DNA Structure, Replication, And Repair - USMLE Step 1

1. Which of the following best describes the function of primase during DNA replication?
   Answer: It synthesizes a short RNA sequence to provide a 3'-OH group for DNA polymerase. 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.
2. 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?
   Answer: DNA mismatch repair 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.
3. This strain most likely has a mutation in the gene encoding which of the following enzymes?
   Answer: DNA polymerase I 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.
4. A defect in which of the following DNA repair mechanisms is the most likely cause of this patient's condition?
   Answer: Nucleotide excision repair 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.
5. This fundamental unit, known as a nucleosome, consists of DNA wrapped around an octamer of which of the following proteins?
   Answer: Histones 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.
6. Which of the following best characterizes the origins of replication in eukaryotes?
   Answer: Multiple, often AT-rich sequences per chromosome Eukaryotic chromosomes contain multiple origins of replication to ensure the entire genome can be replicated during S-phase. These origins are specific DNA sequences where the pre-replication complex assembles. While the sequences can be variable, they are often rich in adenine and thymine (AT-rich). The two hydrogen bonds in an A-T pair make it easier to separate the DNA strands compared to the three hydrogen bonds in a G-C pair, facilitating the initiation of replication.
7. The protein encoded by the mutated gene is a critical sensor for which of the following types of DNA damage?
   Answer: Double-strand breaks 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.
8. This process of replicative aging is most directly caused by the progressive shortening of which of the following?
   Answer: Telomeres Linear eukaryotic chromosomes have a problem with replicating their very ends. DNA polymerase cannot replicate the 3' end of the lagging strand template, leading to a small loss of genetic material from the end of the chromosome with each cell division. Telomeres are repetitive DNA sequences at the ends of chromosomes that buffer against this loss. However, as most somatic cells lack sufficient telomerase activity, telomeres progressively shorten with each division. Once they reach a critical length, a DNA damage signal is triggered, leading to cell cycle arrest and replicative senescence.
9. The activity of which of the following enzymes is most likely upregulated in these cancer cells, contributing to their unlimited replicative potential?
   Answer: Telomerase 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.
10. Which of the following is the first enzyme to act in the base excision repair pathway to correct this lesion?
    Answer: DNA glycosylase 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.
11. Which of the following correctly pairs a nitrogenous base with its structural class?
    Answer: Thymine - Pyrimidine 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.
12. How does the E. coli MMR system distinguish the newly synthesized daughter strand from the parental strand?
    Answer: The daughter strand is transiently unmethylated at GATC sequences. In E. coli, strand discrimination is based on DNA methylation. The enzyme Dam methylase adds a methyl group to the adenine in GATC sequences. This process is slow, so immediately after replication, the parental strand is methylated, but the newly synthesized daughter strand is not (it is hemimethylated). The mismatch repair machinery (MutS, MutL, MutH) recognizes this lack of methylation on the daughter strand, preferentially cleaving it and directing repair to the new strand.
13. Which of the following enzymes is the primary target of this medication in gram-negative bacteria?
    Answer: DNA gyrase 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.
14. 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?
    Answer: Homologous recombination 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.
15. 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?
    Answer: Deoxythymidine monophosphate (dTMP) 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.
16. Which of the following enzymes is responsible for joining the Okazaki fragments on the lagging strand by forming a phosphodiester bond?
    Answer: DNA ligase 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.
17. 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?
    Answer: Acetylation 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.
18. Which of the following enzymatic activities is responsible for the proofreading function of DNA polymerase?
    Answer: 3' to 5' exonuclease activity 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.
19. Which of the following DNA repair mechanisms is predominantly used to repair double-strand breaks in cells during the G1 phase?
    Answer: Non-homologous end joining 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.
20. Which of the following molecular defects would be most prominent in these cells during DNA replication?
    Answer: Accumulation of unjoined Okazaki fragments 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.

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