Week 13 — Quiz (auto-graded) · DNA Structure & Replication

Course: Introduction to Biology — General Biology I (BIOL 101) · Silver Oak University (fictional sample) · Prof. Castellano
Objective tested: Objective 7 — DNA structure (double helix, backbone, antiparallel, complementary base pairing); Chargaff's rule; semiconservative replication; the replication enzymes (helicase, DNA polymerase, ligase).
Points: 10 (1 each) · Assignment group: Quizzes (10% of grade) · Due: end of Module 13.

This is the human-readable quiz with its vetted answer key and feedback. The import-ready Classic QTI is in F-quiz-week-13-qti.xml (generated by the shared validated script — parses with 10 items, every single-answer item exactly one correct). The reusable item-bank entries and the Canvas placement block are at the bottom of this file.

Blueprint

No trick questions; distractors target the Week 13 misconceptions named in the lecture outline (A–G mispairing, "identical" strands, "conservative" replication, %C = %A, polymerase-unzips).

Questions, key, and feedback

Q1 (MC). In double-stranded DNA, the base adenine (A) always pairs with —
- A. guanine (G)
- B. cytosine (C)
- C. thymine (T) ✅
- D. another adenine (A)
Feedback: DNA allows only two base pairs: A–T and G–C. Adenine's one and only partner is thymine. (A–G and A–C never pair; A does not pair with itself.)

Q2 (MC). The two "rails" of the DNA ladder — the backbone that runs down each side — are made of an alternating chain of —
- A. sugar (deoxyribose) and phosphate ✅
- B. adenine and thymine
- C. two strands of protein
- D. hydrogen bonds only
Feedback: The sugar-phosphate backbone forms the rails; the paired bases form the rungs. (Proteins are not part of the DNA strand itself, and hydrogen bonds hold the rungs together — they aren't the backbone.)

Q3 (MC). One strand of DNA reads A–T–G–C. Reading in the same order, its complementary strand is —
- A. A–T–G–C
- B. T–A–C–G ✅
- C. G–C–A–T
- D. T–A–G–C
Feedback: Swap each base for its partner: A→T, T→A, G→C, C→G, giving T–A–C–G. The two strands are complementary, not identical (A is not copied as A).

Q4 (Multiple answer — select all that apply). Which of the following statements about DNA structure are true?
- A. DNA is a double helix (a twisted ladder of two strands) ✅
- B. The two strands are antiparallel (they run in opposite directions) ✅
- C. Adenine pairs with guanine, and cytosine pairs with thymine
- D. The two strands are held together by hydrogen bonds between paired bases ✅
- E. Each strand is identical to the other strand
Feedback: DNA is a double helix (A), the strands are antiparallel (B), and hydrogen bonds between paired bases hold them together (D). The pairing is A–T, G–C (so C is wrong), and the strands are complementary, not identical (so E is wrong).

Q5 (MC). DNA replication is semiconservative. This means that each new DNA double helix —
- A. is built entirely from new strands, while the original molecule stays completely intact
- B. contains one original (old) strand and one newly made strand ✅
- C. has old and new pieces randomly scrambled along every strand
- D. is only half-finished and must be copied again
Feedback: Semiconservative = "half old, half new": each old strand is a template, so every new helix keeps one parental strand and adds one new strand. (A describes the conservative model; C describes the dispersive model — both were ruled out by Meselson and Stahl.)

Q6 (Matching). Match each part of DNA replication to its role.
| Term | Correct role |
|---|---|
| Helicase | Unzips the double helix by breaking hydrogen bonds |
| DNA polymerase | Adds new complementary bases to each template strand |
| DNA ligase | Seals/joins the pieces of a new strand together |
| Template strand | The old strand used as the pattern for the new one |
Feedback: Three machines, one job each: helicase unzips, DNA polymerase adds bases, DNA ligase seals. The template is the old strand whose sequence dictates the new partner. The classic error is thinking polymerase unzips — it doesn't; helicase does.

Q7 (MC). A sample of double-stranded DNA is 30% adenine (A). Using Chargaff's rule (%A = %T, %G = %C, and the four bases total 100%), what percentage is cytosine (C)?
- A. 30%
- B. 70%
- C. 20% ✅
- D. 35%
Feedback: Thymine equals adenine, so %T = 30%. A + T = 60%, leaving 40% for G + C. Since %G = %C, each is 40 ÷ 2 = 20%. (The trap is setting %C = %A = 30%; cytosine equals guanine, not adenine.)

Q8 (True / False). "The two strands of a DNA molecule are identical to each other."
- True
- False ✅
Feedback: False. The strands are complementary, not identical: wherever one strand has A, the other has T; wherever one has G, the other has C. That complementarity is exactly what lets one strand rebuild the other during replication.

Q9 (MC). Which enzyme unzips the DNA double helix by breaking the hydrogen bonds between the two strands, so that copying can begin?
- A. DNA polymerase
- B. DNA ligase
- C. helicase ✅
- D. amylase
Feedback: Helicase separates the strands ("un-helixes" the molecule). DNA polymerase then adds bases, and DNA ligase seals. (Amylase is a digestive enzyme that breaks down starch — nothing to do with DNA.)

Q10 (MC). Four nitrogenous bases make up DNA. Which of the following is NOT one of them?
- A. adenine
- B. cytosine
- C. uracil ✅
- D. guanine
Feedback: DNA's four bases are adenine, thymine, guanine, and cytosine (A, T, G, C). Uracil (U) is found in RNA, where it replaces thymine — a fact you'll use next week. In DNA, the missing partner to A is thymine, not uracil.

Answer key (quick reference)

Quality gate (self-checked): each single-answer item has exactly one correct option; the multiple-answer item lists all three true structure statements (A, B, D) and requires C and E to be left unselected; the matching item pairs four terms to four distinct roles. The one calculation (Q7) was pre-computed and independently re-verified with a Python check that re-derives every Chargaff value (30% A → 30% T, 20% G, 20% C; sum = 100), and the trap answer (30%) is engineered from the %C = %A error — quantitative gate: PASS. No item asserts a fact outside the Week 13 course definitions (transcription, RNA roles, and protein synthesis are deferred to Week 14; uracil appears only as a "not in DNA" distractor).

Item-bank entries (for variants + the midterm/final)

All ten items are tagged course=BIOL101 · week=13 · objective=7 · topic=dna-structure-replication and deposited in Item Bank: Week 13 — DNA Structure & Replication. The final (Week 16) and the per-term variant updates draw fresh items from this bank. (Tags: q1 base-pairing-AT, q2 sugar-phosphate-backbone, q3 complementary-strand, q4 dna-structure-select-all, q5 semiconservative, q6 replication-enzymes-match, q7 chargaff-calc, q8 strands-not-identical, q9 helicase-unzips, q10 not-a-dna-base-uracil.)

Canvas placement block

canvas_object   = Quizzes::Quiz
title           = "Week 13 Quiz — DNA Structure & Replication"
assignment_group = "Quizzes"
points_possible = 10
grading_type    = points
due_offset_days = 6        # 6 days after module start
published       = true
shuffle_answers = true
provenance      = "~ Prof. Castellano's edition · Fall 2026 · built with thecoursemaker.com"