Cellular Processes in Microorganisms and Viral Genetics: Study Notes for Microbiology

Cellular Processes in Eukaryotic and Prokaryotic Microorganisms

Replication, Transcription, Translation, and Control of Gene Expression

This section covers the fundamental cellular processes in both eukaryotic and prokaryotic microorganisms, focusing on how genetic information is replicated, transcribed, translated, and regulated.

• DNA and RNA Comparison: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are nucleic acids with distinct chemical characteristics. DNA is double-stranded and contains deoxyribose sugar, while RNA is usually single-stranded and contains ribose sugar.

• Location, Structure, and Chromosomes: In prokaryotes, DNA is typically found in a single circular chromosome within the nucleoid region. Eukaryotes have multiple linear chromosomes located in the nucleus.

• Processes of Replication, Transcription, and Translation:

  • Replication: The process by which DNA is copied before cell division.

  • Transcription: The synthesis of RNA from a DNA template.

  • Translation: The process by which ribosomes synthesize proteins using mRNA as a template.

• DNA Replication Differences: Eukaryotic replication involves multiple origins of replication and complex machinery, while prokaryotic replication typically starts at a single origin.

• Transcription and Translation Differences: In prokaryotes, transcription and translation are coupled (occur simultaneously), whereas in eukaryotes, they are separated by the nuclear envelope.

• Gene Expression Regulation: Regulation involves promoters, enhancers, repressors, and transcription factors. In prokaryotes, operons (e.g., lac operon) are common regulatory units.

• mRNA Processing: Eukaryotic mRNA undergoes capping, polyadenylation, and splicing; prokaryotic mRNA does not.

• Genetic Code and Translation: The nucleotide sequence of mRNA is translated into amino acids using the genetic code. For example, the codon AUG codes for methionine.

Example: The lac operon in Escherichia coli regulates genes involved in lactose metabolism and is a classic model for gene regulation in prokaryotes.

Control of Gene Expression in Microorganisms

Inducible and Repressible Genes, Operons, and Regulatory Mechanisms

Gene expression in microorganisms is tightly regulated to respond to environmental changes and cellular needs.

• Inducible Genes: Genes that are expressed only in the presence of specific substrates (e.g., lac operon induced by lactose).

• Repressible Genes: Genes that are turned off when a specific product is abundant (e.g., trp operon repressed by tryptophan).

• Operon Model: An operon consists of structural genes, a promoter, an operator, and regulatory genes. The operator is the site where repressors or activators bind to regulate transcription.

• Inducers and Repressors: Inducers activate gene expression, while repressors inhibit it.

• Catabolite Repression: The presence of a preferred carbon source (e.g., glucose) inhibits the expression of genes involved in the metabolism of other sugars.

Example: The trp operon in bacteria is repressed when tryptophan levels are high, preventing unnecessary synthesis.

Viruses: Structure, Genome, and Replication Cycles

General Characteristics and Replication in Host Cells

Viruses are acellular entities that require host cells for replication. Their structure and genome type influence their replication strategies.

• Typical Size Range and Shapes: Viruses vary in size (20-300 nm) and shape (helical, icosahedral, complex).

• Structural Components: All viruses contain a nucleic acid genome (DNA or RNA) and a protein coat (capsid). Some have an envelope derived from host membranes.

• Replication Steps in Animal Viruses:

  1. Adsorption (attachment to host cell)

  2. Penetration (entry into host cell)

  3. Synthesis (replication of viral genome and proteins)

  4. Assembly (formation of new virions)

  5. Release (exit from host cell)

• Positive Sense vs. Negative Sense: Positive-sense RNA can be directly translated by host ribosomes; negative-sense RNA must be converted to positive-sense before translation.

• Replication Cycles: Lytic cycle results in host cell lysis and release of new viruses; lysogenic cycle involves integration of viral genome into host DNA, with later activation.

Example: Bacteriophage lambda can undergo both lytic and lysogenic cycles in Escherichia coli.

Characteristic	Lytic Cycle	Lysogenic Cycle
Viral Genome Integration	No	Yes
Host Cell Fate	Lysis	Survival (until induction)
Viral Replication	Immediate	Delayed
Example Virus	T4 phage	Lambda phage

Genetic Change in Microbes and Microbial Evolution

Mutations and Their Consequences

Genetic mutations are changes in the DNA sequence that can affect microbial evolution and adaptation.

• Types of Mutations:

  • Silent Mutation: Alters DNA sequence without changing the amino acid.

  • Nonsense Mutation: Converts a codon to a stop codon, truncating the protein.

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

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

• Gene Mutation Analysis: By comparing wild-type and mutant sequences, the type of mutation can be determined.

• Mutation Effects: Mutations can result in functional or non-functional proteins, affecting microbial survival and evolution.

Example: A missense mutation in the beta-globin gene leads to sickle cell anemia.

Mutation Type	DNA Change	Protein Effect
Silent	Base substitution	No change in amino acid
Nonsense	Base substitution	Premature stop codon
Missense	Base substitution	Change in one amino acid
Frameshift	Insertion/deletion	Multiple amino acids changed

Key Equations and Concepts

• Central Dogma of Molecular Biology:

$\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

• Genetic Code Translation:

$\text{Codon} \rightarrow \text{Amino Acid}$

• Mutation Rate:

$\text{Mutation Rate} = \frac{\text{Number of mutations}}{\text{Total number of bases}}$

Additional info: Some explanations and examples have been expanded for clarity and completeness.