Microbial Genetics: Study Guide for Chapter 7 (BIO 2350)

Microbial Genetics

Definitions and Structure of Bacterial Genetic Material

Understanding the organization and function of genetic material in bacteria is fundamental to microbiology. Bacterial cells possess unique genetic structures that differ from those of eukaryotes.

• Nucleoid: The region within a bacterial cell where the chromosome (DNA) is located. Unlike eukaryotes, bacteria lack a membrane-bound nucleus; the nucleoid is an irregularly shaped area containing the cell's genetic material.

• Plasmid: Small, circular, double-stranded DNA molecules separate from the chromosomal DNA. Plasmids often carry genes for antibiotic resistance, virulence factors, or metabolic functions.

• Genome: The complete set of genetic material in an organism, including both chromosomal DNA and plasmids in bacteria.

• 5’ versus 3’ end of nucleic acid: DNA and RNA strands have directionality. The 5’ end has a phosphate group attached to the fifth carbon of the sugar, while the 3’ end has a hydroxyl group attached to the third carbon. DNA synthesis proceeds from the 5’ to 3’ direction.

• Bacterial Genome: Typically consists of a single, circular chromosome located in the nucleoid. Some bacteria may have linear chromosomes or multiple chromosomes.

• Types of Genes Found on Plasmids: Plasmids may carry genes for antibiotic resistance (e.g., bla for beta-lactamase), toxin production, heavy metal resistance, and genes enabling conjugation.

Example: The F plasmid in Escherichia coli enables the cell to transfer genetic material via conjugation.

DNA Replication in E. coli

Bacterial DNA replication is a highly regulated process that ensures faithful transmission of genetic information during cell division.

• Circular Chromosome: Most bacteria, including E. coli, have a single circular chromosome.

• Origin of Replication (oriC): Replication begins at a specific site called the origin of replication.

• Bidirectional Replication: Two replication forks move in opposite directions around the chromosome, allowing rapid duplication.

• Replication Time vs. Division Time: Although DNA replication takes about 38 minutes, E. coli can divide every 30 minutes. This is possible because new rounds of replication begin before the previous round is completed (multi-fork replication).

Example: E. coli initiates DNA replication at multiple origins, enabling overlapping cycles of replication.

Equation:

$\text{DNA synthesis proceeds:} \\ 5' \rightarrow 3'$

Mutation in Bacteria

Mutations are changes in the DNA sequence that can affect bacterial survival and evolution.

• Definition: A mutation is a heritable change in the nucleotide sequence of DNA.

• Frequency: Mutations are rare in individual cells but occur frequently in large populations due to rapid division.

• Outcome: Mutations may be beneficial, neutral, or harmful. Beneficial mutations can confer advantages such as antibiotic resistance.

• Types of Mutation:

  • Spontaneous: Occur naturally due to errors in DNA replication or repair.

  • Induced: Caused by external agents (mutagens) such as chemicals or radiation.

Example: Exposure to UV light can induce mutations by causing thymine dimers.

Ames Test for Identifying Mutagens

The Ames test is a widely used assay to determine the mutagenic potential of chemical compounds.

• Purpose: To assess whether a substance causes mutations in the DNA of bacteria.

• Procedure: Uses a strain of Salmonella that cannot synthesize histidine. The test substance is applied, and the number of colonies that regain the ability to grow without histidine (revertants) is counted. An increase in revertants indicates mutagenicity.

Example: Testing industrial chemicals for mutagenic properties before approval.

Horizontal Gene Transfer (HGT) in Bacteria

Horizontal gene transfer is the movement of genetic material between organisms other than by descent. It is a major mechanism for genetic diversity in bacteria.

• Definition: Transfer of genes from one organism to another, not via parent-offspring inheritance.

• Experimental Evidence: Demonstrated by experiments such as Griffith's transformation and studies using molecular markers (see Figure 8.17 in PPT).

• Replicon: Any DNA molecule capable of replication, such as chromosomes or plasmids.

• Homologous Recombination: Integration of transferred DNA into the recipient genome via sequence similarity.

• Permanent Change: For HGT to permanently alter a population, the transferred DNA must be stably maintained and passed to progeny.

• Is HGT Reproduction? No, HGT is not a form of reproduction; it is a mechanism for genetic exchange.

• Benefits: HGT allows bacteria to acquire new traits rapidly, such as antibiotic resistance or virulence factors.

Comparison of Three Types of Horizontal Gene Transfer

All three mechanisms result in genetic recombination, but differ in their processes and outcomes.

Type	Vector Used	Donor Cell Fate	Cell-to-Cell Contact
Transformation	None	May survive	No
Transduction	Virus (bacteriophage)	Often dies	No
Conjugation	Plasmid	Survives	Yes

Additional info: Inferred from standard microbiology textbooks.

Transformation

Transformation is the uptake of naked DNA from the environment by a competent bacterial cell.

• Griffith’s Experiment: Demonstrated transformation in Streptococcus pneumoniae by showing that non-virulent bacteria could become virulent after exposure to heat-killed virulent cells.

• Competency: The ability of a cell to take up DNA; cells that can do this are termed competent.

• Process: DNA binds to the cell surface, is taken up, and may be integrated into the genome by homologous recombination.

Example: Laboratory transformation of E. coli with plasmids for genetic engineering.

Transduction

Transduction is the transfer of DNA from one bacterium to another via a bacteriophage (virus).

• Lytic Cycle: The bacteriophage infects the cell, replicates, and lyses the host, sometimes packaging host DNA into new phage particles.

• Process:

  1. Making transducing phage: Host DNA is mistakenly packaged into phage capsids.

  2. Transduction: The phage infects a new cell, transferring the DNA.

• Example: E. coli O157:H7 acquired toxin genes via transduction.

Conjugation (Plasmid Transfer Only)

Conjugation is the direct transfer of DNA between two bacterial cells via cell-to-cell contact, typically mediated by a pilus.

• Pilus/Pili: Hair-like appendages used to connect donor and recipient cells.

• Process: The donor cell (F+) forms a pilus, attaches to the recipient (F-), and transfers plasmid DNA.

• Differences: Conjugation requires cell contact, unlike transformation and transduction. The donor cell survives, and only plasmid DNA is transferred (in this context).

Example: Spread of antibiotic resistance genes via conjugative plasmids in hospital settings.

Additional info: Some explanations and table entries were expanded and inferred from standard microbiology textbooks to ensure completeness and clarity.