Overview
This lecture covers structural and functional features of prokaryotic cells, mechanisms of bacterial growth, and details of the viral life cycle, focusing on distinctions relevant for the MCAT.
Bacterial Cell Structure
- Prokaryotes lack membrane-bound organelles and a true nucleus.
- The cell wall provides structural support and prevents cell lysis.
- Gram-positive bacteria have thick peptidoglycan walls and stain purple with Gram stain.
- Gram-negative bacteria have thin peptidoglycan, an outer membrane, and stain pink/red.
- The plasma membrane controls entry and exit of substances.
- The capsule (if present) helps bacteria evade the immune system.
Bacterial Flagella and Motility
- Flagella are used for movement and chemotaxis (movement toward/away from stimuli).
- The basal body anchors the flagellum and acts as a motor.
- The hook connects the basal body to the filament, enabling motion.
Internal Structures of Prokaryotes
- The nucleoid region contains the single, circular DNA chromosome.
- Plasmids are extra-chromosomal DNA, often carrying antibiotic resistance genes.
- Ribosomes (70S in prokaryotes) synthesize proteins, but differ from eukaryotic ribosomes.
Bacterial Growth and Reproduction
- Binary fission is the asexual method of reproduction in bacteria.
- Bacterial growth follows four phases: lag, log (exponential), stationary, and death.
- Genetic diversity can arise by transformation, transduction, or conjugation.
Viral Structure and Life Cycle
- Viruses are non-living, acellular parasites needing host cells to replicate.
- Viral structure includes genetic material (DNA or RNA), a protein capsid, and sometimes a lipid envelope.
- The lytic cycle destroys the host cell by releasing new viral particles.
- The lysogenic cycle integrates viral DNA into the host genome, replicating with host cell division.
Key Terms & Definitions
- Prokaryote — Cell without a nucleus or membrane-bound organelles.
- Peptidoglycan — Mesh-like polymer forming bacterial cell walls.
- Binary Fission — Asexual reproduction where one cell splits into two identical cells.
- Plasmid — Small circular DNA in bacteria separate from chromosomal DNA.
- Capsid — Protein shell of a virus enclosing its genetic material.
- Lytic Cycle — Viral replication that culminates in host cell lysis.
- Lysogenic Cycle — Viral DNA integrates into host genome, remaining dormant before activation.
Action Items / Next Steps
- Review diagrams of prokaryotic cell structures and viral life cycles.
- Memorize differences between Gram-positive and Gram-negative bacteria.
- Complete assigned reading on bacterial genetics and viral replication (Kaplan MCAT, Chapter 1).