Bacterial Genetics Lecture 7

Bacterial Genetics

• Lecture 7, MICR20010
• Presented by Dr. Jennifer Mitchell, Microbiology
• School of Biomolecular and Biomedical Science

Lecture 6 (Background)

• Covered metabolic diversity, energy production, storage, and release
• Introduced phototrophs and chemotrophs, further divided into chemoorganotrophs and chemolithotrophs
• Discussed autotrophs, heterotrophs, and photosynthesis

Learning Outcomes

• DNA replication
• Gene structure (transcription)
• Protein synthesis (translation)
• Antibiotics
• The Genetic Code
• Mutations
• Genetic exchange

DNA Structure

• Deoxyribonucleic Acid (DNA)
• Monomers are deoxyribonucleotides
  • 5-carbon sugar deoxyribose
  • A nitrogenous base
  • A phosphate group
• Four nitrogenous bases in DNA: Adenine (A), Guanine (G), Cytosine (C), Thymine (T)
  • Purines: A, G
  • Pyrimidines: C, T
• Double-stranded helix, with complementary base pairing (A with T, G with C)

DNA Strand

• A 5' to 3' arrangement of phosphate-sugar-base units
• The DNA strand consists of a backbone made of repeating sugar and phosphate molecules, which are connected to base pairs extending outward.

DNA Replication

• Process of creating identical copies of genes during cell division
• Carried out with high accuracy by DNA polymerases
• Occasional errors generate slightly altered nucleotide sequences—mutations
• Key stages: initiation, elongation, proofreading, and termination

DNA Polymerase

• Adds nucleotides to the 3' end of the growing DNA strand only
• This creates a 5' → 3' strand
• DNA polymerase is unable to start a new chain

Genetic Code

• Codons: three adjacent nucleotides encoding a specific amino acid
• Specifies the type and sequence of amino acids for protein synthesis in protein synthesis.

Gene Expression

• Transcription: RNA polymerase initiates at a promoter region upstream of a gene, copies the DNA (forming an RNA transcript, mRNA)
• Translation: ribosomes and tRNA decode mRNA to synthesize amino acids and proteins.

Gene Structure

• Sections within DNA of a gene include the promoter region and the stop codon separating the gene from other genes.

Protein Expression

• mRNA polymerase transcribes a gene into mRNA.
• Ribosomes convert mRNA into protein (e.g., enzyme).

Antibiotics and DNA/RNA/Protein

• Some antibiotics target DNA replication, transcription, and translation
• Rifampicin affects RNA polymerase
• Macrolides (erythromycin), Kanamycin, and Tetracycline affect ribosomes and protein synthesis
• Mutations in antibiotic targets lead to bacterial resistance

Ciprofloxacin

• Targets DNA gyrase to prevent replication and cell division
• A quinolone antibiotic

Plasmids

• Circular extrachromosomal DNA
• Replicate independently and can move between cells
• Offer phenotypic advantage to host cells (e.g., antibiotic resistance, virulence genes, metabolic genes).

Hospital-Acquired Infections

• Plasmids with multiple antibiotic resistance genes largely dominate hospital bacteria
• These nosocomial infections are serious, hard to treat
• Antibiotic resistance genes already existed, but became prevalent due to selective pressure
• Illustrates bacterial adaptability

Transmission of AMR genes between Species

• ARGs (antibiotic resistance genes) spread between species through different mechanisms: conjugation, transformation, and transduction that move among bacterial species in different environments.

Mutation

• Most common source of genetic variation
• Spontaneous or induced (by mutagens)
• Types: substitution, deletion, insertion
• Mutations can result in changes in the reading frame of mRNA and can therefore change the resulting protein sequence, potentially causing resistance to antibiotics.

Codons - Mutation

• A mutation in a gene can change the DNA sequence and, consequently, the mRNA sequence in a gene.
• This change in the mRNA sequence might result in
• a different amino acid, a stop codon, or no change at all

Genetic Variation

• Various mutations, including those related to altering temperature sensitivity, drug resistance, and phenotypic traits
• Includes changes that result in differences with other bacteria species

Modes of Genetic Transfer (Mechanisms)

• Transformation: bacteria take up free DNA;
• Conjugation: direct transfer via cell-to-cell contact;
• Transduction: transfer by bacteriophages.

Mutagenesis

• Processes causing mutations, including agents like radiation and chemicals (like base analogues, intercalating agents, and metals/ROS), and biological agents (virus, transposon)

Genetic Exchange

• Modes of genetic transfer between bacterial cells, including transformation, conjugation, and transduction
• Describes how bacteria exchange genetic information

Transformation

• Bacteria uptake free DNA
• Often degraded
• Sometimes integrated into the host genome
• Some bacteria are naturally competent (e.g., Streptococcus pneumoniae)

Conjugation

• Plasmid transfer between bacterial cells
• Requires cell-to-cell contact; can happen between different bacterial species
• Bacteria have specialized channels (tra genes)
• Plasmids replicate in the donor cell first
• Transferred to the recipient cell

Transduction

• DNA transfer between bacteria via bacteriophages (viruses that infect bacteria)
• Phage infects and replicates inside bacterial cell, also packaging some host DNA.
• Host DNA incorporated into phage capsids and transferred to other bacteria

Transposition

• DNA sequences (transposons) jump between sites within a bacterial genome (or to plasmids)
• Transposons carry enzymes
• Can cause mutations and disrupt genes

Genetic Variation and Antibiotic Resistance

• Mutations, transformation, transposition
• Conjugation can all lead to antibiotic resistance in bacteria

Further Reading

• Brock Biology of Microorganisms (Chapter 10, "Bacterial Genetics")

Explore the fundamentals of bacterial genetics in this quiz based on Lecture 7 of MICR20010 by Dr. Jennifer Mitchell. Delve into DNA structure, gene expression, and the impact of mutations on protein synthesis. Test your understanding of key concepts including antibiotics and genetic exchanges.