Defective DNA repair is seen in all EXCEPT?
**Question:** Defective DNA repair is seen in all EXCEPT?
**Core Concept:** DNA repair is a critical process that maintains the integrity of the genetic material in organisms by correcting errors or damages that occur in DNA replication and transcription. Defective DNA repair can lead to genomic instability, which is associated with various diseases and conditions, including cancer.
**Why the Correct Answer is Right:**
The correct answer is **D.** Base excision repair (BER) is a type of DNA repair that corrects small base lesions and damaged bases in DNA. In BER, DNA polymerase reads the correct base and inserts it into the DNA strand, replacing the damaged base. When BER is defective, it leads to genomic instability and an increased risk of cancer.
**Why Each Wrong Option is Incorrect:**
A. Nucleotide excision repair (NER) is a type of DNA repair that corrects bulky DNA lesions and damaged bases. NER is distinct from base excision repair (BER) mentioned in option D.
B. Mismatch repair (MMR) is a type of DNA repair that corrects errors during DNA replication, such as base substitutions, insertions, and deletions. It is distinct from base excision repair (BER) mentioned in option D.
C. Direct reversal repair (DRR) is a type of DNA repair that involves the direct reversal of the damaged base to its normal form. DRR differs from base excision repair (BER) mentioned in option D.
E. DNA replication is the process of synthesizing DNA molecules from the parental DNA molecules. It is a separate process from DNA repair but is crucial for maintaining genomic integrity.
**Clinical Pearl:**
Understanding the different types of DNA repair mechanisms, such as BER, NER, MMR, and DRR, is essential for understanding the pathogenesis of diseases and conditions associated with defective DNA repair, like certain cancers. Knowledge of these mechanisms also aids in interpreting genetic tests and diagnosing genetic disorders, as well as providing guidance on treatment options.
**Correct Answer:** C. Direct reversal repair (DRR) is a type of DNA repair that involves the direct reversal of the damaged base to its normal form. DRR differs from base excision repair (BER) mentioned in option D. Defective DRR would lead to accumulation of damaged bases, not base substitutions, insertions, or deletions.
**Why Each Wrong Option is Incorrect:**
A. Nucleotide excision repair (NER) is a type of DNA repair that targets bulky DNA adducts, such as those caused by UV radiation exposure. NER differs from base excision repair (BER) mentioned in option D. Defective NER would lead to failure to correct bulky DNA lesions, increasing the risk of skin cancer due to DNA damage from UV radiation exposure.
B. Mismatch repair (MMR) is a type of DNA repair that corrects errors in newly synthesized DNA strands. MMR differs from base excision repair (BER) mentioned in option D. Defective MMR would lead to the accumulation of mispaired bases, not base substitutions, insertions, or deletions.
D. Base excision repair