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: Culture Condition Dilution Colonies Minimal medium 10⁻¹ 18 Minimal medium + leucine 10⁻⁷ 9 What is the rate of mutation at the locus associated with leucine biosynthesis?

Key Concepts

Mutation Rate

The mutation rate refers to the frequency at which new mutations occur in a given gene or locus over a specific period or number of cell divisions. It is typically expressed as the number of mutations per cell division or per generation. In this context, understanding the mutation rate is crucial for determining how often leu⁻ cells can revert to leu⁺ cells, which are capable of synthesizing leucine.

Selective Media

Selective media are types of growth media that allow for the growth of certain organisms while inhibiting others. In this experiment, minimal medium only supports the growth of leu⁺ cells, while minimal medium with leucine allows all cells to grow. This distinction is essential for analyzing the results, as it helps differentiate between the original leu⁻ cells and those that have mutated to leu⁺.

Dilution Plating

Dilution plating is a technique used to estimate the concentration of viable microorganisms in a sample. By diluting the original culture and plating it on selective media, researchers can count the number of colonies that grow. This method is important for calculating the mutation rate, as it allows for the quantification of leu⁺ colonies in relation to the total number of cells present in the original culture.