Microbial Genetics: DNA Structure, Replication, Transcription, Translation, Mutation, and Gene Transfer

Microbial Genetics

Introduction

Microbial genetics is the study of how microorganisms inherit and express genetic information. This field explores the structure and function of genetic material, mechanisms of gene transfer, and the processes by which genetic information is replicated, transcribed, and translated into proteins. Understanding microbial genetics is fundamental to microbiology, biotechnology, and medicine.

Key Terms & Definitions

• Genetics: The study of heredity and the variation of inherited characteristics.

• Chromosomes: DNA-containing structures that carry genetic information.

• Genes: Segments of DNA that encode functional products, usually proteins.

• Genome: The complete set of genetic information in a cell.

• Genomics: The study of genomes and their interactions.

• Genetic Code: The set of rules by which nucleotide sequences are translated into amino acid sequences.

• Central Dogma: The flow of genetic information: DNA → RNA → Protein.

• Genotype: The genetic makeup of an organism.

• Phenotype: The observable characteristics resulting from gene expression.

• Short Tandem Repeats (STRs): Repeating sequences of noncoding DNA, useful in forensics.

• Vertical Gene Transfer: Transmission of genes from parent to offspring.

• Horizontal Gene Transfer: Transfer of genes between cells of the same generation.

• Mutation: A permanent change in the DNA sequence.

• Mutagen: An agent that causes mutations.

• Spontaneous Mutation: Mutation occurring without a known mutagen.

• Genetic Recombination: Exchange of genes between DNA molecules.

• Recombinant: A cell that has incorporated foreign DNA.

• Plasmid: A small, self-replicating DNA molecule in bacteria.

• Transposon: A DNA segment that can move within the genome.

• Operon: A cluster of genes under the control of a single promoter and operator.

• Constitutive Genes: Genes that are always expressed.

Genetic Information Flow in Microbes

Expression, Recombination, and Replication

Genetic information in microbes can be expressed, recombined, or replicated. Expression involves using genetic information to synthesize proteins, recombination introduces new gene combinations, and replication ensures genetic information is passed to offspring.

• Expression: DNA is transcribed to RNA and translated to protein, allowing the cell to grow and function.

• Recombination: Genetic information can be exchanged between cells, creating new gene combinations.

• Replication: DNA is copied and passed to the next generation during cell division.

DNA Structure & Replication

DNA Structure

DNA is a double helix composed of two antiparallel strands. Each strand consists of a sugar-phosphate backbone and nitrogenous bases (adenine, thymine, guanine, cytosine). Base pairing follows specific rules: A pairs with T, and G pairs with C, held together by hydrogen bonds.

• Backbone: Deoxyribose sugar and phosphate groups.

• Base Pairing: Adenine (A) pairs with Thymine (T); Guanine (G) pairs with Cytosine (C).

• Antiparallel Strands: One strand runs 5'→3', the other 3'→5'.

• Genetic Instructions: The sequence of bases encodes genetic information.

DNA Replication

DNA replication is semiconservative: each new DNA molecule contains one old and one new strand. In bacteria, replication is bidirectional and highly accurate due to proofreading by DNA polymerase.

• Semiconservative: Each daughter DNA has one parental and one new strand.

• Bidirectional: Replication forks move in opposite directions around the circular chromosome.

• Proofreading: DNA polymerase corrects errors during synthesis.

Steps of DNA Replication

1. Relaxing: Topoisomerase and gyrase relax supercoiled DNA.

2. Unwinding: Helicase separates the DNA strands, forming a replication fork.

3. Priming: RNA primer is added to provide a starting point for DNA polymerase.

4. Elongation (Leading Strand): DNA polymerase synthesizes continuously toward the fork (5'→3').

5. Elongation (Lagging Strand): DNA polymerase synthesizes discontinuously away from the fork, creating Okazaki fragments.

6. Joining: DNA polymerase removes RNA primers; DNA ligase joins Okazaki fragments.

RNA & Transcription

RNA vs. DNA

Feature	DNA	RNA
Strands	Double-stranded	Single-stranded
Sugar	Deoxyribose	Ribose
Bases	A, T, G, C	A, U, G, C
Function	Stores genetic information	Implements genetic instructions

Types of RNA

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

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

• rRNA (Ribosomal RNA): Structural and functional component of ribosomes.

Transcription in Prokaryotes

Transcription is the synthesis of mRNA from a DNA template. In prokaryotes, it occurs in the cytoplasm and involves three main steps: initiation, elongation, and termination.

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

2. Elongation: RNA polymerase synthesizes mRNA in the 5'→3' direction.

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

Transcription in Eukaryotes

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

• Genes contain exons (coding) and introns (noncoding).

• After transcription, introns are removed and exons are spliced together to form mature mRNA.

Memory trick: EXons are EXpressed; INtrons are INterrupting (removed).

Translation

Key Terms

• Codon: Three-nucleotide sequence on mRNA coding for an amino acid.

• Start Codon: AUG (methionine) — signals the start of translation.

• Stop Codons: UAA, UAG, UGA — signal the end of translation.

• Anticodon: Three-nucleotide sequence on tRNA complementary to mRNA codon.

• Degeneracy: Most amino acids are encoded by more than one codon.

• Peptide Bond: Joins amino acids during translation.

• Polypeptide: Chain of amino acids forming a protein.

Ribosome Sites

Site	Name	Function
A site	Aminoacyl site	Entry of tRNA with amino acid
P site	Peptidyl site	Holds tRNA with growing polypeptide
E site	Exit site	tRNA exits ribosome

Steps of Translation

1. Initiation: Ribosome assembles at start codon; first tRNA binds at P site.

2. tRNA Attachment: Second tRNA binds at A site.

3. Peptide Bond Formation: Ribosome forms peptide bond between amino acids.

4. Translocation: Ribosome moves along mRNA; tRNAs shift positions.

5. Elongation: Process repeats, polypeptide grows.

6. Termination: Ribosome reaches stop codon; polypeptide is released.

Mutations

Types of Mutations

Type	What Changes	Effect on Protein
Base substitution (point mutation)	One nucleotide replaced	Missense, nonsense, or silent
Missense mutation	Wrong amino acid coded	Protein function altered
Nonsense mutation	Stop codon introduced	Premature termination
Silent mutation	Codon changes, same amino acid	No effect
Frameshift mutation	Insertion/deletion shifts reading frame	Severely disrupts protein

Chemical and Physical Mutagens

• Nucleoside Analogs: Mimic normal bases, cause mispairing.

• Ionizing Radiation: Breaks DNA backbone.

• UV Radiation: Causes thymine dimers, blocking replication and transcription.

Nucleotide Excision Repair

1. Enzyme cuts DNA around damage (e.g., thymine dimer).

2. DNA polymerase fills in the gap.

3. DNA ligase seals the strand.

Gene Regulation: The Operon Model

Why Regulate Genes?

Gene regulation ensures that proteins are produced only when needed, conserving energy and resources.

• Constitutive Genes: Always on, essential for cell function.

• Inducible Genes: Turned on by an inducer (e.g., substrate).

• Repressible Genes: Turned off by a corepressor (e.g., product).

Operon Components

Component	Function
Promoter	RNA polymerase binding site
Operator	Regulatory site; repressor binding blocks transcription
Structural Genes	Code for proteins
Repressor	Protein that blocks transcription
Inducer	Turns on gene expression
Corepressor	Turns off gene expression

Examples: lac and trp Operons

• lac Operon (Inducible): Off unless lactose (inducer) is present.

• trp Operon (Repressible): On unless tryptophan (corepressor) is abundant.

Genetic Transfer & Recombination

Vertical vs. Horizontal Gene Transfer

• Vertical: Parent to offspring.

• Horizontal: Between cells of the same generation (unique to prokaryotes).

Mechanisms of Horizontal Gene Transfer

Mechanism	Description	Key Details
Transformation	Uptake of naked DNA from environment	No cell contact; demonstrated by Griffith's experiment
Conjugation	Direct transfer via sex pilus	Requires F factor; donor (F+) to recipient (F-)
Transduction	Transfer by bacteriophage (virus)	Generalized or specialized; phage acts as vector

Plasmids and Transposons

• Plasmid: Small, circular DNA; can carry antibiotic resistance or toxin genes.

• Conjugative Plasmid (F factor): Enables conjugation.

• Transposon: DNA segment that can move within the genome.

Griffith's Transformation Experiment

• Smooth (S) strain of Streptococcus pneumoniae: virulent, kills mice.

• Rough (R) strain: non-virulent.

• Heat-killed S strain: non-virulent.

• Live R + heat-killed S: kills mice; R strain transformed by DNA from S strain.

Conclusion: DNA is the transforming principle.

Quick Reference Summary

Central Dogma

Process	What Happens	Key Enzyme/Molecule
DNA Replication	DNA → DNA	DNA polymerase, helicase, ligase, topoisomerase
Transcription	DNA → mRNA	RNA polymerase
Translation	mRNA → Protein	Ribosome, tRNA, mRNA

Codon Reference

• Start Codon: AUG (methionine)

• Stop Codons: UAA, UAG, UGA

• Total Codons: 64 (61 sense, 3 stop)

• Degeneracy: Multiple codons for most amino acids

Mutation Types at a Glance

Mutation	Cause	Result
Missense	Base substitution	Altered protein
Nonsense	Base substitution	Truncated protein
Silent	Base substitution	No change
Frameshift	Insertion/deletion	Disrupted protein

Gene Transfer Comparison

Mechanism	Vehicle for DNA	Direct Contact Required?
Transformation	Naked DNA	No
Conjugation	Plasmid (F factor)	Yes
Transduction	Bacteriophage	No

Additional info: This guide covers the essential concepts of microbial genetics, including DNA structure, replication, gene expression, mutation, and gene transfer mechanisms, as outlined in a typical college-level microbiology curriculum.