Test on DNA Repair and Genetic Recombination

Question 1: Silent mutations, which have no effect on gene function, are considered a primary type of unrepaired DNA damage.

A. Ano

B. Ne

Explanation: The study materials define a 'lesion' as unrepaired DNA damage, and state that 'unrepaired lesions turn into mutations' such as substitutions, insertions, or deletions. Silent mutations are described as having 'no effect on gene function or affect nonessential regions of the DNA', but they are a *type* of mutation that *results* from unrepaired lesions, not a primary type of unrepaired DNA damage itself. The primary types of DNA damage listed are mismatches, abnormal bases, pyrimidine dimers, and backbone lesions.

Question 2: The direct repair mechanism by photolyase is present in humans and placental mammals.

A. Ano

B. Ne

Explanation: Humans and placental mammals have lost the direct repair mechanism by photolyase.

Question 3: In E. coli, how does the mismatch repair system distinguish between the parent and newly synthesized daughter strand to ensure accurate repair of replication errors?

A. The system recognizes the daughter strand by its unmethylated GATC sequences.

B. The parent strand is identified by specific chemical tags applied during replication.

C. MutS binds preferentially to the daughter strand, marking it for repair.

D. The system cleaves both strands, and the correct sequence is determined by a separate proofreading mechanism.

Explanation: In E. coli, the parent strand is methylated at GATC sequences by Dam methylase. After replication, the newly synthesized daughter strand remains unmethylated for a short period, resulting in hemimethylated GATC motifs. The mismatch repair system leverages this difference by cleaving the unmethylated daughter strand, where replication errors are presumed to reside.

Question 4: Which statement accurately describes the relationship between DNA lesions and mutations, according to the provided study materials?

A. A DNA lesion is unrepaired DNA damage, and if unrepaired, it can become a permanent mutation.

B. All DNA lesions immediately result in permanent mutations, even if they are repaired.

C. Silent mutations are a type of DNA lesion that always lead to cancer in eukaryotic cells.

D. Mutations always refer to insertions or deletions, while substitutions are always referred to as lesions.

Explanation: A lesion is defined as unrepaired DNA damage, and these unrepaired lesions can turn into mutations. If an incorrect base is copied to a daughter strand, the DNA change becomes permanent. The study materials state that most lesions are repaired, and less than 1 in 1000 become a mutation. Silent mutations are a type of mutation that has no effect on gene function, not necessarily a lesion that leads to cancer. Mutations can include substitutions, insertions, or deletions, not just insertions or deletions, and lesions are the damage itself, not a type of mutation.

Question 5: Transposons are typically found in most cells.

A. Ano

B. Ne

Explanation: The study materials state that 'Transposons are found in most cells'.