifically targeted).

Plasmids and Transposons

Plasmids
  • Small, circular, double-stranded DNA molecules that carry genes capable of conferring advantages like antibiotic resistance.
  • Plasmids replicate independently of chromosomal DNA and can carry important genes such as F factors (for conjugation) and R factors (for antibiotic resistance).
Transposons
  • Movable DNA elements, often referred to as jumping genes, that can replicate and insert themselves into various locations within the genome.
  • Can carry genes that confer advantageous traits, such as antibiotic resistance, impacting cellular survival and virulence.

Classification of Bacteria

Historical Context

  • LINNAEUS: Originally classified living organisms into two kingdoms: Plant and Animal.
  • With advancements in microbiology, WOESE proposed three domains:
    1. Eukarya
    2. Bacteria
    3. Archaea

Species Definition in Bacteria

  • For sexually reproducing organisms, the concept of a species is straightforward; however, for bacteria, it is defined historically either phenotypically (observable traits) or phylogenetically (DNA similarity).
  • DNA homology of 70% among species using overall genomic comparison, while a specific 16S ribosomal RNA gene must show a 97% similarity for classification.

Tools for Identification

Morphology
  • Classification based on cell shape: coccus (spherical), bacillus (rod-shaped), vibrio (comma-shaped), and spirilla (spiral).
Molecular Methods
  • Phylogenetic methods, such as ribosomal RNA gene sequencing and PCR, help classify and identify bacterial species by examining DNA sequences.
Biochemical Tests
  • Assessing the presence of specific enzymes can provide insight into metabolic capabilities, assisting in the identification process.
MALDI-TOF
  • A method for identifying bacterial species based on protein fingerprints created through mass spectrometry.
Serology
  • Uses antibodies in blood to identify specific microbial infections based on antigen-antibody interactions, demonstrating high specificity for pathogen identification.

Questions & Clarifications

  • The session concluded with a recap of various concepts. Students were encouraged to draw operons and study horizontal gene transfer mechanisms as preparation for upcoming exams and lab work.