Bacterial Genetics: Mutations, Screening, and Gene Transfer

A comprehensive vocabulary review set covering types of mutations, mutagen screening techniques, the Ames test, and gene transfer mechanisms discussed in the lecture.

Point mutation (Base substitution)

A permanent change in DNA where a single base pair is replaced by another, categorized into silent, missense, or nonsense subtypes.

Missense mutation

A base substitution where a codon change results in a different amino acid in the synthesized protein, which may or may not alter protein function depending on the position and nature of the change.

Nonsense mutation

A base substitution that changes an amino acid-encoding codon into a stop codon, terminating protein synthesis prematurely and producing a shortened protein.

Truncated protein

A shortened, incomplete protein produced when a nonsense mutation introduces a premature stop codon during translation.

Silent mutation

A base substitution that causes no change in the amino acid sequence of a protein and therefore produces no alteration in phenotype.

Deletion

A class of mutation where a segment of DNA—ranging from a single base pair up to $10{,}000$ or more base pairs—is removed from the genetic sequence.

Insertion

A class of mutation where additional DNA—ranging from a single base pair to $10{,}000+$ base pairs—is added into the genetic sequence.

Frameshift mutation

A shift in the translational reading frame caused by insertions or deletions of nucleotides in amounts not divisible by $3$, occurring approximately two-thirds ($67\%$) of the time with random-sized mutations.

Reading frame

One of three possible ways nucleotide sequences are grouped into successive three-nucleotide codons during translation, consisting of one correct frame and two incorrect frames ($-1$ and $+1$).

Open reading frame (ORF)

A continuous stretch of DNA or mRNA codons starting with an initiation codon (AUG) and ending with a stop codon that can be translated into a polypeptide.

Spontaneous mutation rate

The baseline error frequency of DNA polymerases during replication, occurring at approximately $1$ mistake per $10^9$ replicated base pairs, or about $1$ mistake per $10^6$ replicated genes.

Mutagen

A chemical or physical agent (such as UV light) that significantly increases the rate of DNA mutations above the spontaneous background rate.

Positive selection (Direct selection)

A method used to identify mutants by directly exposing a population to conditions where only the desired mutant can grow, such as using streptomycin to select for antibiotic resistance.

Negative selection (Indirect selection / Screening)

A method used to detect mutants, such as auxotrophs, by identifying colonies that fail to grow on a specific minimal medium while growing on a complete reference plate.

Auxotroph

A mutant microbial strain that has lost the biochemical ability to synthesize an essential nutrient (such as an amino acid) and requires that nutrient to be added to the medium to grow.

Velvet replicator

A sterile velvet-covered cylinder used in replica plating to stamp bacterial colonies from a master plate onto secondary selective or minimal media plates in an identical pattern.

Ames test

A biological screening assay developed by Dr. Ames using a histidine auxotroph of Salmonella to detect whether a chemical or substance is a mutagen.

Revertant

A mutant cell or colony that undergoes a reverse mutation restoring the wild-type phenotype, such as a histidine-requiring auxotroph mutating back to histidine independence in the Ames test.

Vertical gene transfer

The passage of genetic material from parent organisms to offspring through replication and cell division in sexual or asexual reproduction.

Horizontal gene transfer

The one-way transfer of genetic material between two mature cells or organisms rather than from parent to offspring.

Genetic recombination

The process by which foreign DNA entering a cell becomes physically integrated or inserted into the host cell's existing genome.