Problem 20 - Biology

Question

A scientist creates fruit fly larvae with a mutation that eliminates the exonuclease function of DNA pol III. Which prediction about the mutational load in the adult fruit flies is most likely to be correct? a. The adults with the DNA pol III mutation will have significantly more mutations than average. b. The adults with the DNA pol III mutation will have slightly more mutations than average. c. The adults with the DNA pol III mutation will have the same number of mutations as average. d. The adults with the DNA pol III mutation will have fewer mutations than average.

Step-by-Step Solution

Answer

a. The adults with the DNA pol III mutation will have significantly more mutations than average.

Step 1: Understand DNA Pol III Function

DNA polymerase III (DNA pol III) is crucial for DNA replication in prokaryotes. It has a proofreading ability due to its exonuclease function, which helps correct errors by removing mismatched nucleotides.

Step 2: Identify the Mutation

The mutation described eliminates the exonuclease function of DNA pol III. This means the enzyme can no longer correct errors during DNA replication.

Step 3: Predict Mutational Load Impact

Without the proofreading function, the error rate during DNA replication will increase. This leads to a higher likelihood of mutations being passed on to the daughter cells.

Step 4: Compare Mutation Rates

Given that the error-correcting function is compromised, it is logical to predict that the adult fruit flies will have a higher number of mutations compared to normal flies.

Step 5: Choose the Most Accurate Prediction

Based on these insights, the correct prediction is that the adults with the DNA pol III mutation will have significantly more mutations than average, as the proofreading capability has been lost.

Key Concepts

Error Correction in DNA Replication

DNA replication is a complex and highly accurate process. During replication, the enzyme DNA polymerase III (DNA pol III) plays a crucial role.

It adds nucleotides to the growing DNA strand, ensuring that the genetic information is accurately copied. However, errors can occur when incorrect nucleotides are incorporated.
To minimize these errors, DNA pol III has a built-in proofreading ability.

This proofreading action is known as 'error correction in DNA replication'. It works as follows:

• During DNA synthesis, if DNA pol III detects a mismatched nucleotide (an error), it temporarily pauses.
• The enzyme then activates its exonuclease function to remove the incorrect nucleotide.
• After removal, DNA pol III resumes adding the correct nucleotide.

This error correction mechanism is essential for maintaining genetic stability and preventing mutations that could lead to diseases or malfunctioning proteins.

Exonuclease Function

The exonuclease function of DNA polymerase III is integral to its proofreading capability. The term 'exonuclease' refers to an enzyme that removes nucleotides one at a time from the end of a DNA strand.

For DNA pol III, this function allows the enzyme to backtrack and excise mismatched nucleotides.

• When a nucleotide mismatch occurs, DNA pol III shifts its active site to the mismatch site.
• Its exonuclease activity cleaves the faulty nucleotide from the DNA strand.
• Once the incorrect nucleotide is removed, DNA pol III switches back to its polymerase function, adding the correct nucleotide to continue replication.

This dual functionality of DNA pol III ensures high fidelity during DNA replication, significantly reducing the error rate.

Mutation Rate

The term 'mutation rate' refers to the frequency at which changes or errors occur in the DNA sequence during replication.
Under normal circumstances, the mutation rate is kept low by the proofreading functions of enzymes like DNA polymerase III.

When the exonuclease function is impaired or absent, as described in the exercise, the error correction mechanism fails.
This leads to an increased frequency of errors being introduced into the DNA.

Mutations can have various consequences depending on their nature and location in the genome:

• Some mutations may have no noticeable effect if they occur in non-coding regions or do not alter protein function significantly.
• Other mutations might be detrimental, potentially leading to genetic disorders, malfunctioning proteins, or reduced organism fitness.
• In some rare cases, mutations can be beneficial, providing adaptive advantages in changing environments.

In the case of the fruit flies with the exonuclease function mutation, we would predict a significantly higher mutation rate compared to the average, as the primary error-correcting function is lost.