Viruses, Bacteria, Archaea, and Plant-Microbe Interactions: Study Notes

Viruses: Structure, Replication, and Classification

Eigenschappen virussen (Properties of Viruses)

Viruses are microscopic infectious agents that require a host cell for replication. They are much smaller than bacteria and can only be seen with an electron microscope.

• Structure: Consist of genetic material (DNA or RNA) enclosed in a protein coat (capsid); some have an additional lipid envelope.

• Obligate intracellular parasites: Can only reproduce inside host cells.

• Host specificity: Viruses infect specific hosts and often specific cell types.

• Capsid and envelope: The capsid protects the genetic material; some viruses have a lipid envelope derived from the host cell membrane.

Virus Replication Cycles

Viruses use host cell machinery to replicate. The replication cycle includes attachment, entry, replication, assembly, and release.

• Lytic cycle: Virus replicates rapidly, leading to cell lysis and release of new virions.

• Lysogenic cycle: Viral genome integrates into host DNA and replicates with it; can later enter the lytic cycle.

Types of Viral Genomes

• DNA viruses: Use host DNA polymerase for replication.

• RNA viruses: Use RNA-dependent RNA polymerase; some require reverse transcription (retroviruses).

• Retroviruses: Contain RNA genome; use reverse transcriptase to make DNA from RNA template (e.g., HIV).

Viral Replication Table

Type	Genome	Replication Enzyme	Example
DNA virus	DNA	Host DNA polymerase	Herpesvirus
RNA virus	RNA	RNA-dependent RNA polymerase	Influenza
Retrovirus	RNA	Reverse transcriptase	HIV

Bacteria: Structure, Genetics, and Physiology

Bacterial Cell Structure

Bacteria are prokaryotic cells with diverse shapes and structures.

• Cell wall: Contains peptidoglycan (Gram-positive: thick layer; Gram-negative: thin layer plus outer membrane).

• Flagella: Used for motility; structure differs from eukaryotic flagella.

• Capsule: Protective outer layer in some bacteria.

• Plasma membrane: Controls transport and maintains cell environment.

Bacterial Genetics and Horizontal Gene Transfer (HGT)

Bacteria can exchange genetic material through several mechanisms, increasing genetic diversity.

• Transformation: Uptake of naked DNA from the environment.

• Transduction: Transfer of DNA by bacteriophages (viruses that infect bacteria).

• Conjugation: Direct transfer of DNA via cell-to-cell contact, often involving plasmids.

Plasmids and Antibiotic Resistance

• Plasmids: Small, circular DNA molecules independent of the bacterial chromosome; often carry antibiotic resistance genes.

• Antibiotic resistance: Can be spread via plasmids and HGT, posing a major challenge in medicine.

Bacterial Cell Division

• Binary fission: Asexual reproduction where one cell divides into two identical daughter cells.

Gram Staining

Gram staining differentiates bacteria based on cell wall structure.

• Gram-positive: Thick peptidoglycan, stains purple.

• Gram-negative: Thin peptidoglycan, outer membrane, stains pink/red.

Archaea: Unique Features and Adaptations

Characteristics of Archaea

• Cell wall: Lacks peptidoglycan; may have S-layer proteins.

• Membrane lipids: Ether-linked, more stable in extreme environments.

• Extremophiles: Many archaea live in extreme conditions (high temperature, salinity, acidity).

Plant-Microbe Interactions

Symbiosis and Pathogenesis

Plants interact with a variety of microorganisms, including beneficial symbionts and harmful pathogens.

• Symbiosis: Mutualistic relationships (e.g., mycorrhizae, nitrogen-fixing bacteria).

• Pathogens: Fungi, bacteria, and viruses can infect plants, causing disease.

Plant Defense Mechanisms

• Physical barriers: Cell wall, cuticle.

• Chemical defenses: Production of antimicrobial compounds.

• Immune responses: Recognition of pathogen-associated molecular patterns (PAMPs) and activation of defense pathways.

Laboratory Techniques

Microscopy

• Light microscopy: Used for observing cell structure.

• Electron microscopy: Higher resolution, used for viruses and detailed cell structures.

PCR (Polymerase Chain Reaction)

PCR is a technique to amplify specific DNA sequences using primers and DNA polymerase.

• Steps: Denaturation, annealing, extension.

• Applications: Detection of pathogens, genetic analysis.

Summary Table: Key Differences Between Bacteria, Archaea, and Viruses

Feature	Bacteria	Archaea	Viruses
Cell type	Prokaryotic	Prokaryotic	Non-cellular
Cell wall	Peptidoglycan	No peptidoglycan	None
Genetic material	DNA	DNA	DNA or RNA
Reproduction	Binary fission	Binary fission	Host-dependent
Environment	Various	Often extreme	Host-dependent

Additional info:

• Some context and terminology were inferred from standard General Biology curriculum to clarify fragmented points.

• Tables and diagrams were recreated in text/HTML format for clarity.