Bacterial Genetics - Prof. I. L. Oboro

Practice flashcards covering key concepts from Bacterial Genetics, including genome organization, plasmids, transposable elements, gene transfer mechanisms, bacteriophage lifecycles, bacterial mutations, and their medical implications.

What is defined as the complete set of genetic determinants of an organism?

Genotype

What term refers to the observable or expressed characteristics of an organism?

Phenotype

How is a gene defined in bacterial genetics?

A DNA sequence that encodes a gene product (i.e., a protein, or RNA molecule).

What is the definition of a locus?

Position of a gene on the chromosome

What are the main structural characteristics of the bacterial genome?

The bacterial genome is a single circular chromosome made of double stranded DNA, making it haploid. The DNA strands are covalently linked, supercoiled, and lie unenclosed within the cytoplasm as the nucleoid.

What is the nucleoid in a bacterial cell?

The region in the cytoplasm containing the unenclosed, covalently linked, supercoiled double-stranded bacterial DNA.

What are the two essential functions of the bacterial chromosome?

Replication and expression.

What are the two major classifications of bacterial DNA?

Chromosomal DNA (Genome) and Extra-chromosomal DNA (Plasmids and Transposons).

What are plasmids and what type of DNA structure do they usually possess?

Plasmids are extra-chromosomal pieces of DNA present in some bacteria that encode traits non-essential for viability and replicate independently. Most are circular, double-stranded, supercoiled DNA molecules, though some are linear in Borrelia and Streptomyces species.

In which bacterial species are linear plasmids found?

Borrelia and Streptomyces species.

What is meant by plasmid copy number and plasmid incompatibility?

Copy number is the average number of plasmid molecules per bacterial chromosome. Incompatibility means closely related or identical plasmids cannot be stably maintained in the same bacterial host.

What are the criteria used to classify plasmids?

Transfer properties (conjugative vs. non-conjugative), phenotypic effects (fertility/F plasmids, resistance/R factors, bacteriocinogenic plasmids), and size (small vs. large).

What are the characteristics of small plasmids?

They are $< 7.5\text{ kilobase pairs}$, usually non-conjugative, have high copy numbers (typically $10\text{--}20$ per chromosome), rely on their bacterial host for replication functions, and are distributed randomly between daughter cells.

What are the characteristics of large plasmids?

They are $> 40\text{ kilobase pairs}$, often conjugative, have small copy numbers ($1$ to several per chromosome), code for all functions required for their replication, and partition themselves among daughter cells during cell division similarly to the chromosome.

What medically important properties of pathogenic bacteria are controlled by plasmids?

Resistance to antibiotics (R plasmids/R factors), toxin production (e.g., enterotoxins of Escherichia coli, exfoliative toxin of Staphylococcus aureus, tetanus toxin), synthesis of cell surface structures required for adherence/colonization, increased pathogenicity, increased transmissibility, resistance to vaccines, and usefulness in epidemiological studies.

What are transposable elements?

Non-self-replicating segments of DNA that can change position on a DNA molecule, either within the same DNA molecule or from one DNA molecule to a different one (also known as mobile genetic sequences or jumping genes).

How do Insertion sequences (IS) differ from Transposons (Tn)?

Insertion sequences (IS) are simple and only code for proteins implicated in transposition (transposase), whereas Transposons (Tn) carry other genes in addition to transposition genes, such as antibiotic resistance genes.

What is the importance of transposable elements in bacteria?

They aid bacteria to acquire new genes, contribute to the dissemination of new genes within bacterial populations, cause mutations (altering genetic identity and genome size), and mediate genomic rearrangements.

What are the three main mechanisms of gene transfer among bacteria?

Conjugation, Transformation, and Transduction.

What cellular structures and cells are participating in bacterial conjugation as shown in this image?

An $\text{F}^+$ cell communicating with an $\text{F}^-$ cell via a sex pilus.

How does genetic transfer occur during bacterial conjugation?

Donor ability is determined by fertility plasmids ($\text{F}^+$) conferring sex pili in Gram-negative bacteria or adhesins in Gram-positive bacteria. Cytoplasmic connections form, direct transfer of donor plasmid DNA occurs into recipient cells, and the recipient synthesizes the complementary strand.

What is transduction and what entity acts as the vector?

Transduction is a mechanism where Bacteriophages (viruses that infect bacteria) function as vectors to introduce DNA from donor bacteria into recipient bacteria.

What is the structural composition of a bacteriophage as illustrated in this diagram?

A head, collar, core, helical sheath, hexagonal base plate, and tail pins.

How do virulent and temperate bacteriophages differ in their host outcomes?

Virulent phages destroy host bacterial cells by lysing them (lysis). Temperate phages preserve the host bacterium by replicating as a prophage without forming new virions or lysing the host cell (lysogeny).

What are the stages of phage attachment and nucleic acid injection during host cell infection?

Adsorption (irreversible attachment via tail fibers), base plate attachment, sheath contraction, nucleic acid injection, and DNA uptake.

What is transformation and in which bacterium was it first discovered?

Transformation is the mechanism by which pieces of DNA released from donor bacteria into the extracellular environment are taken up directly by recipient bacteria; it was first discovered in Streptococcus pneumoniae.

What is competence, and how can incompetent bacteria be forced to take up DNA?

Competence is the ability of bacteria to take up extracellular DNA. Incompetent bacteria can be forced through laboratory manipulations such as calcium shock or exposure to a high-voltage electrical pulse (electroporation).

What is a mutation, and what agents can induce it?

A mutation is a heritable change in the genome (spontaneous or induced). It can be induced by chemical and physical agents including X-rays, ultraviolet rays, and nitrous oxide.

What is the difference between wild type and mutant bacteria?

Unmutated bacteria are called wild type, while their progeny with mutations are called mutants.

What are the three structural classifications of mutations?

Substitutions, Insertions, and Deletions.

What defines Missense, Frameshift, and Nonsense mutations?

Missense mutations substitute one amino acid for another in a polypeptide chain. Frameshift mutations involve insertions/deletions that shift the reading frame of nitrogenous bases. Nonsense mutations substitute an amino acid codon with a Stop codon, often causing complete loss of gene function.

What is the difference between true (genotypic) reversions and suppressor mutations?

True reversions restore the exact nucleotide sequence of wild-type DNA, making strains identical genotypically and phenotypically. Suppressor mutations reverse the phenotype back to wild-type without restoring the exact nucleotide sequence.

What are the major health implications of genetic variations in bacteria?

Antibiotic resistance, increased bacterial virulence, increased transmission, outbreaks/epidemics/pandemics, challenges with effective vaccine production/efficacy, and development of gene therapy.