Microbial Genetics: Structure, Function, and Regulation of Genetic Material

Microbial Genetics

Using Microbiology to Discover the Secrets of Life

This section explores foundational experiments and discoveries that revealed how genetic information is stored, transferred, and expressed in microorganisms.

• Gregor Mendel

  • Used pea plants to demonstrate and document the basic patterns of inheritance.

  • Established the principles of segregation and independent assortment.

• Griffith

  • Showed that Streptococcus could transfer hereditary information horizontally.

  • Demonstrated transformation: non-virulent bacteria became virulent when mixed with heat-killed virulent bacteria.

• Avery, MacLeod, McCarty

  • Identified DNA as the macromolecule responsible for hereditary information, building on Griffith's experiment.

  • Used enzymatic treatments to show that only DNA could transform non-virulent bacteria.

• Hershey and Chase

  • Used viruses (bacteriophages) to confirm that DNA, not protein, is the genetic material.

  • Radioactive labeling of DNA and protein showed only DNA entered bacterial cells during infection.

• Chargaff

  • Discovered that the amount of adenine (A) equals thymine (T), and cytosine (C) equals guanine (G) in DNA.

  • Chargaff's rules helped Watson and Crick deduce the double helix structure of DNA.

• Watson/Crick

  • Proposed the double helix structure of DNA based on X-ray crystallography data from Franklin and Wilkins.

Basic Structure of DNA

Components of DNA

DNA is a polymer composed of nucleotides, each consisting of a phosphate group, deoxyribose sugar, and a nitrogenous base.

• Phosphate

• Deoxyribose

• Bases

  • Pyrimidines: Cytosine (C), Thymine (T)

  • Purines: Adenine (A), Guanine (G)

Directionality of DNA

DNA strands have directionality, defined by the 5' and 3' ends of the sugar-phosphate backbone.

• Antiparallel strands: One strand runs 5' to 3', the other 3' to 5'.

• Directionality is crucial during DNA synthesis and replication.

Prokaryote Genome Structure

Organization

Prokaryotic genomes are typically organized as a single, circular chromosome located in the nucleoid region. Genes are often arranged in operons.

• Haploid: One copy of each gene.

• Found in Nucleoid: DNA is not enclosed by a membrane.

• Polycistronic Operons: Multiple genes transcribed together under a single promoter.

Plasmids

Plasmids are small, circular DNA molecules that replicate independently of the chromosome. They often carry genes for specialized functions.

• Types of genes found on plasmids:

  • Fertility factors: Encode for conjugation pili.

  • Resistance genes: Provide antibiotic resistance.

  • Bacteriocin genes: Encode proteins that kill other bacteria.

  • Virulence factors: Enhance pathogenicity.

DNA Replication

Semiconservative Replication

DNA replication is semiconservative, meaning each new DNA molecule consists of one old strand and one newly synthesized strand.

• Experiment using N15 and N14 isotopes proved this model.

Directionality of DNA Synthesis

DNA polymerase can only synthesize DNA in the 5' to 3' direction.

• Nucleotides are added as triphosphates, providing energy for polymerization.

Replication in Prokaryotes

• Replication begins at a single origin of replication.

• Multiple enzymes are involved, each with specific roles (e.g., helicase, primase, DNA polymerase).

• Leading and lagging strands are synthesized differently.

Methylation

Methylation serves multiple functions, including regulation and protection from restriction enzymes. DNA bases that are methylated differ between prokaryotes and eukaryotes.

DNA Transcription

Process Overview

Transcription is the process by which mRNA is synthesized from a DNA template, enabling protein synthesis.

• Stages:

  1. Initiation

  2. Elongation

  3. Termination

• Directionality: Nucleotides are added in the triphosphate form.

• Termination can be rho-dependent or rho-independent.

Transcription in Eukaryotes

• Occurs in the nucleus.

• Includes 5' capping, polyadenylation, and splicing (removal of introns).

Translation

Process Overview

Translation is the process by which ribosomes synthesize proteins using mRNA as a template. In prokaryotes, translation can occur simultaneously with transcription.

• Stages:

  1. Initiation: Ribosome binds to the Shine-Dalgarno sequence; methionine tRNA binds to start codon.

  2. Elongation: Ribosome has three sites (A, P, E); tRNA brings amino acids via complementary base pairing.

  3. Termination: Stop codon recruits termination factor; ribosome dissociates.

Regulation of Transcription and Translation

Organization of Prokaryotic Genome

• Polycistronic operons: Multiple genes under one promoter.

• Promoter, operator, and structural genes are key elements.

lac Operon

The lac operon is an inducible operon that uses both inducers and inhibitors to regulate transcription in response to environmental conditions.

• Inducers (e.g., allolactose) activate transcription.

• Catabolite repression: Glucose inhibits lac operon transcription.

trp Operon

The trp operon is a repressible operon that is turned off in the presence of tryptophan.

Riboswitch

Three-dimensional shapes of mRNA can influence the amount of translation/transcription that occurs.

• Attenuation: Example is trp attenuation, where transcription is prematurely terminated.

Mutations

Types of Mutations

• Point mutations: Affect a single base pair.

  • Silent: No change in amino acid.

  • Missense: Changes one amino acid.

  • Nonsense: Creates a stop codon.

• Insertions/deletions: Disrupt the ribosomal reading frame (frameshift mutations).

Horizontal Gene Transfer

Mechanisms

• Transformation: Uptake of naked DNA from the environment.

• Transduction: Transfer of DNA via bacteriophages.

• Conjugation: Direct transfer of DNA between bacteria via pilus.

Key Table: Types of Plasmid Genes

This table summarizes the main types of genes found on plasmids and their functions.

Type of Plasmid Gene	Function
Fertility Factor	Encodes conjugation pili for DNA transfer
Resistance Gene	Provides antibiotic resistance
Bacteriocin Gene	Produces proteins that kill other bacteria
Virulence Factor	Enhances pathogenicity

Key Equations

• Chargaff's Rule: $[A] = [T]$ $[C] = [G]$

• Directionality of DNA Synthesis: $\text{DNA Polymerase adds nucleotides in the 5' \rightarrow 3' direction}$

Additional info: Some explanations and context have been expanded for clarity and completeness, including details on classic experiments, operon regulation, and mutation types.