Microbial Genetics

Vocabulary practice flashcards covering mutation types, mutagens, repair mechanisms, mutant selection, Ames test, plasmids, and horizontal gene transfer (transformation, conjugation, transduction, transposition) from Microbial Genetics.

Mutation

A permanent change in the base sequence of DNA that is passed to subsequent generations through replication, transcribed into mRNA, and capable of altering the amino acid sequence during translation.

Central Dogma

The framework describing the flow of genetic information: DNA replicates and is transcribed into mRNA, which is translated into a sequence of amino acids during protein synthesis.

Beneficial Mutation

A mutation that increases an organism's fitness, allowing it to grow and survive more efficiently, such as altering antibiotic targets to confer resistance or modifying bacterial membranes to resist phagocytosis.

Harmful Mutation

A mutation that compromises the survival of an organism, such as a mutation in an essential gene or an alteration in genes encoding the bacterial capsule that promotes phagocytosis.

Neutral Mutation

A mutation that causes no functional or phenotypic change, often due to the degenerate and redundant nature of the genetic code where different codons code for the same amino acid.

Mutagen

A chemical or physical agent (such as UV radiation, ionizing radiation, X-rays, or 5-bromouracil) that directly or indirectly increases the frequency of mutations.

Spontaneous Mutation

A mutation that occurs naturally in the absence of a mutagen, primarily resulting from replication errors or the insertion of an incorrect base by DNA polymerase enzymes.

Induced Mutation

A mutation caused by deliberate exposure to mutagens, typically conducted in a laboratory setting for scientific research.

Silent Mutation

A point mutation (base substitution) that changes a codon without changing the resulting amino acid, leaving the protein's structure and function unaffected.

Missense Mutation

A base substitution that results in the insertion of a different amino acid, which may or may not disrupt protein folding, stability, or catalytic function.

Nonsense Mutation

A base substitution that generates a premature stop codon, yielding a truncated protein whose functionality depends on how many amino acids are lost.

Frameshift Mutation

An insertion or deletion of one or more nucleotide pairs (not divisible by three) that shifts the translational triplet reading frame and usually completely alters downstream amino acid sequences.

Nitrous Acid ($\text{HNO}_2$)

A deaminating chemical mutagen that removes amino groups from DNA bases, causing adenine to pair with cytosine instead of thymine and promoting $A \rightarrow C$ base substitutions.

Nucleoside Analog

A chemical mutagen structurally similar to normal nitrogenous bases that integrates into replicating DNA and causes base-pairing mistakes.

5-Bromouracil

A nucleoside analog resembling thymine that incorporates into DNA in place of thymine and mistakenly pairs with cytosine, altering an $A\text{–}T$ pair into a $G\text{–}C$ pair.

Aflatoxin

A bulky chemical mutagen produced by Aspergillus mold that inserts into DNA to induce frameshift mutations and drive carcinogenesis.

Thymine Dimers

Covalent cross-links formed between adjacent thymine bases on the same DNA strand due to ultraviolet (UV) radiation exposure, disrupting base pairing, replication, and transcription.

Photolyases

Light-repair enzymes that absorb visible light energy to split UV-induced thymine dimers and restore normal DNA pairing.

Nucleotide Excision Repair

An enzymatic repair process wherein an endonuclease and exonuclease cut and remove damaged DNA segments, DNA polymerase synthesizes replacement DNA, and DNA ligase seals the remaining gap.

Auxotroph

A mutant microorganism that has acquired a nutritional requirement absent in its parent strain, such as a loss of the ability to synthesize histidine or metabolize lactose.

Prototroph

The wild-type (unmutated) organism that retains normal metabolic capabilities and lacks the specific nutritional deficiencies of an auxotroph.

Positive (Direct) Selection

A selection technique that detects mutant cells directly by establishing conditions where only the mutants survive or grow, such as plating on media containing antibiotics.

Negative (Indirect) Selection

A screening process that identifies mutant cells unable to perform a specific function or synthesize a nutrient (auxotrophs), commonly performed using replica plating.

Replica Plating

A technique using sterile velvet to transfer identical colony patterns from a complete master plate to selective media plates lacking specific nutrients to identify auxotrophs.

Ames Test

A mutagenicity assay that exposes histidine-dependent (his-) Salmonella auxotrophs to a suspected chemical and rat liver extract to measure the reversion rate back to histidine synthesizers (his+).

Spontaneous Mutation Rate

The natural frequency of mutation occurrence, approximating 1 mutation per $10^9$ replicated base pairs or 1 per $10^6$ replicated genes, which mutagens can increase by $10$ to $1,000$ times.

Vertical Gene Transfer

The transmission of genetic material directly from an organism to its offspring through cellular division.

Horizontal Gene Transfer

The non-reproductive transfer of DNA from a donor cell to a recipient cell belonging to the same generation, occurring via transformation, conjugation, or transduction.

Genetic Recombination

The exchange of nucleotide sequences between two DNA molecules mediated by the RecA protein to integrate donor DNA into a recipient chromosome, forming a recombinant cell.

Transformation

A horizontal gene transfer mechanism where a competent bacterial cell takes up naked DNA fragments or autonomous plasmids directly from the environment.

Competence

The physiological capacity of a bacterial cell to take up exogenous naked DNA, which can happen naturally or be artificially induced by CaCl2 treatment or electroporation.

Plasmids

Extrachromosomal, circular, self-replicating DNA elements measuring 1% to 5% the size of a bacterial chromosome that carry nonessential but advantageous genes.

Resistance Factors (R Factors)

Plasmids carrying Resistance Transfer Factor (RTF) genes for transfer and replication, alongside r-determinant genes that code for resistance against antibiotics or heavy metals.

Conjugation

A horizontal gene transfer process requiring direct cell-to-cell contact between opposite mating types, utilizing sex pili in Gram-negative bacteria or sticky cellular surfaces in Gram-positive bacteria.

F Plasmid

A fertility plasmid possessing an origin of replication (ori), an origin of transfer, and a tra region that directs the expression of sex pili, enabling $F^+$ donor cells to transfer plasmid copies to $F^-$ recipients.

Bacteriophage

A bacterial virus structured with a protein capsid head containing viral DNA, a contractile sheath, tail fibers, a baseplate, and pins used to attach to and infect host cells.

Virulent (Lytic) Phage

A bacteriophage (such as T-even phages) that exclusively replicates via the lytic cycle, degrading host DNA, commandeering cellular synthesis, and inducing host cell lysis and death.

Temperate (Lysogenic) Phage

A bacteriophage (such as Lambdavirus) capable of entering the lysogenic cycle, integrating its DNA into the host chromosome as a latent prophage until an induction event initiates the lytic cycle.

Generalized Transduction

A process mediated by lytic phages wherein host bacterial chromosome fragments are randomly and mistakenly packaged into viral capsids and delivered to a recipient bacterium.

Specialized Transduction

A process mediated by temperate phages where imprecise excision of a prophage carries adjacent bacterial genes (such as the gal gene) into newly formed virions to be transferred to recipient cells.

Transposons

Mobile DNA segments ('jumping genes') bounded by insertion sequences (IS) that code for transposase, allowing them to excise and integrate into different sites on chromosomes or plasmids.

Transposase

An enzyme encoded within the insertion sequences of transposable elements that catalyzes the cutting, movement, and resealing of DNA during transposition.