Microbial Genetics: Core Concepts and Mechanisms

Microbial Genetics

Genetics: The Big Picture

Microbial genetics is the study of how genes function, are regulated, and are inherited in microorganisms. Understanding these principles is essential for exploring microbial physiology, adaptation, and evolution.

• The science of heredity: Genetics explains how traits are passed from one generation to the next.

• Central dogma of molecular biology: Describes the flow of genetic information from DNA to RNA to protein.

• Gene regulation: Genes are controlled by various mechanisms to ensure proper expression.

• Expression of functional genes: Genes are expressed to produce proteins and enzymes necessary for cellular function.

• Mutation and recombination: Changes in genetic material can lead to variation and evolution.

• Applications: Includes biotechnology, medicine, and research.

Terminology

Key genetic terms are foundational for understanding microbial genetics.

• Gene: A segment of DNA that encodes information for a specific trait or function.

• Genome: The complete set of genetic material in an organism.

• Genotype: The genetic makeup of an organism.

• Phenotype: The observable characteristics resulting from gene expression.

• Chromosome: A DNA molecule containing part or all of the genetic material of an organism.

• Plasmid: Small, circular DNA molecules found in bacteria, often carrying accessory genes.

Structure and Function of Genetic Material

Central Dogma

The central dogma describes the flow of genetic information within a cell.

• DNA: Stores genetic information.

• RNA: Acts as a messenger and functional molecule.

• Protein: Performs cellular functions.

Process: DNA is transcribed into RNA, which is then translated into protein.

DNA Structure

DNA is a double helix composed of nucleotides.

• Nucleotides: Consist of a phosphate group, deoxyribose sugar, and nitrogenous base (A, T, C, G).

• Base pairing: Adenine pairs with Thymine, Cytosine pairs with Guanine.

• Antiparallel strands: DNA strands run in opposite directions (5' to 3' and 3' to 5').

DNA Replication

Mechanism of Replication

DNA replication is the process by which a cell duplicates its DNA before cell division.

• Semiconservative replication: Each new DNA molecule consists of one old strand and one new strand.

• Initiation: Begins at the origin of replication.

• Enzymes involved: DNA polymerase synthesizes new DNA; helicase unwinds the helix; primase synthesizes RNA primers.

• Leading and lagging strands: Leading strand is synthesized continuously; lagging strand is synthesized in Okazaki fragments.

Equation:

$\text{DNA}_{\text{parent}} \xrightarrow{\text{Replication}} \text{DNA}_{\text{daughter}}$

RNA and Protein Synthesis

Types of RNA

RNA molecules play various roles in protein synthesis.

• Messenger RNA (mRNA): Carries genetic information from DNA to ribosomes.

• Transfer RNA (tRNA): Brings amino acids to the ribosome during translation.

• Ribosomal RNA (rRNA): Forms the core of ribosome structure and catalyzes protein synthesis.

Transcription

Transcription is the synthesis of RNA from a DNA template.

• Initiation: RNA polymerase binds to the promoter region of DNA.

• Elongation: RNA polymerase synthesizes a complementary RNA strand.

• Termination: Transcription stops when a terminator sequence is reached.

Equation:

$\text{DNA} \xrightarrow{\text{Transcription}} \text{RNA}$

Translation

Translation is the process by which mRNA is decoded to synthesize proteins.

• Initiation: Ribosome assembles at the start codon of mRNA.

• Elongation: tRNA brings amino acids, which are joined to form a polypeptide chain.

• Termination: Occurs when a stop codon is reached.

• Genetic code: Triplet codons in mRNA specify amino acids.

Equation:

$\text{RNA} \xrightarrow{\text{Translation}} \text{Protein}$

Transcription in Eukaryotes

Differences from Prokaryotes

Transcription and translation are spatially and temporally separated in eukaryotes.

• Transcription occurs in the nucleus; translation occurs in the cytoplasm.

• RNA processing (splicing, capping, polyadenylation) occurs before translation.

• Exons are coding regions; introns are non-coding and are removed.

Regulation of Bacterial Gene Expression

Gene Regulation Mechanisms

Bacteria regulate gene expression to adapt to environmental changes and conserve resources.

• Constitutive genes: Expressed continuously.

• Inducible genes: Expressed only when needed.

• Repressible genes: Expression can be turned off.

Pre-Transcriptional Control

Gene expression can be regulated before transcription begins.

• Regulatory proteins bind DNA to activate or repress transcription.

• Operons are clusters of genes regulated together.

Operon Models

Operons are genetic units that control the expression of multiple genes in bacteria.

• Inducible operon (e.g., lac operon): Activated in the presence of an inducer (e.g., lactose).

• Repressible operon (e.g., trp operon): Repressed in the presence of a corepressor (e.g., tryptophan).

Operon Type	Regulation	Example
Inducible	Turned on by substrate	lac operon
Repressible	Turned off by product	trp operon

Positive Regulation

Some operons require activator proteins for transcription.

• Catabolite activator protein (CAP): Binds to DNA in the presence of cAMP to enhance transcription.

• cAMP: Cyclic AMP levels increase when glucose is low, promoting expression of certain operons.

Mutation

Types and Effects of Mutation

Mutations are changes in the genetic material that can affect phenotype and genotype.

• Base substitution: Replacement of one nucleotide with another.

• Frameshift mutation: Insertion or deletion of nucleotides that alters the reading frame.

• Spontaneous mutation: Occurs naturally without mutagen exposure.

• Induced mutation: Caused by mutagens such as chemicals or radiation.

Mutation Type	Description	Effect
Base substitution	Single nucleotide change	May be silent, missense, or nonsense
Frameshift	Insertion/deletion	Alters downstream amino acid sequence

Consequences of Mutation

• Can be beneficial, neutral, or harmful.

• Source of genetic diversity and evolution.

Example: Antibiotic resistance in bacteria often arises from mutations in genes encoding drug targets.

Additional info: These notes expand on the original slides by providing definitions, examples, and context for each topic, ensuring a comprehensive overview suitable for college-level microbiology students.