Microbial Genetics and DNA Processes: Study Notes

Microbial Cell Structure and DNA

Bacterial Cell Structure

Bacterial cells are prokaryotic organisms characterized by their simple structure and unique genetic organization.

• Chromosome: Typically a single, circular DNA molecule that is haploid and located in the nucleoid region of the cell.

Structure of DNA Molecule

DNA is a double-stranded helical molecule composed of nucleotides, each containing a sugar, phosphate group, and nitrogenous base.

• Nucleotide: Consists of a phosphate group, 5-carbon sugar (deoxyribose), and a nitrogenous base.

• Sugar-phosphate backbone: Alternating deoxyribose sugars and phosphate groups form the structural framework of DNA.

• Hydrogen bonds: Hold complementary base pairs together (A-T and C-G).

• Complementary base pairing: Adenine (A) pairs with Thymine (T); Cytosine (C) pairs with Guanine (G).

Comparison: DNA vs. RNA

DNA and RNA are nucleic acids with distinct structures and functions.

• DNA: Double-stranded helix, contains deoxyribose sugar, bases A, T, C, G.

• RNA: Single-stranded, contains ribose sugar, bases A, U, C, G (Uracil replaces Thymine).

• RNA is less stable than DNA and can form complex shapes.

DNA Replication

Semi-Conservative Replication

DNA replication is semi-conservative, meaning each new DNA molecule consists of one original (parental) strand and one newly synthesized strand.

• Allows for genetic consistency and error checking.

• Reduces the risk of mutations during cell division.

Steps of Prokaryotic DNA Replication

DNA replication in prokaryotes occurs in a series of coordinated steps:

1. Initiation: Topoisomerase binds to the origin of replication, relieving supercoiling.

2. Unwinding: Helicase unwinds the double helix by breaking hydrogen bonds.

3. Priming: Primase synthesizes short RNA primers to initiate DNA synthesis.

4. Elongation: DNA polymerase III adds nucleotides to the 3' end of the new strand.

5. Leading and Lagging Strands: Leading strand is synthesized continuously; lagging strand is synthesized in Okazaki fragments.

6. Primer Removal: DNA polymerase I removes RNA primers and replaces them with DNA.

7. Ligation: DNA ligase joins Okazaki fragments to form a continuous strand.

Differences in Eukaryotic Replication

• Multiple origins of replication.

• Linear chromosomes.

• Different DNA polymerases and shorter Okazaki fragments.

Plasmids

Structure and Function

Plasmids are small, circular, double-stranded DNA molecules found in bacteria, separate from the chromosomal DNA.

• Carry extra genetic information, such as antibiotic resistance genes.

• Replicate independently of the bacterial chromosome.

Gene Expression: Transcription and Translation

Transcription (DNA to RNA)

Transcription is the process by which RNA is synthesized from a DNA template.

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• RNA polymerase: Enzyme that synthesizes RNA from the DNA template.

• Product: mRNA, complementary to the DNA template strand.

Transcription occurs in three main steps:

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

2. Elongation: RNA polymerase synthesizes the RNA strand by adding nucleotides.

3. Termination: RNA polymerase releases the completed RNA transcript.

Translation (RNA to Protein)

Translation is the process by which proteins are synthesized from mRNA templates, involving ribosomes and tRNA.

• Occurs in the cytoplasm.

• Follows the genetic code to assemble amino acids into polypeptides.

Regulation of Gene Expression

Promoters and Transcriptional Control

Promoters are DNA sequences that control the initiation of transcription by providing binding sites for RNA polymerase.

Operons: Inducible and Repressible

Operons are clusters of genes under the control of a single promoter and regulatory elements.

• Inducible operons: Usually inactive; require an inducer to activate gene expression (e.g., lac operon).

• Repressible operons: Usually active; can be turned off by a repressor (e.g., trp operon).

Example: lac Operon (Inducible)

• Active when the repressor is not bound to the operator.

• Inducer (allolactose) binds to the repressor, causing it to release from the operator, allowing transcription.

Example: trp Operon (Repressible)

• Active when the repressor is inactive.

• Co-repressor (tryptophan) binds to the repressor, activating it to bind the operator and block transcription.

Mutations and DNA Repair

Types of Mutations

• Spontaneous mutations: Occur naturally during DNA replication or due to background radiation; random and not caused by external agents.

• Induced mutations: Caused by external factors such as chemicals (mutagens) or physical agents (UV light, X-rays).

Effects of Mutagens

• UV radiation: Causes thymine dimers, leading to errors in DNA replication.

• Nucleotide analogs: Chemicals that resemble DNA bases and can be incorporated into DNA, causing base-pairing errors.

DNA Repair Mechanisms

• DNA polymerase proofreading: Corrects errors during DNA replication.

• Mismatch repair: Detects and repairs incorrectly paired bases.

• Photoreactivation: Uses visible light to repair UV-induced thymine dimers.

• Nucleotide excision repair: Removes damaged DNA segments and fills in the correct bases.

Genetic Exchange in Bacteria

Horizontal Gene Transfer Mechanisms

Mechanism	Description	Key Features
Transformation	Uptake of naked DNA fragments from the environment by a bacterial cell.	No direct cell-to-cell contact; DNA can recombine with recipient genome.
Transduction	Transfer of bacterial DNA by a bacteriophage (virus that infects bacteria).	No direct contact; DNA transferred via viral infection.
Conjugation	Direct transfer of DNA from one bacterium to another via a pilus (mating bridge).	Requires cell-to-cell contact; usually involves plasmid transfer (e.g., F plasmid).

Specialized vs. Generalized Transduction

• Generalized transduction: Any gene, random, lytic cycle.

• Specialized transduction: Specific genes, near prophage, lysogenic cycle.

Spread of F Plasmid in Bacterial Populations

1. F+ and F- Cells: F+ cells contain the F plasmid; F- cells do not.

2. Formation of Sex Pilus: F+ cell forms a sex pilus to connect to an F- cell.

3. DNA Transfer: F plasmid is replicated and transferred to the F- cell.

4. Conversion: Recipient F- cell becomes F+ and can transfer the plasmid to others.

Result: The F plasmid can rapidly spread through a bacterial population.

Competency and Transformation

• Only competent cells can undergo transformation.

• Competency can be natural (some bacteria become competent at certain growth stages) or induced (chemicals or electrical methods in the lab).

• Competency is essential for transformation; without it, bacteria cannot take up new genetic material from their environment.

Key Terms and Definitions

• Genotype: The genetic makeup of an organism.

• Phenotype: Observable characteristics determined by genotype.

• Promoter: DNA sequence where RNA polymerase binds to start transcription.

• Operator: DNA segment where regulatory proteins bind to control gene expression.

• Repressor: Protein that binds to the operator to inhibit transcription.

• Inducer: Molecule that inactivates a repressor, allowing gene expression.

• Co-repressor: Molecule that activates a repressor to inhibit gene expression.

Formulas and Equations

• Base Pairing Rule:

$A = T$ $C = G$

• Central Dogma of Molecular Biology:

$\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

Summary Table: Horizontal Gene Transfer Mechanisms

Mechanism	DNA Source	Requirement	Example
Transformation	Naked DNA from environment	Competent cells	Streptococcus pneumoniae
Transduction	Bacteriophage-mediated	Phage infection	Generalized or specialized transduction
Conjugation	Plasmid DNA	Cell-to-cell contact (sex pilus)	F plasmid transfer in E. coli

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard microbiology curriculum.