Microbiology and Molecular Biology: Prokaryotes, Viruses, and Genetic Mechanisms

Introduction to Microbiology

Overview of Microbiology

Microbiology is the scientific study of microscopic organisms, including bacteria, viruses, fungi, and protozoa. These organisms are fundamental to ecological processes, human health, and various industrial applications.

• Microorganisms are ubiquitous and play essential roles in nutrient cycling, disease, and biotechnology.

• Understanding their structure and function is crucial for advances in medicine, agriculture, and environmental science.

Prokaryotes

Characteristics of Prokaryotes

Prokaryotes are unicellular organisms lacking a nucleus and membrane-bound organelles. They are classified into two domains: Bacteria and Archaea.

• Cell Structure: Simple organization without compartmentalization.

• Genetic Material: DNA is typically a single, circular chromosome located in the nucleoid region.

• Reproduction: Most reproduce by binary fission, a rapid and efficient process.

Cell-Surface Structures

• Capsule: A dense, well-defined polysaccharide or protein layer that protects against desiccation and immune attack; aids in adherence.

• Slime Layer: Loosely attached, easily removed; also aids in adherence.

• Fimbriae: Short, hair-like projections for attachment to surfaces or other cells.

• Pili (Sex Pili): Longer appendages involved in DNA transfer between cells (conjugation).

Motility

• Taxis: Directed movement toward or away from stimuli.

• Chemotaxis: Movement in response to chemical gradients.

• Phototaxis: Movement toward light.

• Magnetotaxis: Movement in response to magnetic fields.

• Aerotaxis: Movement in response to oxygen concentration.

• Flagella: Main organelles for motility; structurally distinct from eukaryotic flagella.

Internal Organization and DNA

• Specialized Membranes: Some prokaryotes have infolded plasma membranes for metabolic functions (e.g., thylakoids in cyanobacteria).

• Plasmids: Small, circular DNA molecules that replicate independently and often carry advantageous genes (e.g., antibiotic resistance).

Genetic Diversity in Prokaryotes

Despite their simplicity, prokaryotes exhibit high genetic diversity due to rapid reproduction, mutation, and genetic recombination.

• Rapid Reproduction: Short generation times increase mutation rates.

• Mutation: Primary source of genetic variation.

• Genetic Recombination: Exchange of DNA between cells via transformation, transduction, and conjugation.

• Horizontal Gene Transfer: Movement of genes between different species.

• F Factor: Plasmid required for conjugation; F+ cells are donors, F− are recipients.

• R Plasmids: Carry antibiotic resistance genes.

Nutritional and Metabolic Adaptations

Prokaryotes are metabolically diverse and can be classified by their energy and carbon sources.

Mode	Energy Source	Carbon Source	Examples
Photoautotroph	Light	CO2, HCO3−	Photosynthetic prokaryotes (e.g., cyanobacteria), plants, some protists
Chemoautotroph	Inorganic chemicals	CO2, HCO3−	Certain prokaryotes (e.g., Sulfolobus)
Photoheterotroph	Light	Organic compounds	Certain aquatic and salt-loving prokaryotes (e.g., Rhodobacter, Chloroflexus)
Chemoheterotroph	Organic compounds	Organic compounds	Many prokaryotes (e.g., Clostridium), fungi, animals, some plants

• Role of Oxygen:

  • Obligate aerobes: Require O2 for respiration.

  • Obligate anaerobes: Poisoned by O2; use fermentation or anaerobic respiration.

  • Facultative anaerobes: Can use O2 or switch to anaerobic metabolism.

Ecological Interactions

• Symbiosis: Close association between two species.

• Mutualism: Both partners benefit.

• Commensalism: One benefits, the other is unaffected.

• Parasitism: One benefits at the expense of the other; disease-causing parasites are called pathogens.

Viruses

Structure and Nature of Viruses

Viruses are non-cellular infectious particles that require a host cell for replication. They are much simpler than cells and exist in a state of 'borrowed life.'

• Virion: The complete, extracellular virus particle.

• Nucleic Acid: DNA or RNA, single- or double-stranded, linear or circular.

• Capsid: Protein shell made of capsomeres; can be helical or icosahedral.

• Envelope: Some viruses have a lipid membrane derived from the host, with viral glycoproteins.

• Bacteriophages (Phages): Viruses that infect bacteria; have a head (capsid) and tail apparatus.

Viral Replication

• Obligate Intracellular Parasites: Can only replicate inside host cells.

• Host Range: Specificity for certain hosts or cell types.

• General Replicative Cycle:

  1. Entry and uncoating of viral genome.

  2. Replication of viral genome and synthesis of viral proteins.

  3. Self-assembly of new virions.

  4. Release of new viruses, often by cell lysis.

Phage Replicative Cycles

• Lytic Cycle: Results in host cell death and release of new phages.

  1. Attachment

  2. Entry of phage DNA and degradation of host DNA

  3. Synthesis of viral components

  4. Assembly of new phages

  5. Release (lysis)

• Lysogenic Cycle: Phage DNA integrates into host genome as a prophage; can later enter lytic cycle.

• Temperate Phages: Can use both cycles.

Bacterial Defenses Against Phages

• Surface Protein Mutations: Prevent phage attachment.

• Restriction Enzymes: Cut foreign DNA.

• Methylation: Protects host DNA from restriction enzymes.

Molecular Basis of Inheritance: Genes to Proteins

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information: DNA → RNA → Protein.

• DNA: Stores genetic information.

• RNA: Acts as a messenger and functional molecule.

• Protein: Performs cellular functions.

DNA Structure

• Discovery: Key experiments by Miescher, Avery, Hershey & Chase, Chargaff, and Franklin led to the double helix model by Watson and Crick.

• Nucleotide: Composed of deoxyribose sugar, phosphate group, and nitrogenous base (A, T, G, C).

• Double Helix: Two antiparallel strands held by hydrogen bonds (A-T: 2 bonds, G-C: 3 bonds).

• Purines: Adenine (A), Guanine (G); Pyrimidines: Cytosine (C), Thymine (T).

• DNA vs. RNA: DNA has deoxyribose and thymine; RNA has ribose and uracil, and is usually single-stranded.

DNA Packaging

• Genome: Complete set of genetic material.

• Prokaryotes: Single, circular chromosome; organized by supercoiling and DNA-binding proteins.

• Eukaryotes: Multiple, linear chromosomes; DNA wrapped around histones to form nucleosomes.

DNA Replication

DNA replication is semi-conservative: each new molecule contains one old and one new strand.

• Initiation: Begins at origins of replication; DNA unwinds.

• Elongation: DNA polymerases synthesize new strands in the 5' → 3' direction.

  • Leading Strand: Synthesized continuously.

  • Lagging Strand: Synthesized in Okazaki fragments.

• Termination: Replication ends when forks meet or at termination sequences.

• Key Enzymes: Helicase, primase, DNA polymerase, ligase, single-strand binding proteins.

• Prokaryotic vs. Eukaryotic Replication: Prokaryotes have a single origin; eukaryotes have multiple origins and more complex regulation.

Gene Expression: Transcription and Translation

Transcription

• Definition: Synthesis of RNA from a DNA template.

• Location: Nucleus (eukaryotes), cytoplasm (prokaryotes).

• Stages: Initiation, elongation, termination.

• RNA Polymerase: Binds promoter, synthesizes RNA.

• RNA Processing (Eukaryotes):

  • 5' Capping: Modified guanine added to 5' end.

  • 3' Poly-A Tail: Adenine nucleotides added to 3' end.

  • RNA Splicing: Removal of introns, joining of exons.

Translation

• Definition: Synthesis of polypeptides from mRNA template.

• Location: Cytoplasm, on ribosomes.

• Key Components:

  • mRNA: Carries genetic code in codons (triplets).

  • tRNA: Brings amino acids; has anticodon complementary to mRNA codon.

  • Ribosomes: rRNA and proteins; sites for translation (A, P, E sites).

• Stages: Initiation, elongation, termination.

• Genetic Code: Universal, redundant, and unambiguous; codons specify amino acids.

Mutations and Their Effects

• Mutation: Change in DNA sequence.

• Types:

  • Substitution: One base replaced by another.

    • Silent: No amino acid change.

    • Missense: Different amino acid.

    • Nonsense: Creates stop codon; premature termination.

  • Insertion/Deletion: Addition or loss of bases; may cause frameshift if not in multiples of three.

Cell Division: Binary Fission and Mitosis

Binary Fission in Prokaryotes

• Definition: Asexual reproduction where one cell divides into two identical cells.

• Process: DNA replication, segregation, and division of cytoplasm.

• Features: No spindle apparatus; rapid and efficient.

The Eukaryotic Cell Cycle

• Interphase: G1 (growth), S (DNA synthesis), G2 (preparation for division).

• Mitotic (M) Phase: Mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Mitosis

• Purpose: Produces two genetically identical daughter nuclei.

• Stages:

  1. Prophase: Chromosomes condense; spindle forms.

  2. Prometaphase: Nuclear envelope breaks down; spindle attaches to kinetochores.

  3. Metaphase: Chromosomes align at metaphase plate.

  4. Anaphase: Sister chromatids separate to opposite poles.

  5. Telophase: Chromosomes decondense; nuclear envelopes reform.

• Cytokinesis: Division of cytoplasm; cleavage furrow in animals, cell plate in plants.

Key Terms and Concepts

• Mitotic Spindle: Microtubule structure that segregates chromosomes.

• Centrosome: Microtubule organizing center.

• Kinetochores: Protein complexes on chromosomes where spindle fibers attach.

Example: Binary Fission vs. Mitosis

• Binary Fission: Prokaryotic, simple, rapid, no spindle apparatus.

• Mitosis: Eukaryotic, complex, involves spindle, multiple stages.

Additional info: The above notes integrate foundational concepts from microbiology and molecular biology, providing a comprehensive overview suitable for introductory college biology courses.