Microbial Genetics: Structure, Function, and Genetic Change

Microbial Genetics

Genome Structure and Organization in Prokaryotes vs. Eukaryotes

Understanding the differences between prokaryotic and eukaryotic cells is fundamental to microbial genetics. These differences influence how genetic information is stored, replicated, and expressed.

• Genome Structure:

  • Prokaryotes: Typically possess a single, circular DNA molecule (chromosome) located in the nucleoid region. Plasmids (small, circular DNA) may also be present.

  • Eukaryotes: Contain multiple, linear chromosomes housed within a membrane-bound nucleus. DNA is associated with histone proteins, forming chromatin.

• Location of Genetic Processes:

  • DNA Replication: Occurs in the cytoplasm of prokaryotes and in the nucleus of eukaryotes.

  • Transcription: Takes place in the cytoplasm (prokaryotes) and nucleus (eukaryotes).

  • Translation: Occurs in the cytoplasm for both, but in eukaryotes, mRNA must be exported from the nucleus to the cytoplasm.

Example: In Escherichia coli (a prokaryote), transcription and translation can occur simultaneously, while in human cells (eukaryotes), these processes are separated by the nuclear envelope.

DNA Replication in Cells

DNA replication is the process by which a cell duplicates its DNA before cell division. It ensures genetic continuity between generations.

• Purpose: To produce two identical copies of the genome for daughter cells.

• Key Steps:

  1. Initiation: Replication begins at specific sites called origins of replication.

  2. Elongation: DNA polymerase synthesizes new DNA strands by adding nucleotides complementary to the template strand.

  3. Termination: Replication ends when the entire molecule has been copied.

• Key Enzymes and Proteins: DNA helicase (unwinds DNA), single-strand binding proteins, primase (synthesizes RNA primers), DNA polymerase, DNA ligase (joins Okazaki fragments).

Equation:

$\text{(dNTP)}_n + dNTP \rightarrow \text{(dNTP)}_{n+1} + PP_i$

Example: In prokaryotes, replication is bidirectional from a single origin; in eukaryotes, multiple origins are used.

Protein Synthesis: Transcription and Translation

Protein synthesis involves two main processes: transcription (DNA to RNA) and translation (RNA to protein). These processes differ between prokaryotes and eukaryotes.

• Transcription: Synthesis of messenger RNA (mRNA) from a DNA template by RNA polymerase.

• Translation: Ribosomes read mRNA to assemble amino acids into a polypeptide chain.

• Prokaryotes vs. Eukaryotes:

  • In prokaryotes, transcription and translation are coupled (occur simultaneously in the cytoplasm).

  • In eukaryotes, transcription occurs in the nucleus, and translation occurs in the cytoplasm; mRNA undergoes processing (capping, polyadenylation, splicing) before translation.

Example: In Bacillus subtilis, ribosomes can begin translating mRNA while it is still being transcribed.

Operons: Inducible and Repressible Systems

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

• Inducible Operons: Usually off but can be turned on (induced) in response to a substrate. Example: lac operon in E. coli is induced by lactose.

• Repressible Operons: Usually on but can be turned off (repressed) when the end product is abundant. Example: trp operon is repressed by tryptophan.

• Key Components:

  • Promoter: Site where RNA polymerase binds.

  • Operator: Regulatory sequence where repressors or activators bind.

  • Structural Genes: Encode proteins with related functions.

  • Regulatory Genes: Encode proteins (repressors/activators) that control operon activity.

Operon Type	Default State	Regulation Mechanism	Example
Inducible	Off	Induced by substrate presence	lac operon
Repressible	On	Repressed by end product	trp operon

Mutations: Types and Effects

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

• Types of Mutations:

  • Point Mutation: Change in a single nucleotide (substitution).

  • Insertion/Deletion (Indel): Addition or loss of one or more nucleotides.

  • Frameshift Mutation: Indels that alter the reading frame of a gene.

  • Silent Mutation: No change in amino acid sequence.

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

  • Nonsense Mutation: Introduces a premature stop codon.

• Immediate Effects: Can range from no effect (silent) to loss of function or gain of new function.

Example: Sickle cell anemia is caused by a missense mutation in the hemoglobin gene (in eukaryotes).

Genetic Transfer: Vertical vs. Horizontal

Genetic information can be transferred from one generation to the next (vertical) or between organisms of the same generation (horizontal), especially in prokaryotes.

• Vertical Gene Transfer: Transmission of genetic material from parent to offspring during reproduction.

• Horizontal Gene Transfer (HGT): Movement of genetic material between organisms other than by descent.

  • Transformation: Uptake of free DNA from the environment.

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

  • Conjugation: Direct transfer of DNA between bacteria via cell-to-cell contact (often involves plasmids).

Transfer Type	Mechanism	Example
Vertical	Parent to offspring	Binary fission in bacteria
Horizontal: Transformation	Uptake of naked DNA	Griffith's experiment with Streptococcus pneumoniae
Horizontal: Transduction	Phage-mediated transfer	Generalized/specialized transduction
Horizontal: Conjugation	Direct cell contact	F plasmid transfer in E. coli

Additional info: Horizontal gene transfer is a major driver of genetic diversity and antibiotic resistance in microbial populations.