Microbiology Study Guide: DNA Replication, Viruses, and Pathogenicity Mechanisms

DNA Replication and Gene Expression

DNA Replication Mechanisms

DNA replication is the process by which a cell duplicates its DNA before cell division. It is essential for genetic continuity and occurs in a semiconservative manner, meaning each new DNA molecule contains one original and one new strand.

• Semiconservative Replication: Each daughter DNA molecule consists of one parental and one newly synthesized strand.

• Leading vs. Lagging Strand Synthesis: The leading strand is synthesized continuously, while the lagging strand is synthesized in short fragments called Okazaki fragments.

• Key Enzymes:

  • DNA Polymerase: Synthesizes new DNA strands by adding nucleotides.

  • Ligase: Joins Okazaki fragments on the lagging strand.

  • Primase: Synthesizes RNA primers needed to start DNA synthesis.

  • Helicase: Unwinds the DNA double helix.

  • Gyrase/Topoisomerase: Relieves supercoiling ahead of the replication fork.

• Replication Direction: DNA synthesis always proceeds in the 5' to 3' direction.

Example: In Escherichia coli, DNA replication begins at a single origin and proceeds bidirectionally.

Gene Expression: Transcription and Translation

Gene expression involves two main processes: transcription (DNA to RNA) and translation (RNA to protein).

• Transcription: DNA is used as a template to synthesize messenger RNA (mRNA) by RNA polymerase.

• Translation: mRNA is decoded by ribosomes to assemble amino acids into a polypeptide chain. Transfer RNA (tRNA) brings amino acids, and codon-anticodon pairing ensures correct sequence.

• Central Dogma: The flow of genetic information is summarized as: $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

Example: The codon AUG on mRNA pairs with the anticodon UAC on tRNA, inserting methionine into the growing protein.

Mutations and Their Effects

Mutations are changes in the DNA sequence that can affect protein function and phenotype.

• Silent Mutation: No change in amino acid sequence.

• Missense Mutation: Changes one amino acid in the protein.

• Nonsense Mutation: Introduces a premature stop codon.

• Frameshift Mutation: Insertion or deletion shifts the reading frame, altering downstream amino acids.

Example: Changing the codon GAG (glutamic acid) to GUG (valine) is a missense mutation, as seen in sickle cell anemia.

Horizontal Gene Transfer in Prokaryotes

Horizontal gene transfer allows bacteria to acquire new genetic traits from other organisms, contributing to genetic diversity and antibiotic resistance.

• Transformation: Uptake of naked DNA from the environment.

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

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

Example: The spread of antibiotic resistance genes among bacteria often occurs via conjugation.

Operons and Gene Regulation

Operons are clusters of genes under the control of a single promoter, allowing coordinated expression in prokaryotes.

• Inducible Operons: Usually off, turned on in response to a substrate (e.g., lac operon).

• Repressible Operons: Usually on, turned off when the end product is abundant (e.g., trp operon).

Example: The lac operon is induced in the presence of lactose, allowing E. coli to metabolize it.

Viruses: Structure, Classification, and Replication

Virus Structure and Classification

Viruses are acellular infectious agents composed of genetic material (DNA or RNA), a protein coat (capsid), and sometimes a lipid envelope with spikes.

• Capsid: Protein shell that encases the viral genome.

• Envelope: Lipid membrane derived from host cell, present in some viruses.

• Spikes: Glycoproteins for attachment to host cells.

• Classification: Based on nucleic acid type (DNA or RNA), strandedness (single or double), and presence of envelope.

Example: Influenza virus is an enveloped, single-stranded RNA virus.

Viral Replication Cycles

Viruses replicate by hijacking host cell machinery. Two main cycles are observed in bacteriophages:

• Lytic Cycle: Virus replicates rapidly, lyses host cell, and releases new virions.

• Lysogenic Cycle: Viral genome integrates into host DNA as a prophage, replicates with host, and can later enter the lytic cycle.

Animal Virus Replication Stages:

1. Attachment

2. Penetration

3. Uncoating

4. Biosynthesis

5. Maturation

6. Release

Example: HIV is a retrovirus that uses reverse transcriptase to convert its RNA genome into DNA.

Retroviruses and Reverse Transcriptase

Retroviruses are RNA viruses that replicate through a DNA intermediate using the enzyme reverse transcriptase.

• Reverse Transcriptase: Synthesizes DNA from an RNA template.

• Integration: Viral DNA integrates into host genome, forming a provirus.

Example: HIV uses reverse transcriptase to establish persistent infections.

Effects of Viral Infection on Host Cells

Viral infections can cause various cytopathic effects (CPE) in host cells.

• Cell Death (Lysis): Destruction of host cell.

• Syncytium Formation: Fusion of infected cells into multinucleated giant cells.

• Inclusion Bodies: Aggregates of viral particles or altered host cell components.

Example: Rabies virus causes Negri bodies (inclusion bodies) in neurons.

Exotoxins vs. Endotoxins

Bacterial toxins contribute to pathogenicity and can be classified as exotoxins or endotoxins.

Feature	Exotoxins	Endotoxins
Source	Mostly Gram-positive bacteria	Gram-negative bacteria (outer membrane)
Chemical Nature	Proteins	Lipopolysaccharide (LPS)
Heat Stability	Unstable (destroyed by heat)	Stable (withstands heat)
Potency	Very potent, specific effects	Less potent, general effects (fever, shock)

Example: Tetanus toxin (exotoxin) vs. LPS from Escherichia coli (endotoxin).

Mechanisms of Pathogenicity

Portals of Entry and Virulence Factors

Pathogens enter the host through specific portals and use virulence factors to establish infection.

• Portals of Entry: Mucous membranes (respiratory, gastrointestinal, urogenital), skin, parenteral route (injury).

• Virulence Factors:

  • Adhesins: Surface molecules for attachment (e.g., fimbriae, spikes).

  • Capsules: Prevent phagocytosis by host immune cells.

  • Enzymes: Coagulase (clots blood), collagenase (breaks down collagen), proteases.

  • Antigenic Variation: Alteration of surface proteins to evade immune response.

Example: Streptococcus pneumoniae uses a capsule to avoid phagocytosis.

Measuring Virulence: LD50 and ID50

Virulence is quantified by the infectious dose (ID50) and lethal dose (LD50).

• ID50: Dose required to infect 50% of a population.

• LD50: Dose required to kill 50% of a population.

Example: If Pathogen A has LD50 = 100 and Pathogen B has LD50 = 10,000, Pathogen A is more virulent.

Host Cell Damage and Immune Evasion

Pathogens damage host cells directly (toxins, enzymes) or indirectly (immune response modulation).

• Cytopathic Effects: Cell lysis, syncytia, inclusion bodies.

• Immune Modulation: Suppression or evasion of host immune responses.

Example: HIV depletes CD4+ T cells, weakening the immune system.

Comparisons and Application Scenarios

DNA vs. RNA Viruses

Feature	DNA Viruses	RNA Viruses
Genome	DNA (single or double stranded)	RNA (single or double stranded)
Replication Site	Nucleus (usually)	Cytoplasm (usually)
Mutation Rate	Lower	Higher
Examples	Herpesviruses, Adenoviruses	Influenza, HIV

Enveloped vs. Naked Viruses

Feature	Enveloped Viruses	Naked Viruses
Structure	Lipid envelope + capsid	Capsid only
Stability	Less stable (sensitive to detergents, drying)	More stable (resistant to harsh conditions)
Release	Budding (host cell may survive)	Cell lysis (host cell destroyed)
Examples	HIV, Influenza	Poliovirus, Adenovirus

Practice Application: Mutation Analysis

Given a codon change from GAG (glutamic acid) to GUG (valine), this is a missense mutation. The result may be altered protein function, as seen in diseases like sickle cell anemia.

Practice Application: Pathogen Strategy

Pathogens with capsules are more resistant to phagocytosis than those without, giving them a survival advantage in the host.

Practice Application: Virulence Comparison

If Pathogen A has LD50 = 100 and Pathogen B has LD50 = 10,000, Pathogen A is more virulent because a lower dose is required to cause death in 50% of the population.

Summary Table: Horizontal Gene Transfer Mechanisms

Mechanism	Description	Key Features
Transformation	Uptake of naked DNA from environment	Competent cells, no cell-to-cell contact
Conjugation	Direct DNA transfer via pilus	Requires cell-to-cell contact, plasmids
Transduction	DNA transfer by bacteriophage	Generalized or specialized, virus-mediated

Study Tips and Application Skills

• Draw and label the Central Dogma pathway: DNA → RNA → Protein.

• Practice matching enzymes to their roles in DNA replication and gene expression.

• Compare and contrast lytic vs. lysogenic cycles, exotoxins vs. endotoxins, and DNA vs. RNA viruses.

• Use flow diagrams and side-by-side tables for quick comparisons.

• Understand the mechanisms behind each process, not just the facts.

Additional info: Some content was inferred and expanded for clarity and completeness, including detailed tables and examples for application scenarios.