In a bacterial culture in which all cells are unable to synthesize leucine (leu⁻), a potent mutagen is added, and the cells are allowed to undergo one round of replication. At that point, samples are taken, a series of dilutions are made, and the cells are plated on either minimal medium or minimal medium containing leucine. The first culture condition (minimal medium) allows the growth of only leu⁺ cells, while the second culture condition (minimal medium with leucine added) allows growth of all cells. The results of the experiment are as follows:

What is the rate of mutation at the locus associated with leucine biosynthesis?

Verified step by step guidance

1. Identify the two types of colonies counted: colonies on minimal medium represent the number of \( leu^{+} \) revertants (mutants that regained the ability to synthesize leucine), while colonies on minimal medium plus leucine represent the total viable cells.
2. Calculate the number of \( leu^{+} \) revertant cells per mL by adjusting for the dilution factor using the formula: \[ \text{Revertant cells per mL} = \frac{\text{Number of colonies on minimal medium}}{\text{Dilution factor}} \] In this case, the dilution factor is \( 10^{-1} \).
3. Calculate the total number of viable cells per mL by adjusting for the dilution factor for the colonies grown on minimal medium plus leucine: \[ \text{Total viable cells per mL} = \frac{\text{Number of colonies on minimal medium + leucine}}{\text{Dilution factor}} \] Here, the dilution factor is \( 10^{-7} \).
4. Determine the mutation rate at the leucine biosynthesis locus by dividing the number of revertant cells per mL by the total viable cells per mL: \[ \text{Mutation rate} = \frac{\text{Revertant cells per mL}}{\text{Total viable cells per mL}} \]
5. Interpret the mutation rate as the frequency of mutations that restore leucine synthesis ability in the bacterial population after one round of replication in the presence of the mutagen.

Key Concepts

Mutation Rate Calculation

Mutation rate is the frequency at which a specific mutation occurs in a given population or number of cells. It is often calculated by dividing the number of mutant colonies by the total number of viable cells, adjusted for dilution factors. Understanding how to interpret colony counts on selective and non-selective media is essential for accurate calculation.

Selective and Non-Selective Media in Microbial Genetics

Selective media allow growth of only certain genotypes, such as mutants that can synthesize leucine (leu⁺), while non-selective media support growth of all cells regardless of genotype. Comparing colony counts on these media helps identify the number of mutants versus total viable cells, which is critical for mutation rate determination.

Role of Dilution in Microbial Colony Counting

Dilution is used to reduce cell concentration to a countable number of colonies on plates. The dilution factor must be accounted for when calculating the original number of cells or mutants in the culture. Accurate interpretation of dilution factors ensures correct estimation of mutation frequency from colony counts.