Microbial Genetics, Antibiotic Resistance, and Gene Regulation: Study Notes

Microbial Genetics and Antibiotic Resistance

Staphylococcus aureus and Antibiotic Resistance

Staphylococcus aureus is a common bacterial pathogen that can cause a variety of infections. The widespread use of antibiotics has led to the emergence of antibiotic-resistant strains, such as methicillin-resistant S. aureus (MRSA).

• Antibiotic resistance occurs when bacteria acquire mutations or genes that allow them to survive in the presence of antibiotics.

• Penicillin resistance in S. aureus is often due to mutations in the penicillin binding protein (PBP), which can be transferred between bacteria via horizontal gene transfer.

• MRSA stands for methicillin-resistant Staphylococcus aureus, a major clinical concern due to its resistance to multiple antibiotics.

Central Dogma: DNA to Protein

Transcription: DNA to mRNA

Transcription is the process by which a segment of DNA is used as a template to synthesize messenger RNA (mRNA).

• Enzyme involved: RNA polymerase catalyzes the synthesis of mRNA from the DNA template.

• Direction: mRNA is synthesized in the 5' to 3' direction, using the 3' to 5' DNA template strand.

• Example: For the template strand 3'-CTCTAGAATCTATTTATGGCTTA-5', the mRNA sequence produced would be 5'-GAGAUCUUAGAUAAUACCGA-3'.

Translation: mRNA to Protein

Translation is the process by which ribosomes synthesize proteins using the sequence of codons in mRNA.

• Codons: Each set of three nucleotides (codon) in mRNA specifies an amino acid.

• Genetic code: The genetic code table is used to determine which amino acid corresponds to each codon.

• Example: Using the mRNA sequence above, the amino acid sequence can be determined by reading each codon and matching it to the code table.

Mutation Types

Mutations are changes in the DNA sequence that can affect the resulting protein.

• Point mutation: A single nucleotide change.

• Missense mutation: A change that results in a different amino acid.

• Nonsense mutation: A change that introduces a stop codon, truncating the protein.

• Silent mutation: A change that does not affect the amino acid sequence.

Genetic Code Table

The genetic code table is used to translate mRNA codons into amino acids. Below is a simplified representation:

First Letter	Second Letter	Third Letter	Amino Acid
U	U	U	Phe
A	U	G	Met (Start)
U	A	A	Stop
U	A	G	Stop
U	G	G	Trp
C	G	U	Arg

Transcription vs. Translation: Prokaryotes vs. Eukaryotes

Key Differences

• Location: In prokaryotes, both processes occur in the cytoplasm; in eukaryotes, transcription occurs in the nucleus and translation in the cytoplasm.

• RNA Processing: Eukaryotic mRNA undergoes splicing, capping, and polyadenylation; prokaryotic mRNA does not.

• Coupling: In prokaryotes, translation can begin before transcription is complete; in eukaryotes, these processes are separated.

Gene Regulation: The mecA Gene and Repressors

Repressor Proteins and Operons

Gene expression in bacteria is often regulated by repressor proteins that bind to specific DNA sequences, preventing transcription.

• Repressor protein: Binds to the operator region of DNA to block RNA polymerase.

• Constitutive gene: Expressed continuously.

• Repressible gene: Can be turned off by a repressor.

• Inducible gene: Can be turned on in response to an inducer.

mecA Gene Regulation in MRSA

• mecA gene: Encodes a penicillin-binding protein that confers resistance to methicillin.

• Repressor protein MecI: Binds to the operator region to prevent transcription of mecA.

• Inducer: A molecule that binds to the repressor, causing it to release from DNA and allowing transcription.

• Example: In the lac operon, allolactose acts as an inducer; similarly, an inducer can remove MecI from DNA.

Bacteriophages: Structure and Function

Basic Structure of Naked Viruses (Bacteriophages)

Bacteriophages are viruses that infect bacteria. The simplest naked virus consists of:

• Capsid: Protein shell that encloses the viral genome.

• Nucleic acid: DNA or RNA genome inside the capsid.

• Example: The T4 phage has a head (capsid), tail, and tail fibers, but the simplest phages may only have a capsid and genome.

Additional info:

• Some context and explanations have been expanded for clarity and completeness.

• Table entries and some examples are inferred for illustration purposes.