Chapter 7: DNA

DNA Replication, Transcription, and Translation

These fundamental processes are central to genetic information flow in all cells. Understanding their mechanisms is essential for microbiology.

• DNA Replication: The process by which a cell copies its DNA before cell division. Key enzymes include DNA polymerase, helicase, and primase.

• Transcription: The synthesis of RNA from a DNA template, catalyzed by RNA polymerase.

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

• Example: In Escherichia coli, DNA replication occurs bidirectionally from the origin of replication (oriC).

Plasmid Structure and Replication

Plasmids are extrachromosomal DNA elements found in bacteria, often carrying beneficial genes.

• ori sites: Origin of replication, where DNA synthesis begins.

• Types of genes present: Commonly include antibiotic resistance, nutrient biosynthesis, and virulence factors.

• Replication mechanisms: Rolling circle replication produces single-stranded DNA intermediates; bidirectional replication involves two replication forks moving in opposite directions.

• Example: The F plasmid in E. coli uses rolling circle replication during conjugation.

Chapter 8: Gene Expression

mRNA Function and Bacterial Promoters

Messenger RNA (mRNA) carries genetic information from DNA to ribosomes for protein synthesis.

• Bacterial promoters: DNA sequences where RNA polymerase binds to initiate transcription. Key regions are the -35 and -10 consensus sequences.

• Example: The -10 (Pribnow box) sequence is typically TATAAT.

Transcription Termination

Transcription ends by two mechanisms in bacteria:

• Rho-dependent termination: Requires the Rho protein to release the RNA transcript.

• Rho-independent termination: Relies on a GC-rich hairpin followed by a poly-U sequence in the RNA.

Polysomes

A polysome is a complex of multiple ribosomes translating a single mRNA simultaneously, increasing efficiency of protein synthesis.

Chapter 9: Genetic Variation

Genetic Code and Gene Structure

The genetic code is a set of rules by which nucleotide sequences are translated into amino acids.

• Reading the Genetic Code: Each codon (three nucleotides) specifies an amino acid.

• Gene parts: Includes promoter, coding region, and terminator.

• Translation: The coding region is translated into protein.

Mutation Types

Mutations are changes in DNA sequence. Types include:

• Point mutations: Single nucleotide changes (missense, nonsense, silent).

• Insertions/Deletions: Addition or removal of nucleotides, potentially causing frameshifts.

• Example: A missense mutation changes one amino acid in a protein.

Ames Test

The Ames test assesses the mutagenic potential of compounds using bacteria.

• Principle: Measures the rate of mutation in a strain of Salmonella unable to synthesize histidine.

Gene Transfer in Bacteria

Bacteria transfer genes by three main mechanisms:

• Transformation: Uptake of naked DNA from the environment.

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

• Transduction: Transfer of DNA by bacteriophages.

Discovery of Conjugation

Dr. Lederberg mixed two mutant strains of bacteria, which could not grow alone, but grew together, demonstrating gene transfer.

• Explanation: Complementation of mutations via DNA exchange enabled growth.

RecA and Nanotubes

• RecA: Protein essential for homologous recombination during transformation.

• Nanotubes: Structures that facilitate direct transfer of cytoplasmic contents between cells.

F Plasmid Concepts

• Pilus encoding: The tra genes of the F plasmid encode the pilus.

• Insertion sequence: Short DNA sequences that can move within the genome, facilitating integration of plasmids.

• Replication origin: DNA replication begins at the oriT site during conjugation.

• Experimental verification: Use of selectable markers and growth on selective media to confirm DNA transfer.

• F- cell changes: Conjugation with Hfr or F+ cells can convert F- cells to F+ or introduce chromosomal genes.

• F' plasmid: An F plasmid carrying additional chromosomal genes due to improper excision.

Transduction

• Generalized transduction: Any bacterial gene can be transferred by a phage.

• Specialized transduction: Only specific genes near the prophage insertion site are transferred.

CRISPR Defense Mechanism

CRISPR systems provide adaptive immunity against foreign DNA in bacteria.

• Mechanism: Incorporation of foreign DNA sequences (spacers) into CRISPR loci, guiding Cas proteins to target and cleave matching DNA.

2-Component Mechanism, Chemotaxis, and Quorum Sensing

• 2-component mechanism: Signal transduction system involving a sensor kinase and a response regulator.

• Chemotaxis: Movement of cells toward or away from chemical stimuli.

• Quorum sensing: Regulation of gene expression in response to cell density.

Chapter 10: Gene Regulation

2-Component Signal Transduction System

This system allows bacteria to sense and respond to environmental changes.

• Sensor kinase: Detects environmental signals and autophosphorylates.

• Response regulator: Receives phosphate and modulates gene expression.

Operon Regulation: lac, trp, and ara Operons

Operons are clusters of genes regulated together.

• lac operon: Inducible; activated in presence of lactose.

• trp operon: Repressible; turned off when tryptophan is abundant.

• ara operon: Regulated by arabinose; can be both activated and repressed.

Riboswitches and Regulatory RNAs

• Riboswitches: RNA elements in mRNA that bind metabolites and regulate gene expression.

• sRNA vs asRNA: sRNA (small RNA) regulates gene expression post-transcriptionally; asRNA (antisense RNA) binds complementary mRNA to inhibit translation.

Chemotaxis Mechanism

Bacterial movement is controlled by flagellar rotation, which is affected by ligand binding to methyl-accepting chemotaxis proteins (MCPs).

• Ligand binding: Alters MCP conformation, affecting flagellar motor direction.

Chapter 11: Viral Genetics

Phage Lambda Structure and Replication

Phage lambda is a model bacteriophage with well-studied genetics.

• Structure: Contains head, tail, and tail fibers.

• Ori site: Origin of replication.

• cos site: Cohesive ends for DNA packaging.

• Rolling circle replication: Produces long concatemers of DNA; differs from conventional rolling circle by generating linear DNA for packaging.

Lysogeny vs Lytic Cycle

• Lysogeny: Phage DNA integrates into host genome and is replicated passively.

• Lytic cycle: Phage replicates, assembles, and lyses host cell.

• Genetic mechanisms: Controlled by regulatory proteins (e.g., cI repressor).

Influenza Structure and Replication

Influenza virus is an enveloped virus with a segmented RNA genome.

• Structure: Contains hemagglutinin and neuraminidase proteins.

• Genome: Segmented, negative-sense RNA.

• Replication cycle: Entry, uncoating, replication in nucleus, assembly, and release.

Summary Table: Bacterial Gene Transfer Mechanisms

Summary Table: Operon Regulation

Key Equations

• Central Dogma:

• Genetic Code Translation:

Additional info: Academic context was added to clarify mechanisms, gene regulation, and viral genetics based on standard microbiology textbooks.