Microbial Genetics: DNA Replication, Transcription, Translation, Regulation, Mutations, and Recombination

Chapter 11: Microbial Genetics

Overview

This chapter covers the fundamental processes and mechanisms of microbial genetics, including DNA replication, transcription, translation, gene regulation via operons, mutations, and genetic recombination. These topics are essential for understanding how genetic information is maintained, expressed, and altered in microorganisms.

DNA Replication

Semiconservative Replication

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

• Definition: Each daughter DNA molecule retains one original (parental) strand and one new strand.

• Example: After replication, two DNA molecules are formed, each with one old and one new strand.

Origin of Replication

Replication begins at a specific site called the origin of replication, which is typically AT-rich in prokaryotes.

• AT-rich regions: Easier to separate due to fewer hydrogen bonds between A-T pairs.

• Replication forks: Two replication forks form, allowing bidirectional synthesis.

Enzymes and Steps in DNA Replication

• Helicase: Unzips the DNA helix by breaking hydrogen bonds.

• Topoisomerase/Gyrase: Relieves supercoiling ahead of the replication fork.

• Single-stranded binding proteins: Stabilize unwound DNA.

• Primase: Synthesizes RNA primers to provide a starting point for DNA polymerase.

• DNA Polymerase III: Main enzyme that adds nucleotides in the 5' to 3' direction.

• DNA Polymerase I: Removes RNA primers and replaces them with DNA.

• Ligase: Seals gaps between Okazaki fragments on the lagging strand.

Leading vs. Lagging Strand

• Leading strand: Synthesized continuously toward the replication fork.

• Lagging strand: Synthesized discontinuously in short fragments (Okazaki fragments) away from the fork.

• Reason for fragments: DNA polymerase can only add nucleotides in the 5' to 3' direction.

DNA Replication Table

Enzyme/Factor	Function
Helicase	Unzips DNA helix
Topoisomerase/Gyrase	Relieves supercoiling
Single-stranded binding proteins	Stabilize unwound DNA
Primase	Synthesizes RNA primer
DNA Polymerase III	Main DNA synthesizer
DNA Polymerase I	Removes RNA primer, replaces with DNA
Ligase	Seals Okazaki fragments

Transcription and Translation

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information: DNA → RNA → Protein.

• Transcription: DNA is used as a template to synthesize RNA.

• Translation: RNA is used to synthesize proteins.

Transcription

• Initiation: RNA polymerase binds to the promoter region, often with the help of a sigma factor.

• Elongation: RNA polymerase synthesizes mRNA in the 5' to 3' direction.

• Termination: RNA polymerase reaches a terminator sequence and releases the mRNA.

• mRNA: Messenger RNA carries the genetic code from DNA to ribosomes.

Translation

• Initiation: Ribosome assembles at the start codon (AUG, codes for methionine).

• Elongation: tRNAs bring amino acids to the ribosome, matching codons with anticodons.

• Termination: Stop codons (UAA, UAG, UGA) signal the end of translation; release factors disassemble the complex.

• Polyribosomal complexes: Multiple ribosomes can translate a single mRNA simultaneously in prokaryotes.

Types of RNA

RNA Type	Function
mRNA	Carries genetic code for proteins
tRNA	Transfers amino acids, matches codons with anticodons
rRNA	Structural and catalytic component of ribosomes

Gene Expression Differences: Prokaryotes vs. Eukaryotes

Feature	Prokaryotes	Eukaryotes
Transcription/Translation	Simultaneous	Separate (nucleus/cytoplasm)
First amino acid	Formyl-methionine	Methionine
mRNA structure	Polycistronic, no introns	Monocistronic, introns/exons, 5' cap, poly-A tail

Gene Regulation and Operons

Operon Structure and Function

Operons are clusters of genes regulated as a single unit, common in prokaryotes.

• Inducible operons: Turned ON by substrate (e.g., lac operon for lactose metabolism).

• Repressible operons: Turned OFF by product (e.g., arginine operon for amino acid synthesis).

Lac Operon Example

• No lactose: Repressor binds operator, blocks transcription.

• Lactose present: Lactose binds repressor, inactivates it, allowing transcription.

Arginine Operon Example

• No arginine: Repressor inactive, transcription occurs.

• Arginine present: Arginine activates repressor, blocks transcription.

Mutations: a permeant change in nucleotide sequence of DNA

Types of Mutations

Wild Type: natural non muted characteristic (wild strand)

Mutant Strand: an organism that has a mutation, they can show variance in morphology, nutritional characteristics

genetic control

• Spontaneous mutations: Occur naturally due to errors in replication.

• Induced mutations: Caused by mutagens (chemicals, radiation).

• Ionizing radiation: deep penetrating power sufficient energy to cause electrons to leave orbit (breaks DNA)

• Non-ionizing: little penetrating power, interferes with replication

Mutation Classifications

Type	Description
Silent	No change in amino acid (no effect on protein sequence/ only alters base)
Missense	Change in one amino acid
Nonsense	Change to stop codon
Frameshift	Insertion/deletion alters reading frame
Back-mutation	Reversion to original sequence

Mutation Detection and Repair

• Ames test: Detects mutagenic potential of chemicals using Salmonella typhimurium mutants (mutation rate associated with exposure

  to carcinogenic compounds) (if it turns back to wild type then it is a mutegenic)

  • Auxotrophs: mutants that lost the ability to synthesize certain substances

• DNA repair mechanisms: DNA polymerase: proofreading, mismatch repair, light repair: using photolyase, excision repair: removes segment of DNA

  and then adding correct nucleotides

  Not all are bad some are beneficial for the environment they are in

Genetic Recombination

Genetic variability is created by mutations or Genetic recombination

Horizontal Gene Transfer Mechanisms

• Conjugation: Direct transfer of plasmid or chromosomal DNA via cell-to-cell contact (sex pilus).

• Transformation: Uptake of free DNA fragments from the environment by competent cells.

• Transduction: Transfer of DNA via bacteriophage (virus).

Fates of Donor DNA

• Integration into host genome

• Degradation

• Maintenance as plasmid

• Partial diploidy (in some cases)

Transposons

• Definition: DNA segments capable of moving within the genome.

• Function: Can cause rearrangement, gene disruption, or transfer between chromosome and plasmid.

Summary Table: Horizontal Gene Transfer

Mechanism	Description	Key Features
Conjugation	Direct cell-to-cell transfer	Requires sex pilus, plasmid (F factor)
Transformation	Uptake of free DNA	Requires competence
Transduction	Phage-mediated transfer	Generalized or specialized

Key Equations and Concepts

• Base pairing: $A = T$, $C = G$

• Direction of synthesis: $5' \rightarrow 3'$

• Central Dogma: $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

Additional info: These notes are based on lecture slides and study guide prompts for a college-level microbiology course, focusing on microbial genetics. All major processes and mechanisms are explained with academic context and examples.