Lecture 3: Calculating Mutation Rate & The Fluctuation Test

Vocabulary flashcards covering the Luria-Delbrück fluctuation experiment, spontaneous versus directed mutations, mathematical calculation of mutation rates using the Poisson distribution, and specific mutagens and their mechanisms.

Directed Mutations Hypothesis

The hypothesis proposing that exposure to an environmental agent specifically directs or induces genetic changes that confer resistance, predicting that every cell population has an equal opportunity to develop resistance and yielding similar numbers of resistant colonies across cultures.

Spontaneous Mutations Hypothesis

The hypothesis proposing that mutations arise randomly before exposure to an environmental agent, predicting widely fluctuating numbers of resistant colonies across independent cultures depending on when mutations occurred during growth.

TonB

A bacterial outer membrane protein in wild-type E. coli that serves as the receptor for phage T1 attachment, leading to infection, cell lysis, and release of new phage particles.

$$TonB^R$$

A mutant form of E. coli harboring an altered TonB receptor to which phage T1 cannot attach, preventing viral infection and allowing the bacterium to survive on media containing phage T1.

Luria-Delbrück Fluctuation Test

A 1943 experimental assay designed by Salvador Luria and Max Delbrück to test whether bacterial resistance to phage T1 arises via directed physiological adaptation or random spontaneous mutation prior to selection.

Fluctuation Test Control (Experiment 1)

The plating of multiple aliquots from a single bulk culture onto phage T1 plates; it produces a low variance ($\text{Variance} = 18.23$, $\text{Mean} = 16.7$) because pre-existing mutants are mixed thoroughly in one flask, making the sampling probability identical across aliquots.

Fluctuation Test Individual Cultures (Experiment 2)

The plating of independent small cultures grown in the absence of phage onto separate plates containing phage T1; it produces high variance ($\text{Variance} = 752.38$, $\text{Mean} = 11.35$), proving that mutations occur spontaneously prior to selection.

Jackpot Culture

An independent culture in the fluctuation test that yields an unusually high number of resistant colonies because a spontaneous mutation occurred very early in population growth prior to phage exposure and was exponentially propagated.

Mutation Rate ($a$)

The probability of a mutation occurring per cell division, calculated as $a = \frac{m_2 - m_1}{N_2 - N_1}$ or $a = \frac{m}{N}$, where $m$ is the number of mutations per culture and $N$ is the total number of cell divisions.

Poisson Equation (Mutational Probabilities)

The discrete probability distribution governing random mutational occurrences: $P_i = \frac{m^i e^{-m}}{i!}$, where $P_i$ is the probability of having $i$ mutations in a culture and $m$ is the average number of mutations per culture.

$P_0$ Method

A mathematical method using the zero-class of the Poisson distribution ($P_0 = e^{-m}$) to calculate the average number of mutations per tube as $m = -\ln(P_0)$, based on the proportion of cultures containing zero mutants.

UV Irradiation (Mutagen)

A physical mutagen that induces pyrimidine dimers in DNA, triggering error-prone SOS repair and resulting primarily in $\text{G-C} \rightarrow \text{A-T}$ transition mutations.

Bromouracil

A base analogue mutagen that incorporates into DNA and causes both $\text{G-C} \rightarrow \text{A-T}$ and $\text{A-T} \rightarrow \text{G-C}$ transition mutations.

N-methyl-N'-nitro-N-nitrosoguanidine (NTG)

An alkylating mutagen that generates $O^6$-methylguanine, producing $\text{G-C} \rightarrow \text{A-T}$ transitions and typically inducing multiple closely spaced mutations in E. coli.

ICR-191

An intercalating chemical mutagen that stacks between looped-out bases, primarily causing frameshift mutations through insertions or deletions within runs of G or C bases.