Chapter 8 Lecture Focused

Microbial Genetics

Introduction to Microbial Genetics

Microbial genetics is the study of how genes are structured, how they carry information, how this information is expressed, and how genes are replicated and transferred. Advances in DNA technology have revolutionized medicine, agriculture, and biotechnology, enabling the manipulation and transfer of genetic material across species.

• Genetics: The study of genes, their functions, and inheritance.

• Gene: A segment of DNA encoding a functional product, usually a protein.

• Genome: All genetic material in a cell.

• Genomics: Molecular study of genomes.

• Genotype: The genetic makeup of an organism.

• Phenotype: The expression of the genotype as observable traits.

• Example: Escherichia coli DNA is highly packed, allowing a large genome to fit inside a small cell.

Structure of DNA

DNA is a double helix composed of nucleotides, each containing a phosphate group, deoxyribose sugar, and a nitrogenous base. The two strands are held together by hydrogen bonds and are antiparallel.

• Nitrogenous Bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G)

• Base Pairing: A pairs with T (double hydrogen bond), C pairs with G (triple hydrogen bond)

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

• Polymer of Nucleotides: Repeating units of phosphate-deoxyribose-base

DNA Replication

DNA replication is a semi-conservative process where each new DNA molecule contains one old strand and one new strand. Replication occurs in the 5' to 3' direction and is initiated by an RNA primer.

• Replication Fork: The site where DNA strands separate for replication.

• Leading Strand: Synthesized continuously in the 5' to 3' direction.

• Lagging Strand: Synthesized discontinuously, forming Okazaki fragments.

• Enzymes: DNA polymerase (synthesizes DNA), RNA polymerase (adds primer), DNA ligase (joins fragments)

• Equation: $\text{DNA}_{\text{old}} + \text{Nucleotides} \xrightarrow{\text{DNA Polymerase}} \text{DNA}_{\text{new}}$

Transcription

Transcription is the process by which DNA is used as a template to synthesize RNA. In eukaryotes, introns are removed from the RNA transcript before translation.

• RNA Types: Messenger RNA (mRNA), Transfer RNA (tRNA), Ribosomal RNA (rRNA)

• Initiation: RNA polymerase binds to the promoter sequence.

• Direction: Synthesized in the 5' to 3' direction.

• Termination: Transcription stops at the terminator sequence.

• Introns and Exons: Introns are removed by ribozymes; exons are expressed.

• Equation: $\text{DNA} \xrightarrow{\text{RNA Polymerase}} \text{RNA}$

Translation

Translation is the process by which mRNA is decoded to synthesize proteins. It involves mRNA, tRNA, and rRNA, and occurs at the ribosome.

• Start Codon: AUG (codes for methionine)

• Stop Codons: UAA, UAG, UGA

• Process: Ribosome reads mRNA codons, tRNA brings corresponding amino acids, peptide bonds form between amino acids.

• Sites: P site (peptidyl), A site (aminoacyl)

• Equation: $\text{mRNA} + \text{tRNA} + \text{rRNA} \xrightarrow{\text{Ribosome}} \text{Protein}$

• Example: In prokaryotes, transcription and translation can occur simultaneously in the cytoplasm.

Regulation of Gene Expression: Operons

Gene expression in bacteria is regulated by operons, which consist of a promoter, operator, structural genes, and regulatory genes. Operons can be inducible or repressible.

• Constitutive Enzymes: Expressed at a fixed rate (e.g., enzymes for glucose metabolism)

• Inducible Operon: Transcription is turned on by an inducer (e.g., lactose operon)

• Repressible Operon: Transcription is turned off by a repressor (e.g., tryptophan operon)

• Regulatory Gene: Produces repressor protein that can block transcription

• Example: The lac operon is induced by allolactose, which inactivates the repressor protein.

Mutations

Mutations are changes in the genetic material. They can be spontaneous or induced by mutagens such as chemicals or radiation. Mutations may be neutral, beneficial, or harmful.

• Types of Mutations:

  • Base Substitution (Point Mutation): One base is replaced by another.

  • Missense Mutation: Results in a different amino acid.

  • Nonsense Mutation: Results in a stop codon, terminating protein synthesis prematurely.

  • Frameshift Mutation: Insertion or deletion of nucleotides shifts the reading frame.

• Mutagens: Agents that cause mutations (e.g., carcinogenic chemicals, ionizing radiation, UV light)

• DNA Repair: Enzymes recognize and repair damaged DNA (e.g., light repair for thymine dimers, excision repair)

• Surveillance Genes: Genes like P53 on chromosome 17 help monitor and repair DNA damage.

Selection of Mutant Cells

Mutant cells can be identified using positive (direct) or negative (indirect) selection methods.

• Positive Selection: Detects mutants that grow or appear different (e.g., antibiotic resistance)

• Negative Selection: Detects mutants that do not grow (e.g., histidine-dependent cells using replica plating)

• Ames Test: Identifies mutagenic chemicals by observing reverse mutations in histidine-dependent Salmonella

Genetic Transfer and Recombination

Genetic information can be transferred vertically (parent to offspring) or horizontally (between cells of the same generation). Horizontal gene transfer leads to genetic recombination.

• Transformation: Uptake of naked DNA fragments from the environment (e.g., Griffith's experiment with encapsulated and non-encapsulated bacteria)

• Conjugation: Transfer of plasmids via direct cell-to-cell contact using sex pili (F factor, HFR cells)

• Transduction: Transfer of DNA by bacteriophages (viruses that infect bacteria)

Plasmids and Transposons

Plasmids are small, circular DNA molecules that enhance bacterial traits but are not essential for survival. Transposons are DNA segments that can move within the genome.

• Conjugated Plasmids: Carry genes for sex pili and plasmid transfer

• Dissimilation Plasmids: Encode enzymes for catabolism of unusual compounds

• R Factors: Encode antibiotic resistance

• Transposons: Segments of DNA that move from one region to another, containing insertion sequences for cutting and resealing DNA

Summary Table: Types of Mutations

Type	Description	Effect	Example
Missense Mutation	Base substitution changes amino acid	Protein may function differently	Valine instead of Glutamic acid in hemoglobin
Nonsense Mutation	Base substitution creates stop codon	Protein synthesis stops prematurely	UAG codon in mRNA
Frameshift Mutation	Insertion/deletion shifts reading frame	Protein sequence altered	Deletion of a base in a gene

Summary Table: Horizontal Gene Transfer Mechanisms

Mechanism	Description	Key Features	Example
Transformation	Uptake of naked DNA	Recipient incorporates DNA into genome	Griffith's experiment
Conjugation	Direct transfer via pilus	F factor, plasmid transfer	F+ to F- cell
Transduction	Transfer by bacteriophage	Virus packages bacterial DNA	Bacteriophage infects new cell

Additional info: Academic context was added to clarify mechanisms, provide definitions, and summarize key concepts for exam preparation.