A software engineer presents to the OPD with 'complaints of easy fatigability. He reports sitting in front of a computer for 12-14 hours a day consuming junk food, and eating few fruits and vegetables. CBC results show hemoglobin (Hb) concentration of $7 \mathrm{gm} \%$ and MCV of 120 fL . What is the most likely cause of anemia?
Q2
An investigator isolates bacteria from a patient who presented with dysuria and urinary frequency. These bacteria grow rapidly in pink colonies on MacConkey agar. During replication of these bacteria, the DNA strands are unwound at the origin of replication, forming two Y-shaped replication forks that open in opposite directions. At each replication fork, daughter strands are synthesized from the template strands in a 5′ to 3′ direction. On one strand, the DNA is synthesized continuously; on the other strand, the DNA is synthesized in short segments. The investigator finds that three enzymes are directly involved in elongating the DNA of the lagging strand in these bacteria. One of these enzymes has an additional function that the others do not possess. Which of the following steps in DNA replication is unique to this enzyme?
Q3
An investigator is studying the replication of bacterial DNA with modified nucleotides. After unwinding, the double-stranded DNA forms a Y-shaped replication fork that separates into two strands. At each of these strands, daughter strands are synthesized. One strand is continuously extended from the template strands in a 5′ to 3′ direction. Which of the following is exclusively associated with the strand being synthesized away from the replication fork?
Q4
While performing a Western blot, a graduate student spilled a small amount of the radiolabeled antibody on her left forearm. Although very little harm was done to the skin, the radiation did cause minor damage to the DNA of the exposed skin by severing covalent bonds between the nitrogenous bases and the deoxyribose sugar, leaving several apurinic/apyrimidinic sites. Damaged cells would most likely repair these sites by which of the following mechanisms?
Q5
A 54-year-old woman with breast cancer comes to the physician because of redness and pain in the right breast. She has been undergoing ionizing radiation therapy daily for the past 2 weeks as adjuvant treatment for her breast cancer. Physical examination shows erythema, edema, and superficial desquamation of the skin along the right breast at the site of radiation. Sensation to light touch is intact. Which of the following is the primary mechanism of DNA repair responsible for preventing radiation-induced damage to neighboring neurons?
Q6
As part of a clinical research study, the characteristics of neoplastic and normal cells are being analyzed in culture. It is observed that neoplastic cell division is aided by an enzyme which repairs progressive chromosomal shortening, which is not the case in normal cells. Due to the lack of chromosomal shortening, these neoplastic cells divide more rapidly than the normal cells. Which of the following enzymes is most likely involved?
Q7
An investigator studying DNA mutation mechanisms isolates single-stranded DNA from a recombinant bacteriophage and sequences it. The investigator then mixes it with a buffer solution and incubates the resulting mixture at 70°C for 16 hours. Subsequent DNA resequencing shows that 3.7 per 1,000 cytosine residues have mutated to uracil. Which of the following best describes the role of the enzyme that is responsible for the initial step in repairing these types of mutations in living cells?
Q8
A 47-year-old man presents to his primary care physician for fatigue. Over the past 3 months, his tiredness has impacted his ability to work as a corporate lawyer. He denies any changes to his diet, exercise regimen, bowel movements, or urinary frequency. His past medical history is notable for obesity, type II diabetes mellitus, and hypertension. He takes metformin and enalapril. His family history is notable for colorectal cancer in his father and paternal grandfather and endometrial cancer in his paternal aunt. He has a 20-pack-year smoking history and drinks one 6-pack of beer a week. His temperature is 98.8°F (37.1°C), blood pressure is 129/71 mmHg, pulse is 82/min, and respirations are 17/min. On exam, he has conjunctival pallor. A stool sample is positive for occult blood. A colonoscopy reveals a small hemorrhagic mass at the junction of the ascending and transverse colon. Which of the following processes is likely impaired in this patient?
Q9
An investigator is comparing DNA replication in prokaryotes and eukaryotes. He finds that the entire genome of E. coli (4 × 106 base pairs) is replicated in approximately 30 minutes. A mammalian genome (3 × 109 base pairs) is usually replicated within 3 hours. Which of the following characteristics of eukaryotic DNA replication is the most accurate explanation for this finding?
Q10
A 19-year-old man presents to his primary care physician for evaluation before going off to college. Specifically, he wants to know how to stay healthy while living outside his home. Since childhood he has suffered severe sunburns even when he goes outside for a small period of time. He has also developed many freckles and rough-surfaced growths starting at the same age. Finally, his eyes are very sensitive and become irritated, bloodshot, and painful after being outside. A defect in a protein with which of the following functions is most likely responsible for this patient's symptoms?
DNA repair US Medical PG Practice Questions and MCQs
Question 1: A software engineer presents to the OPD with 'complaints of easy fatigability. He reports sitting in front of a computer for 12-14 hours a day consuming junk food, and eating few fruits and vegetables. CBC results show hemoglobin (Hb) concentration of $7 \mathrm{gm} \%$ and MCV of 120 fL . What is the most likely cause of anemia?
A. Cyanocobalamin deficiency
B. Acute blood loss
C. Sideroblastic anemia
D. Folate deficiency (Correct Answer)
E. Iron deficiency anemia
Explanation: ***Folate deficiency***
- A **macrocytic anemia** with an **MCV of 120 fL** is characteristic of folate deficiency, as folate is vital for **DNA synthesis** in red blood cell production.
- The patient's diet of **junk food** and few fruits/vegetables suggests poor nutritional intake, as folate is abundant in leafy greens and fresh produce.
*Cyanocobalamin deficiency*
- While also causing **macrocytic anemia** with high MCV, cyanocobalamin (Vitamin B12) deficiency often presents with **neurological symptoms** (e.g., neuropathy, cognitive changes) which are not mentioned.
- Dietary sources of B12 are primarily **animal products**, and while junk food is poor, a strict vegetarian/vegan diet is a stronger indicator of B12 deficiency.
*Acute blood loss*
- Acute blood loss typically causes **normocytic, normochromic anemia**, characterized by a normal MCV in the initial stages.
- While severe blood loss can lead to fatigue, the **elevated MCV** of 120 fL makes this diagnosis unlikely unless there's a pre-existing macrocytic condition.
*Sideroblastic anemia*
- Sideroblastic anemia can be **microcytic, normocytic, or macrocytic**, but it is primarily characterized by the presence of **ring sideroblasts** in the bone marrow and iron overload.
- It's often associated with **alcoholism, lead poisoning, or myelodysplastic syndromes**, and the typical features of the patient's diet and MCV do not point towards this condition.
*Iron deficiency anemia*
- Iron deficiency anemia presents with **microcytic, hypochromic anemia** with a **low MCV** (typically <80 fL), not macrocytic anemia.
- While iron deficiency is the most common cause of anemia worldwide and can result from poor diet, the **elevated MCV of 120 fL** clearly excludes this diagnosis.
Question 2: An investigator isolates bacteria from a patient who presented with dysuria and urinary frequency. These bacteria grow rapidly in pink colonies on MacConkey agar. During replication of these bacteria, the DNA strands are unwound at the origin of replication, forming two Y-shaped replication forks that open in opposite directions. At each replication fork, daughter strands are synthesized from the template strands in a 5′ to 3′ direction. On one strand, the DNA is synthesized continuously; on the other strand, the DNA is synthesized in short segments. The investigator finds that three enzymes are directly involved in elongating the DNA of the lagging strand in these bacteria. One of these enzymes has an additional function that the others do not possess. Which of the following steps in DNA replication is unique to this enzyme?
A. Elongation of lagging strand in 5'→3' direction
B. Excision of nucleotides with 5'→3' exonuclease activity (Correct Answer)
C. Prevention of reannealing of the leading strand and the lagging strand
D. Creation of ribonucleotide primers
E. Proofreading for mismatched nucleotides
Explanation: ***Excision of nucleotides with 5'→3' exonuclease activity***
- **DNA polymerase I** possesses unique **5'→3' exonuclease activity** that allows it to remove RNA primers synthesized by primase.
- After primer removal, DNA polymerase I synthesizes DNA in the 5'→3' direction to fill the gap.
*Elongation of lagging strand in 5'→3' direction*
- While **DNA polymerase I** elongates the lagging strand, this 5'→3' synthesis function is also shared by **DNA polymerase III**, which is the primary enzyme for DNA synthesis.
- Therefore, this specific function is not unique to the enzyme in question (DNA polymerase I) as DNA polymerase III also performs 5'→3' elongation.
*Prevention of reannealing of the leading strand and the lagging strand*
- This function is carried out by **single-strand binding proteins (SSBs)**, which bind to the separated DNA strands to prevent them from reannealing and protect them from degradation.
- This is not a function of any DNA polymerase.
*Creation of ribonucleotide primers*
- The synthesis of short RNA primers required for initiation of DNA synthesis is performed by **primase**, an RNA polymerase.
- DNA polymerases do not create primers but rather extend them.
*Proofreading for mismatched nucleotides*
- **DNA polymerase I** and **DNA polymerase III** both possess **3'→5' exonuclease activity** for proofreading, allowing them to remove incorrectly incorporated nucleotides.
- Since this function is shared by DNA polymerase III, it is not unique to DNA polymerase I.
Question 3: An investigator is studying the replication of bacterial DNA with modified nucleotides. After unwinding, the double-stranded DNA forms a Y-shaped replication fork that separates into two strands. At each of these strands, daughter strands are synthesized. One strand is continuously extended from the template strands in a 5′ to 3′ direction. Which of the following is exclusively associated with the strand being synthesized away from the replication fork?
A. Reverse transcriptase activity
B. Repeated activity of ligase (Correct Answer)
C. Elongation in the 3'→5' direction
D. Synthesis of short RNA sequences
E. 5' → 3' exonuclease activity
Explanation: ***Repeated activity of ligase***
- The lagging strand, synthesized away from the replication fork, is made in fragments (**Okazaki fragments**) due to the 5' to 3' synthesis direction of DNA polymerase.
- **DNA ligase** repeatedly joins these Okazaki fragments together, forming a continuous strand.
*Reverse transcriptase activity*
- **Reverse transcriptase** synthesizes DNA from an RNA template, which is not involved in normal bacterial DNA replication.
- This enzyme is characteristic of **retroviruses** and certain eukaryotic telomere maintenance.
*Elongation in the 3'→5' direction*
- DNA polymerases only synthesize new DNA strands in the **5' to 3' direction**.
- While reading the template strand in the 3' to 5' direction, the daughter strand is always built from 5' to 3'.
*Synthesis of short RNA sequences*
- **RNA primers** are synthesized by **primase** on both the leading and lagging strands to initiate DNA synthesis.
- This process is not exclusive to the strand synthesized away from the replication fork; the leading strand also requires an initial RNA primer.
*5' → 3' exonuclease activity*
- The **5' to 3' exonuclease activity** of DNA polymerase I in bacteria is responsible for removing RNA primers.
- This activity occurs on both the leading and lagging strands, as both require primer removal.
Question 4: While performing a Western blot, a graduate student spilled a small amount of the radiolabeled antibody on her left forearm. Although very little harm was done to the skin, the radiation did cause minor damage to the DNA of the exposed skin by severing covalent bonds between the nitrogenous bases and the deoxyribose sugar, leaving several apurinic/apyrimidinic sites. Damaged cells would most likely repair these sites by which of the following mechanisms?
A. Nucleotide excision repair
B. Nonhomologous end joining repair
C. Homologous recombination
D. Mismatch repair
E. Base excision repair (Correct Answer)
Explanation: **Base excision repair**
- This mechanism is specifically involved in correcting **single-base DNA damage** or **modified bases**, such as **apurinic/apyrimidinic (AP) sites**.
- It involves removing the damaged base by a **DNA glycosylase**, creating an AP site, which is then processed by an **AP endonuclease** to cleave the phosphodiester backbone, followed by DNA polymerase and ligase.
*Nucleotide excision repair*
- Primarily repairs **bulky DNA lesions**, such as **thymine dimers** caused by UV radiation, or damage from chemical adducts that distort the DNA helix.
- It involves excising a larger oligonucleotide containing the damage, not just a single base.
*Nonhomologous end joining repair*
- This pathway is used to repair **double-strand DNA breaks**, where both strands of the DNA molecule are broken.
- It is a "quick-and-dirty" repair mechanism that ligates the broken ends together, often leading to small insertions or deletions.
*Homologous recombination*
- A repair mechanism for **double-strand DNA breaks** that uses a homologous DNA template (e.g., sister chromatid) to accurately repair the break.
- This process is highly accurate but occurs only when a homologous template is available, typically during the S and G2 phases of the cell cycle.
*Mismatch repair*
- Corrects **base-pair mismatches** and **small insertions/deletions** that occur during DNA replication, which were not corrected by DNA polymerase proofreading.
- It targets newly synthesized DNA strands based on methylation patterns in the parental strand.
Question 5: A 54-year-old woman with breast cancer comes to the physician because of redness and pain in the right breast. She has been undergoing ionizing radiation therapy daily for the past 2 weeks as adjuvant treatment for her breast cancer. Physical examination shows erythema, edema, and superficial desquamation of the skin along the right breast at the site of radiation. Sensation to light touch is intact. Which of the following is the primary mechanism of DNA repair responsible for preventing radiation-induced damage to neighboring neurons?
A. Homology-directed repair
B. Base excision repair
C. Nonhomologous end joining repair (Correct Answer)
D. DNA mismatch repair
E. Nucleotide excision repair
Explanation: ***Nonhomologous end joining repair***
- This pathway is crucial for repairing **double-strand DNA breaks**, which are a major form of damage caused by **ionizing radiation**.
- It directly ligates the broken DNA ends without requiring a homologous template, making it an efficient but potentially error-prone repair mechanism.
*Homology-directed repair*
- This pathway is also used to repair **double-strand DNA breaks** but requires a **homologous DNA template** (usually a sister chromatid) for accurate repair.
- While highly accurate, it is typically active during the S and G2 phases of the cell cycle and is generally slower and less dominant than NHEJ for immediate radiation-induced damage in non-dividing cells like neurons.
*Base excision repair*
- This mechanism primarily corrects damage to individual DNA bases, such as **oxidative damage**, alkylation, or deamination.
- It is not the primary mechanism for repairing the **double-strand breaks** induced by ionizing radiation.
*DNA mismatch repair*
- This pathway corrects errors that arise during **DNA replication**, specifically mismatched base pairs or small insertions/deletions.
- It is not involved in repairing radiation-induced DNA damage like **double-strand breaks**.
*Nucleotide excision repair*
- This pathway repairs bulky DNA lesions, such as those caused by **UV radiation** (e.g., pyrimidine dimers) or chemical mutagens.
- It removes a segment of DNA containing the damage but is not the primary repair mechanism for **double-strand breaks** caused by ionizing radiation.
Question 6: As part of a clinical research study, the characteristics of neoplastic and normal cells are being analyzed in culture. It is observed that neoplastic cell division is aided by an enzyme which repairs progressive chromosomal shortening, which is not the case in normal cells. Due to the lack of chromosomal shortening, these neoplastic cells divide more rapidly than the normal cells. Which of the following enzymes is most likely involved?
A. Topoisomerase
B. DNA polymerase
C. Reverse transcriptase
D. Protein kinase
E. Telomerase (Correct Answer)
Explanation: ***Telomerase***
- **Telomerase** is an enzyme that adds repetitive nucleotide sequences (telomeres) to the ends of chromosomes, counteracting their progressive shortening during DNA replication. This activity is crucial for the continuous division of neoplastic cells.
- In normal somatic cells, **telomerase activity is typically low or absent**, leading to telomere shortening with each division, eventually triggering cellular senescence or apoptosis. The presence of telomerase in neoplastic cells allows them to bypass these natural limits on proliferation.
*Topoisomerase*
- **Topoisomerases** are enzymes that regulate the supercoiling of DNA by breaking and rejoining DNA strands, which is essential during replication and transcription to relieve torsional stress.
- They do not directly repair chromosomal shortening but rather manage the topological state of DNA.
*DNA polymerase*
- **DNA polymerase** is primarily responsible for synthesizing new DNA strands by adding nucleotides, thereby elongating the DNA molecule during replication and DNA repair processes.
- While essential for DNA replication, it cannot fully replicate the very ends of linear chromosomes, leading to the **end-replication problem** and telomere shortening.
*Reverse transcriptase*
- **Reverse transcriptase** is an enzyme that synthesizes DNA from an RNA template, a process central to retroviruses and some eukaryotic elements like retrotransposons.
- Although telomerase itself is a specialized reverse transcriptase (using an RNA template to synthesize DNA telomeres), the general term "reverse transcriptase" does not specifically refer to the enzyme that repairs chromosomal shortening in the context of cell division.
*Protein kinase*
- **Protein kinases** are enzymes that add phosphate groups to proteins, a process known as phosphorylation. This modification can alter protein activity, localization, or stability, playing a critical role in signal transduction pathways.
- They are involved in regulating various cellular processes, including cell growth and division, but do not directly repair chromosomal shortening.
Question 7: An investigator studying DNA mutation mechanisms isolates single-stranded DNA from a recombinant bacteriophage and sequences it. The investigator then mixes it with a buffer solution and incubates the resulting mixture at 70°C for 16 hours. Subsequent DNA resequencing shows that 3.7 per 1,000 cytosine residues have mutated to uracil. Which of the following best describes the role of the enzyme that is responsible for the initial step in repairing these types of mutations in living cells?
A. Connecting the phosphodiester backbone
B. Cleavage of the phosphodiester bond 3' of damaged site
C. Creation of abasic site (Correct Answer)
D. Release of the damaged nucleotide
E. Addition of free nucleotides to 3' end
Explanation: ***Creation of abasic site***
- The mutation of **cytosine to uracil** is an example of **deamination**, which is repaired by the **base excision repair (BER)** pathway.
- The initial step in BER involves **DNA glycosylase**, which *removes* the damaged base (uracil) from the sugar-phosphate backbone by hydrolyzing the **N-glycosidic bond**, creating an **abasic site**.
*Connecting the phosphodiester backbone*
- This is the function of **DNA ligase**, which acts at the *final step* of DNA repair pathways to seal the nicks in the backbone.
- It does not initiate the repair process for deaminated bases.
*Cleavage of the phosphodiester bond 3' of damaged site*
- This is typically performed by an **AP endonuclease (APE1)** after the abasic site has been created.
- It is a *subsequent step* in BER, not the initial one for removing the damaged base itself.
*Release of the damaged nucleotide*
- While the damaged base is eventually *released*, the initial enzyme (DNA glycosylase) specifically removes the **base**, leaving the sugar and phosphate intact.
- The entire nucleotide (base, sugar, and phosphate) is typically removed later by an **AP lyase** or APE1, after the initial glycosylase action.
*Addition of free nucleotides to 3' end*
- This is the function of **DNA polymerase**, which fills in the gap after the damaged nucleotide and surrounding region have been excised.
- This occurs *after* the initial recognition and removal of the damaged base, not as the primary repair step.
Question 8: A 47-year-old man presents to his primary care physician for fatigue. Over the past 3 months, his tiredness has impacted his ability to work as a corporate lawyer. He denies any changes to his diet, exercise regimen, bowel movements, or urinary frequency. His past medical history is notable for obesity, type II diabetes mellitus, and hypertension. He takes metformin and enalapril. His family history is notable for colorectal cancer in his father and paternal grandfather and endometrial cancer in his paternal aunt. He has a 20-pack-year smoking history and drinks one 6-pack of beer a week. His temperature is 98.8°F (37.1°C), blood pressure is 129/71 mmHg, pulse is 82/min, and respirations are 17/min. On exam, he has conjunctival pallor. A stool sample is positive for occult blood. A colonoscopy reveals a small hemorrhagic mass at the junction of the ascending and transverse colon. Which of the following processes is likely impaired in this patient?
A. Mismatch repair (Correct Answer)
B. Homologous recombination
C. Non-homologous end joining
D. Nucleotide excision repair
E. Base excision repair
Explanation: ***Mismatch repair***
- The patient's presentation with **colorectal cancer** at a relatively young age and a strong family history of various cancers (colorectal, endometrial) in **first-degree and second-degree relatives** suggests Lynch syndrome (Hereditary Nonpolyposis Colorectal Cancer).
- **Lynch syndrome** is caused by inherited mutations in genes responsible for **DNA mismatch repair**, leading to an accumulation of errors and increased cancer risk.
*Homologous recombination*
- This repair mechanism is crucial for fixing **double-strand DNA breaks** using a homologous DNA template, important for genetic stability and primarily associated with genes like BRCA1/2.
- While defects in homologous recombination can lead to cancer (e.g., **breast and ovarian cancers**), it is not the primary mechanism implicated in Lynch syndrome or the patient's specific presentation of colorectal and endometrial cancer families.
*Non-homologous end joining*
- This is another major pathway for repairing **double-strand DNA breaks**, but it does so by directly ligating the broken ends, often with some loss of genetic information, and does not rely on a homologous template.
- Defects in non-homologous end joining are not typically linked to the specific spectrum of cancers seen in **Lynch syndrome**.
*Nucleotide excision repair*
- This pathway is responsible for removing bulky DNA lesions, such as those caused by **UV light (e.g., pyrimidine dimers)** or certain chemical mutagens, and its defects are associated with conditions like xeroderma pigmentosum.
- The clinical picture and family history are not characteristic of disorders related to impaired **nucleotide excision repair**.
*Base excision repair*
- This repair pathway primarily corrects small, non-bulky DNA lesions, such as **oxidized, alkylated, or deaminated bases**, that do not distort the DNA helix.
- While important for maintaining genomic integrity, defects in base excision repair are typically associated with different cancer susceptibilities and not the specific features of **Lynch syndrome**.
Question 9: An investigator is comparing DNA replication in prokaryotes and eukaryotes. He finds that the entire genome of E. coli (4 × 106 base pairs) is replicated in approximately 30 minutes. A mammalian genome (3 × 109 base pairs) is usually replicated within 3 hours. Which of the following characteristics of eukaryotic DNA replication is the most accurate explanation for this finding?
A. Replication inhibition at checkpoint
B. Absence of telomerase enzyme activity
C. DNA compaction in chromatin
D. Simultaneous replication at multiple origins (Correct Answer)
E. More efficient DNA polymerase activity
Explanation: ***Simultaneous replication at multiple origins***
- Eukaryotic DNA replication initializes at **multiple origins of replication** along each chromosome, allowing synthesis to occur concurrently in many places.
- This strategy compensates for the much larger eukaryotic genome size, enabling its complete replication within a reasonable timeframe despite slower polymerase speed compared to prokaryotes.
*Replication inhibition at checkpoint*
- **Cell cycle checkpoints**, such as those in G1, S, and G2 phases, ensure the integrity of DNA replication and repair.
- While these checkpoints can *pause* or *inhibit* replication if errors occur, they do not fundamentally explain the *speed* or **efficiency** of replication across the entire genome.
*Absence of telomerase enzyme activity*
- **Telomerase** is an enzyme that maintains the ends of eukaryotic chromosomes (telomeres) by adding repetitive DNA sequences.
- Its presence or absence is related to telomere length regulation and cellular aging, not the overall speed of genome replication.
*DNA compaction in chromatin*
- Eukaryotic DNA is compact and organized into **chromatin** within the nucleus, which presents a challenge to replication by limiting access to the DNA.
- While enzymes must overcome this compaction, it is a *hindrance* rather than an enabler of replication speed. If anything, it would slow down replication.
*More efficient DNA polymerase activity*
- In actuality, **prokaryotic DNA polymerases** (e.g., DNA Pol III in *E. coli*) are generally more processive and faster than eukaryotic DNA polymerases.
- Therefore, more efficient polymerase activity is not a characteristic that would explain the relatively fast replication of a larger eukaryotic genome.
Question 10: A 19-year-old man presents to his primary care physician for evaluation before going off to college. Specifically, he wants to know how to stay healthy while living outside his home. Since childhood he has suffered severe sunburns even when he goes outside for a small period of time. He has also developed many freckles and rough-surfaced growths starting at the same age. Finally, his eyes are very sensitive and become irritated, bloodshot, and painful after being outside. A defect in a protein with which of the following functions is most likely responsible for this patient's symptoms?
A. Distinguishing methylated from unmethylated strands
B. Recognition of chemically dimerized bases (Correct Answer)
C. Endonucleolytic removal of bases from backbone
D. Recognition of mismatched bases
E. Sister chromatid binding and recombination
Explanation: ***Recognition of chemically dimerized bases***
- The patient's symptoms (severe sunburns, numerous freckles, rough growths, and ocular irritation) are highly suggestive of **Xeroderma Pigmentosum (XP)**, a genetic disorder characterized by extreme sensitivity to UV light.
- XP is caused by a defect in **nucleotide excision repair (NER)**, specifically the proteins involved in **recognizing and excising DNA damage** like thymine dimers formed by UV radiation.
*Distinguishing methylated from unmethylated strands*
- This function is crucial for **DNA mismatch repair**, which corrects errors incorporated during DNA replication.
- Defects in mismatch repair are associated with conditions like **Hereditary Nonpolyposis Colorectal Cancer (HNPCC)**, which presents differently from the patient's symptoms.
*Endonucleolytic removal of bases from backbone*
- This describes a step in **base excision repair (BER)**, which primarily handles small, non-helix-distorting base lesions.
- While important for DNA repair, defects in BER would not typically lead to the severe UV sensitivity and skin manifestations seen in this patient.
*Recognition of mismatched bases*
- This is a key step in **DNA mismatch repair**, where enzymes identify incorrectly paired bases after DNA replication.
- Deficiencies in this pathway lead to increased mutation rates and specific cancer syndromes, but not the severe UV sensitivity of Xeroderma Pigmentosum.
*Sister chromatid binding and recombination*
- This function is essential for proper chromosome segregation during cell division and for DNA repair via **homologous recombination**.
- Defects here can lead to disorders with chromosomal instability but do not directly explain the extreme UV sensitivity and skin cancers characteristic of Xeroderma Pigmentosum.
