USMLE Step 1 Quiz
Practice Dna Structure Replication And Repair in USMLE Step 1 with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.
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A 28-year-old woman is treated with ciprofloxacin for a complicated urinary tract infection caused by E. coli. The antibiotic is effective, and her symptoms resolve within a few days. Ciprofloxacin is a fluoroquinolone antibiotic that interferes with bacterial DNA synthesis.
Which of the following enzymes is the primary target of this medication in gram-negative bacteria?
This quiz focuses on Dna Structure Replication And Repair, giving you a quick way to practice the rules, question types, and explanations that matter most for USMLE Step 1.
Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.
A 28-year-old woman is treated with ciprofloxacin for a complicated urinary tract infection caused by E. coli. The antibiotic is effective, and her symptoms resolve within a few days. Ciprofloxacin is a fluoroquinolone antibiotic that interferes with bacterial DNA synthesis.
Which of the following enzymes is the primary target of this medication in gram-negative bacteria?
Explanation: Fluoroquinolones, such as ciprofloxacin, inhibit prokaryotic topoisomerases. In gram-negative bacteria like E. coli, the primary target is DNA gyrase (a type II topoisomerase), which is responsible for introducing negative supercoils into DNA to relieve the strain of unwinding during replication. In gram-positive bacteria, the primary target is topoisomerase IV. By inhibiting these enzymes, fluoroquinolones prevent DNA replication and lead to bacterial cell death.
A 35-year-old woman seeks genetic counseling due to a strong family history of cancer. Her mother and a maternal aunt were diagnosed with breast cancer before age 50, and her maternal grandmother died from ovarian cancer. Genetic testing reveals a pathogenic mutation in the BRCA1 gene.
This patient has an increased lifetime risk of developing breast and ovarian cancer due to an impaired ability to repair DNA damage via which of the following pathways?
Explanation: The BRCA1 and BRCA2 genes encode proteins that are critical for the homologous recombination (HR) pathway, a high-fidelity mechanism for repairing double-strand DNA breaks. Mutations in these genes impair this repair process, leading to genomic instability and a significantly increased risk for breast, ovarian, prostate, and pancreatic cancers. This inherited predisposition is known as Hereditary Breast and Ovarian Cancer syndrome.
A patient with a rare genetic immunodeficiency is found to have a mutation in the gene encoding DNA ligase I. Cells from this patient are cultured in the lab and analyzed during the S phase of the cell cycle.
Which of the following molecular defects would be most prominent in these cells during DNA replication?
Explanation: DNA ligase plays a crucial role in joining DNA fragments together by forming phosphodiester bonds. During DNA replication, its primary function is to seal the nicks between Okazaki fragments on the lagging strand after the RNA primers have been removed and replaced with DNA. A deficiency in DNA ligase would therefore lead to the accumulation of thousands of unjoined, short DNA fragments, severely compromising the integrity of the newly synthesized lagging strand.
A molecular biologist is studying DNA replication in E. coli. A mutant strain is identified which successfully synthesizes most of the new DNA strands but accumulates a large number of Okazaki fragments containing RNA segments. Further analysis shows that the 5' to 3' exonuclease activity is deficient, but the 5' to 3' polymerase activity is intact.
This strain most likely has a mutation in the gene encoding which of the following enzymes?
Explanation: In E. coli, DNA polymerase I has a unique role in processing Okazaki fragments. It uses its 5' to 3' exonuclease activity to remove the RNA primer of the preceding fragment while simultaneously using its 5' to 3' polymerase activity to fill the resulting gap with DNA. DNA polymerase III is the main replicative enzyme but lacks this 5' to 3' exonuclease activity. A deficiency in DNA polymerase I's 5' to 3' exonuclease activity would prevent RNA primer removal, leading to the accumulation of RNA-containing Okazaki fragments.
A cell biology researcher is investigating the regulation of gene expression. She treats cultured cells with a histone deacetylase (HDAC) inhibitor. She observes that this treatment leads to an increase in the transcription of several target genes.
The increased gene transcription observed is most likely due to a change in chromatin structure from a condensed to a more relaxed state. This change is directly promoted by which of the following histone modifications?
Explanation: Histone acetylation is a key epigenetic modification associated with transcriptionally active chromatin (euchromatin). Acetyl groups are added to lysine residues on histone tails by histone acetyltransferases (HATs). This modification neutralizes the positive charge of the lysines, weakening the interaction between the histones and the negatively charged DNA, resulting in a more relaxed, open chromatin structure that is accessible to transcription factors and RNA polymerase. HDAC inhibitors prevent histone deacetylases from removing acetyl groups, thereby maintaining high levels of histone acetylation and promoting gene expression.
A 5-year-old fair-skinned boy is brought to the pediatrician due to multiple blistering sunburns after brief, minimal sun exposure. His mother reports that he has been developing an excessive number of freckle-like lesions on his face and arms. Physical examination reveals multiple hyperpigmented and hypopigmented macules on sun-exposed skin. His developmental milestones are normal. A skin biopsy is performed, and genetic testing is ordered.
A defect in which of the following DNA repair mechanisms is the most likely cause of this patient's condition?
Explanation: This patient's presentation of extreme photosensitivity and skin changes is classic for xeroderma pigmentosum (XP). XP is an autosomal recessive disorder caused by a defect in nucleotide excision repair (NER). The NER pathway is responsible for repairing bulky, helix-distorting DNA lesions, such as pyrimidine dimers formed by ultraviolet (UV) light exposure. Failure to repair this damage leads to a high rate of mutations and an increased risk of skin cancers.
A 45-year-old woman is diagnosed with colorectal adenocarcinoma. Her family history is significant for her father having colon cancer at age 50 and a paternal aunt having endometrial cancer at age 48. Genetic analysis of her tumor cells reveals microsatellite instability. An inherited mutation is suspected.
A germline mutation in a gene responsible for which of the following cellular processes is the most likely underlying cause of this patient's condition?
Explanation: This patient's personal and family history is highly suggestive of Lynch syndrome, also known as Hereditary Non-Polyposis Colorectal Cancer (HNPCC). Lynch syndrome is an autosomal dominant condition caused by germline mutations in DNA mismatch repair (MMR) genes (e.g., MSH2, MLH1, MSH6, PMS2). Defective MMR leads to microsatellite instability and an increased risk of colorectal, endometrial, ovarian, and other cancers at a young age.
A 62-year-old man with extensive-stage small cell lung cancer is receiving chemotherapy with a regimen that includes etoposide. This drug is known to cause DNA damage, leading to apoptosis in rapidly dividing cancer cells.
Etoposide exerts its cytotoxic effect by inhibiting which of the following enzymes, leading to the accumulation of double-strand DNA breaks?
Explanation: Etoposide is a chemotherapeutic agent that specifically targets human topoisomerase II. This enzyme creates transient double-strand breaks in DNA to manage tangles and supercoils during replication. Etoposide stabilizes the covalent complex between topoisomerase II and the cleaved DNA, preventing the re-ligation of the strands. This leads to the accumulation of permanent double-strand breaks, which triggers cell cycle arrest and apoptosis.
Researchers are studying a highly aggressive melanoma cell line in culture. They observe that these cells do not undergo replicative senescence and can divide indefinitely, a property known as cellular immortality. This characteristic is crucial for tumor growth and metastasis.
The activity of which of the following enzymes is most likely upregulated in these cancer cells, contributing to their unlimited replicative potential?
Explanation: Telomerase is a reverse transcriptase enzyme that maintains the length of telomeres, which are repetitive nucleotide sequences at the ends of linear chromosomes. In most somatic cells, telomerase activity is low, leading to progressive telomere shortening with each cell division, eventually triggering senescence or apoptosis. Many cancer cells, including melanoma, upregulate telomerase activity, allowing them to overcome this limit and achieve cellular immortality.
During DNA replication, the process must be highly accurate to prevent the accumulation of mutations. The primary DNA polymerase involved in replication has an intrinsic mechanism to correct errors as it synthesizes the new strand.
Which of the following enzymatic activities is responsible for the proofreading function of DNA polymerase?
Explanation: DNA polymerases (such as DNA polymerase III in prokaryotes and DNA polymerases δ and ε in eukaryotes) have an intrinsic proofreading capability mediated by their 3' to 5' exonuclease activity. If an incorrect nucleotide is incorporated, the polymerase pauses, the 3' to 5' exonuclease activity removes the mismatched base, and the polymerase then inserts the correct nucleotide before proceeding.
A molecular biology student is studying the process of eukaryotic DNA replication. She focuses on the synthesis of the lagging strand, which is synthesized discontinuously in short segments away from the replication fork. These segments are later joined to form a continuous strand.
Which of the following enzymes is responsible for joining the Okazaki fragments on the lagging strand by forming a phosphodiester bond?
Explanation: During lagging strand synthesis, DNA is made in short pieces called Okazaki fragments. Each fragment is initiated by an RNA primer. After the RNA primers are removed (by RNase H and FEN1 in eukaryotes, or DNA Polymerase I in prokaryotes) and the gap is filled with DNA, there remains a nick in the sugar-phosphate backbone. DNA ligase is the enzyme that seals this nick by catalyzing the formation of a phosphodiester bond, joining the fragments into a continuous DNA strand.
Spontaneous deamination of cytosine to uracil is a common form of DNA damage that occurs in cells. If left unrepaired, this change would lead to a C-G to T-A transition mutation during subsequent replication. The cell has a specific pathway to correct this type of damage.
Which of the following is the first enzyme to act in the base excision repair pathway to correct this lesion?
Explanation: The base excision repair (BER) pathway is initiated by a DNA glycosylase. In the case of cytosine deamination, uracil DNA glycosylase recognizes the uracil base (which should not be in DNA) and cleaves the N-glycosidic bond between the base and the deoxyribose sugar. This creates an apurinic/apyrimidinic (AP) site. Subsequently, an AP endonuclease cuts the phosphodiester backbone, DNA polymerase fills the gap, and DNA ligase seals the nick.
An industrial worker is accidentally exposed to a high dose of ionizing radiation, which is known to cause double-strand breaks in DNA. The worker's cells are assessed in the laboratory and are found to be in the G1 phase of the cell cycle. The cells initiate a repair process to deal with this lethal form of DNA damage.
Which of the following DNA repair mechanisms is predominantly used to repair double-strand breaks in cells during the G1 phase?
Explanation: Double-strand breaks (DSBs) are repaired by two main pathways: non-homologous end joining (NHEJ) and homologous recombination (HR). HR is a high-fidelity pathway that uses an undamaged sister chromatid as a template, so it is primarily active in the S and G2 phases of the cell cycle. In the G1 phase, when no sister chromatid is available, the cell predominantly relies on NHEJ. This pathway directly ligates the broken ends together and is often error-prone, potentially leading to small insertions or deletions.
A researcher is studying the structure of DNA. The double helix is composed of two antiparallel strands held together by hydrogen bonds between complementary base pairs. The four bases are classified as either purines or pyrimidines based on their chemical structure.
Which of the following correctly pairs a nitrogenous base with its structural class?
Explanation: Nitrogenous bases are categorized into two classes. Purines have a two-ring structure and include Adenine (A) and Guanine (G). Pyrimidines have a single-ring structure and include Cytosine (C), Thymine (T, in DNA), and Uracil (U, in RNA). Therefore, thymine is correctly classified as a pyrimidine.
DNA replication is a semi-conservative process that requires the coordinated action of numerous enzymes. Before DNA polymerase can begin synthesizing a new strand, a starting point must be established on the template strand.
Which of the following best describes the function of primase during DNA replication?
Explanation: DNA polymerases cannot initiate DNA synthesis de novo; they can only add nucleotides to a pre-existing 3'-hydroxyl (-OH) group. The enzyme primase, a type of RNA polymerase, synthesizes a short RNA primer (typically 5-10 nucleotides long) that is complementary to the DNA template. This RNA primer provides the necessary 3'-OH group for DNA polymerase to begin synthesizing the new DNA strand.
A research team discovers a rare genetic disorder characterized by severely impaired DNA replication, leading to bone marrow failure. Laboratory studies of patient cells show that while replication can be initiated, the replication forks fail to advance. The process of separating the two parental DNA strands is found to be deficient.
The protein most likely to be defective in this disorder is responsible for which of the following functions?
Explanation: The description points to a defect in the enzyme helicase. Helicase is responsible for unwinding the DNA double helix at the replication fork, separating the two parental strands to make them available as templates for replication. This process requires ATP. A defect in helicase would prevent the advancement of the replication fork, leading to a stall in DNA replication.
A 4-year-old child presents with progressive difficulty with balance and walking, jerky eye movements, and prominent, dilated blood vessels on the sclerae and skin of the ears. The patient has a history of recurrent sinopulmonary infections and is found to have elevated alpha-fetoprotein levels. This condition is caused by a mutation in the ATM gene.
The protein encoded by the mutated gene is a critical sensor for which of the following types of DNA damage?
Explanation: This clinical presentation is characteristic of ataxia-telangiectasia, an autosomal recessive disorder caused by mutations in the ATM gene. The ATM protein is a serine/threonine kinase that plays a central role in detecting DNA double-strand breaks (DSBs) and activating the cellular response, which includes cell cycle arrest and DNA repair (primarily through homologous recombination and non-homologous end joining). The hypersensitivity to ionizing radiation and predisposition to cancer in these patients is due to this defective DSB response.
A 68-year-old man with metastatic colon cancer is being treated with a chemotherapy regimen that includes 5-fluorouracil (5-FU). A common side effect of this medication is mucositis, which reflects the drug's impact on rapidly dividing cells of the gastrointestinal mucosa.
The cytotoxic effect of 5-fluorouracil is primarily due to the direct inhibition of an enzyme required for the synthesis of which of the following nucleotides?
Explanation: 5-Fluorouracil (5-FU) is a pyrimidine analog that is converted in the body to 5-fluorodeoxyuridine monophosphate (5-FdUMP). This metabolite forms a stable covalent complex with thymidylate synthase and its cofactor, methylene tetrahydrofolate. This inhibition blocks the conversion of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP), a crucial precursor for DNA synthesis. The resulting "thymineless death" is particularly effective against rapidly dividing cancer cells.
A 55-year-old woman with severe rheumatoid arthritis is treated with low-dose weekly methotrexate. This drug is a folate analog that interferes with nucleotide synthesis. Her physician monitors her complete blood count regularly due to the risk of myelosuppression.
By inhibiting dihydrofolate reductase, methotrexate ultimately limits the pool of precursors available for the synthesis of which of the following?
Explanation: Methotrexate competitively inhibits dihydrofolate reductase (DHFR), the enzyme that reduces dihydrofolate to tetrahydrofolate (THF). THF is a crucial carrier of one-carbon units required for the synthesis of both purine bases (adenine and guanine) and thymidine monophosphate (dTMP). By depleting the THF pool, methotrexate starves rapidly dividing cells of the necessary building blocks for DNA synthesis, explaining its efficacy in cancer and its side effects like myelosuppression.
Eukaryotic DNA is organized into a compact structure called chromatin, which must be dynamically regulated to allow for processes like transcription and replication. The fundamental repeating unit of chromatin has a 'beads-on-a-string' appearance under an electron microscope.
This fundamental unit, known as a nucleosome, consists of DNA wrapped around an octamer of which of the following proteins?
Explanation: The basic structural unit of chromatin is the nucleosome. Each nucleosome is composed of a core particle consisting of approximately 147 base pairs of DNA wrapped around an octamer of core histone proteins. This octamer contains two copies each of four different histones: H2A, H2B, H3, and H4. Another histone, H1, is a linker histone that binds to the DNA between nucleosomes and helps to compact the chromatin further.