Question

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 is 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: $$\begin{array}{lcc} \text { Culture Condition } & \text { Dilution } & \text { Colonies } \\ \text { Minimal medium } & 10^{-1} & 18 \\ \text { Minimal medium + leucine } & 10^{-7} & 6 \end{array}$$ What is the rate of mutation at the locus associated with leucine biosynthesis?

Short answer

Answer: The rate of mutation at the locus associated with leucine biosynthesis is 3 x $$10^{-6}$$.

Step-by-step solution

Identify the correct formula for calculating the rate of mutation

To find the rate of mutation, we will use the following formula: Rate of mutation = $$\frac{\text{Number of mutants}}{\text{Total number of cells plated}}$$

Calculate the number of mutants

The number of mutants can be determined from the minimal medium culture condition, as it allows the growth of only leu' cells. There were 18 colonies grown in the minimal medium at a dilution of $$10^{-1}$$. Number of mutants = Colonies grown in minimal medium x Dilution factor Number of mutants = 18 x $$10^{1}$$ = 180

Calculate the total number of cells plated

The total number of cells plated can be determined from the minimal medium + leucine culture condition, as it allows growth of all cells. There were 6 colonies grown in the minimal medium + leucine at a dilution of $$10^{-7}$$. Total number of cells plated = Colonies grown in minimal medium + leucine x Dilution factor Total number of cells plated = 6 x $$10^{7}$$ = 60,000,000

Calculate the rate of mutation

Now that we have the number of mutants (180) and the total number of cells plated (60,000,000), we can calculate the rate of mutation using the formula from Step 1. Rate of mutation = $$\frac{\text{Number of mutants}}{\text{Total number of cells plated}}$$ Rate of mutation = $$\frac{180}{60,000,000}$$ = 3 x $$10^{-6}$$ Therefore, the rate of mutation at the locus associated with leucine biosynthesis is 3 x $$10^{-6}$$.

Key concepts

Leucine Biosynthesis

Leucine is an essential amino acid, but certain cells, like some bacteria, need to synthesize it because they can't get it from their environment. These bacteria use a series of chemical reactions called leucine biosynthesis. This process transforms simple molecules into leucine through a step-by-step method. Each step requires specific enzymes, which are proteins that help in speeding up the reactions.
When the genes that encode these enzymes undergo mutations (changes in their DNA), the bacteria may lose their ability to produce leucine. Such cells cannot survive on minimal media unless leucine is added externally. Understanding this biosynthesis pathway helps in determining how bacterial cells can survive and adapt when changes occur in their genetic material.

Bacterial Culture Experiment

In the context of experimentation, a bacterial culture is a method where bacteria are grown under controlled conditions. In our exercise, the bacteria with impaired leucine synthesis were exposed to a mutagen. This agent potentially induces genetic mutations.
The experiment involved two types of culture media:

• **Minimal Medium:** This medium does not contain leucine. Only bacteria that somehow regain the ability to synthesize leucine (due to mutations) would grow here.
• **Minimal Medium with Leucine:** This medium adds leucine, allowing all bacteria to grow, regardless of their ability to synthesize leucine themselves.

Culturing under these conditions allows us to differentiate between mutant and non-mutant strains and to evaluate the effectiveness of a mutagen in causing mutations.

Genetic Mutation Analysis

Genetic mutation analysis involves studying changes in the DNA sequence of an organism. In our context, analyzing these mutations helps in identifying how often they occur under certain experimental conditions. To determine the mutation rate, we compare the number of mutants to the total number of cells.
In the provided exercise, using a calculated mutation rate helps us quantify the likelihood of genetic changes at the specific site responsible for leucine biosynthesis. Important calculations include:

• Mutant Cells Identification: Utilized by counting colonies on minimal medium. These are cells that regained leucine synthesis due to mutation.
• Total Cells Plated: Estimated by counting colonies on minimal medium plus leucine, indicating growth of all cells.
• Mutation Rate Formula: \[\text{Rate of mutation} = \frac{\text{Number of mutants}}{\text{Total number of cells plated}}\]This step uses real data to provide insights into the frequency of gene mutations, crucial for genetic and microbiological analysis.