Bacterial Genetics: Genetic Transfer and Mutations

Comprehensive practice flashcards covering bacterial genome structure, horizontal gene transfer mechanisms (conjugation, transformation, transduction), mutagens, repair pathways, and clinical resistance selection.

What is a genome?

The comprehensive collection of all genetic material residing within an organism or virus.

Where is the genome distributed across eukaryotic cells?

Primarily inside the nucleus as chromosomal DNA, and inside mitochondria and chloroplasts as organellar DNA.

What two structures contain genomic DNA in prokaryotic cells?

The primary bacterial chromosome and extrachromosomal plasmids.

What is a bacterial plasmid?

A small, circular, self-replicating loop of extrachromosomal DNA located outside the main chromosome.

Where does the viral genome reside, and what types of nucleic acids can it contain?

It resides exclusively in the core and consists of single- or double-stranded DNA or RNA.

What is the biological relationship between chromosomes and genes?

Chromosomes represent the structural packaging of DNA divided into distinct functional units called genes.

What is the primary role of structural genes?

To encode instructions for synthesizing cellular proteins that perform enzymatic, catalytic, and structural functions.

What is the function of RNA-encoding genes?

To code for functional RNA molecules (like rRNA or tRNA) that serve as regulatory elements.

What is the role of regulatory control genes?

Non-coding sequences that act as genetic switches to turn other genomic regions on or off.

What is a genotype?

The fundamental genetic makeup of an organism, defined by its specific sequence of nucleotide bases.

What are the three subcomponents of a single DNA nucleotide?

Deoxyribose sugar, a phosphate group, and a nitrogenous base.

What chemical components construct the backbone and rungs of the DNA ladder?

Upright sides: Repeating units of deoxyribose sugar and phosphate; Cross-rungs: Hydrogen-bonded nitrogenous base pairs.

What complementary base pairing rules govern DNA?

Adenine pairs with Thymine (A-T) and Cytosine pairs with Guanine (C-G).

How does RNA structurally differ from DNA?

RNA is primarily a single-stranded nucleic acid molecule transcribed from a DNA template.

What is a phenotype?

The observable physical, physiological, and structural properties resulting from selective genotype expression.

What are examples of bacterial phenotypes?

Presence or absence of flagella and fimbriae, pigment and exotoxin production, cell wall architecture, and metabolic pathways.

Why does bacterial binary fission fail to generate novel genetic diversity?

It is an asexual process that replicates parental DNA to yield genetically identical daughter cells.

What two mechanisms enable bacteria to diversify and acquire novel traits?

Genetic recombination and mutation.

What is the difference between Vertical Gene Transfer (VGT) and Horizontal Gene Transfer (HGT)?

VGT represents direct inheritance from parent to offspring via binary fission, whereas HGT is direct DNA transfer between co-existing cells in the same generation.

What must happen to chromosomal DNA during recombination to avoid degradation?

It must physically integrate into the recipient chromosome via homologous recombination.

What clinical advantages do bacteria gain via horizontal gene transfer?

Antimicrobial drug resistance, exotoxin production, capsule synthesis, and heavy metal tolerance.

What physical requirement is essential for bacterial conjugation?

Direct, physical cell-to-cell contact between donor and recipient cells.

What is an F factor (fertility plasmid)?

A specialized plasmid carrying genes required to synthesize a sex pilus and initiate transfer.

What characterizes F+ donor and F- recipient cells?

F+ donor: Possesses the F plasmid and produces a sex pilus; F- recipient: Lacks the F plasmid and cannot build a pilus.

What are the sequential steps of standard F+ × F- conjugation?

1. Donor pilus binds to recipient and retracts to form a bridge; 2. F plasmid replicates via rolling-circle mechanism; 3. Single-stranded copy transfers across bridge; 4. Recipient resynthesizes complementary strand, converting F- to F+.

Why is F+ × F- conjugation described as a conservative process?

The donor retains an intact copy of the F plasmid (remains F+) while converting the recipient to F+.

How does a High-Frequency Recombination (HFR) cell form?

An F factor plasmid integrates directly into the primary bacterial chromosome.

What are the steps of High-Frequency Recombination (HFR) conjugation?

1. HFR cell builds a pilus and binds to an F- recipient; 2. Replication begins at the F site and moves into chromosomal DNA; 3. Chromosome transfer begins across fragile bridge; 4. Bridge breaks early due to agitation, leaving recipient F-.

Why does the recipient cell usually remain F- after HFR conjugation?

The fragile conjugation bridge breaks before the trailing end carrying the complete F factor can transfer.

What are resistance plasmids (R factors)?

Plasmids carrying clustered resistance genes against antibiotics, heavy metals, or toxins.

What is natural bacterial transformation?

The uptake of naked extracellular DNA fragments from the environment without direct contact or a viral vector.

What happens to donor and recipient cells during natural transformation?

Donor: Lyses and dies, releasing fragmented naked DNA; Recipient: Surface DNA-binding proteins import naked DNA into the cytoplasm.

What is the primary natural reason bacteria import extracellular DNA?

As a nutritional scavenging mechanism to break nucleic acids into nucleotides for recycling.

What are the mechanisms of electroporation and thermal shock in laboratory transformation?

Electroporation: Uses electrical pulses to open transient membrane pores; Thermal shock: Uses rapid temperature shifts to soften the cell wall matrix.

What is bacteriophage-mediated transduction?

The transfer of host bacterial DNA between cells mediated by an infecting bacteriophage vector.

What is the difference between generalized and specialized transduction?

Generalized: Randomly mispackages arbitrary host DNA fragments; Specialized: Packages specific, localized host chromosomal genes.

What are the sequential steps of bacteriophage transduction?

1. Phage infects donor and degrades host chromosome; 2. Assembling capsids accidentally package donor bacterial DNA; 3. Cell lyses, releasing defective transducing phages; 4. Transducing phage injects bacterial DNA into recipient cell.

Why does a recipient cell not lyse after infection by a transducing phage?

The phage delivered donor bacterial DNA and lacked the viral genome needed to direct viral replication and lysis.

How did Escherichia coli O157:H7 acquire its Shiga toxin genes?

Through bacteriophage-mediated transduction transferring toxin gene clusters from Shigella.

Why could bacteriophages transfer genes between Shigella and E. coli?

Both organisms belong to the family Enterobacteriaceae and share susceptibility to the same bacteriophages.

What epidemiological difference exists between Shigella and E. coli O157:H7?

Shigella spreads via localized person-to-person contact, while E. coli contaminates broad animal reservoirs and food distribution systems.

What is a mutation?

Any permanent alteration in the primary nucleotide sequence of an organism's genome.

What is the difference between spontaneous and induced mutations?

Spontaneous: Baseline replication errors by DNA polymerase; Induced: DNA alterations caused by external physical, chemical, or radiological mutagens.

How does ultraviolet (UV) radiation physically damage DNA?

Induces abnormal covalent cross-links between adjacent thymines, creating thymine dimers.

How do thymine dimers disrupt replication and transcription?

They create physical kinks that stall polymerases, cause base skipping, or trigger mismatched insertions.

What is the primary physical limitation of UV light as a disinfectant?

Extremely poor penetration; it is blocked by solid barriers, glass, plastic, and shaded surfaces.

How do X-rays cause genomic damage?

They generate reactive free radicals from water that cleave phosphodiester backbones, causing double-strand breaks.

How does nitrous acid induce chemical mutations?

It chemically modifies base structures to force abnormal base pairing, such as Adenine pairing with Cytosine.

Why do RNA viruses mutate far more rapidly than DNA viruses?

RNA replication lacks proofreading enzymes to correct mismatched nucleotides during genome synthesis.

How does the light repair mechanism (photolyase) work?

Photolyase absorbs visible light energy to locate thymine dimers and directly cleave the aberrant covalent bonds.

What are the five sequential steps of excision repair?

1. Recognition: Enzymes locate structural deformity; 2. Cleavage: Endonucleases cut upstream and downstream of damage; 3. Excision: Defective oligonucleotide segment is removed; 4. Resynthesis: DNA Polymerase synthesizes replacement strand; 5. Ligation: DNA Ligase seals phosphodiester nicks.

What are the three phenotypic categories of mutations?

Silent/Neutral: Zero observable effect due to codon degeneracy; Harmful/Lethal: Impairs critical metabolic or replication pathways; Beneficial: Confers survival advantage under environmental pressure.

What caused the Day $8$ relapse in the amoxicillin strep throat case?

Amoxicillin eliminated susceptible bacterial competitors, allowing pre-existing resistant mutants to survive and expand uninhibited into a clinically dominant infection.

Why is stating 'the patient is resistant to amoxicillin' biologically incorrect?

Resistance is acquired by the bacterial pathogen; human cells lack peptidoglycan cell walls entirely.

What are the three primary frameworks of microbial control?

Physical control agents (heat, radiation, filtration), chemical control agents (disinfectants, antiseptics), and chemotherapeutic agents (systemic antimicrobial drugs).