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13. DNA, RNA, and DNA Replication — Practice Questions 2
This chapter introduces the structure and functions of nucleic acids and the process of DNA replication.
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(Multiple Choice — Click to Reveal Answer)
1. Which of the following molecules contains a hydroxyl group on the 2′ carbon of its sugar?
(A) DNA
(B) RNA
(C) Both DNA and RNA
(D) Neither DNA nor RNA
Answer
(B) — RNA has a hydroxyl group at the 2′ position of its ribose sugar, while DNA has only hydrogen.
2. What enzyme is responsible for separating the strands of the DNA double helix during replication?
(A) Ligase
(B) Helicase
(C) DNA polymerase
(D) RNA polymerase
Answer
(B) — Helicase unwinds the double helix, opening the strands for replication.
3. Which process directly uses ribose-containing nucleotides?
(A) DNA replication
(B) Transcription
(C) Translation
(D) Reverse transcription
Answer
(B) — RNA is synthesized during transcription, using ribose-containing nucleotides.
4. Which term best describes the orientation of the two strands in DNA?
(A) Coiled
(B) Complementary
(C) Antiparallel
(D) Redundant
Answer
(C) — DNA strands run in opposite 5′ to 3′ directions — this is called antiparallel.
5. Which of the following occurs in both prokaryotic and eukaryotic DNA replication?
(A) DNA packaged in linear chromosomes
(B) Multiple origins of replication
(C) Use of RNA primers
(D) DNA synthesis in both 3′ to 5′ and 5′ to 3′ directions
Answer
(C) — Both types of organisms use RNA primers to initiate DNA synthesis.
6. Which of the following is NOT a component of a DNA nucleotide?
(A) Phosphate group
(B) Deoxyribose sugar
(C) Uracil
(D) Nitrogenous base
Answer
(C) — Uracil is found in RNA, not DNA.
7. What stabilizes the double helix structure of DNA?
(A) Covalent bonds between base pairs
(B) Ionic bonds between phosphate groups
(C) Hydrogen bonds between nitrogenous bases
(D) Peptide bonds between nucleotides
Answer
(C) — Hydrogen bonds form between complementary nitrogenous bases.
8. What would happen if DNA polymerase added nucleotides in the 3′ to 5′ direction?
(A) Replication would proceed faster
(B) RNA would replace DNA
(C) DNA would be synthesized incorrectly
(D) Replication would not occur
Answer
(D) — DNA polymerase cannot synthesize in the 3′ to 5′ direction, so replication would not occur.
9. Which of the following best describes plasmids?
(A) Linear protein segments
(B) Circular segments of RNA
(C) Small circular DNA molecules
(D) Membrane-bound organelles
Answer
(C) — Plasmids are small circular DNA molecules, often found in prokaryotes.
10. Which of the following enzymes is responsible for removing RNA primers from DNA?
(A) Ligase
(B) Helicase
(C) DNA polymerase I
(D) RNA polymerase
Answer
(C) — DNA polymerase I removes RNA primers and replaces them with DNA.
11. Which type of RNA carries amino acids to the ribosome?
(A) mRNA
(B) tRNA
(C) rRNA
(D) snRNA
Answer
(B) — Transfer RNA (tRNA) brings amino acids to the ribosome during translation.
12. Which of the following is the complementary DNA sequence to 5′-GATTACA-3′?
(A) 5′-CTAATGT-3′
(B) 3′-CTAATGT-5′
(C) 3′-GATTACA-5′
(D) 5′-TGTAATC-3′
Answer
(D) — Complementary to GATTACA is 3′-CTAATGT-5′, which is written 5′-TGTAATC-3′.
13. In which cellular compartment does transcription occur in eukaryotic cells?
(A) Cytoplasm
(B) Nucleus
(C) Mitochondria
(D) Ribosome
Answer
(B) — Transcription occurs in the nucleus where DNA is located.
14. Which sugar is found in the backbone of RNA?
(A) Glucose
(B) Ribose
(C) Deoxyribose
(D) Galactose
Answer
(B) — RNA contains ribose, which has a hydroxyl group at the 2′ carbon.
15. Which strand of DNA is synthesized in Okazaki fragments?
(A) Leading strand
(B) Lagging strand
(C) Template strand
(D) Sense strand
Answer
(B) — The lagging strand is synthesized discontinuously in short segments called Okazaki fragments.
16. Which enzyme creates the phosphodiester bonds that connect DNA fragments together?
(A) DNA polymerase
(B) Ligase
(C) Primase
(D) Helicase
Answer
(B) — Ligase seals the gaps between Okazaki fragments by forming phosphodiester bonds.
17. What is the main reason RNA is less stable than DNA?
(A) It is single-stranded
(B) It lacks thymine
(C) It contains uracil
(D) It has a 2′ hydroxyl group
Answer
(D) — The hydroxyl group at the 2′ carbon in ribose makes RNA more prone to hydrolysis.
18. What happens if a mutation prevents the production of topoisomerase?
(A) Helicase cannot bind DNA
(B) DNA supercoiling will hinder replication
(C) DNA cannot form Okazaki fragments
(D) DNA will not unwind at all
Answer
(B) — Without topoisomerase, supercoiling would block replication fork progression.
19. Which of the following best defines a purine?
(A) A base with one ring
(B) A sugar-phosphate backbone
(C) A base with two rings
(D) A type of RNA
Answer
(C) — Purines like adenine and guanine consist of two ring structures.
20. What is the function of primase during DNA replication?
(A) Ligates Okazaki fragments
(B) Adds DNA nucleotides
(C) Synthesizes RNA primers
(D) Unwinds DNA
Answer
(C) — Primase synthesizes a short RNA primer so DNA polymerase can begin replication.
21. Which of the following base pairs is found in DNA but not in RNA?
(A) A–U
(B) G–C
(C) A–T
(D) C–G
Answer
(C) — Adenine–thymine base pairing occurs in DNA, while RNA uses uracil instead of thymine.
22. Which structure is used as the template in semiconservative DNA replication?
(A) Ribosomal RNA
(B) Original DNA strand
(C) RNA primer
(D) Helicase enzyme
Answer
(B) — Each original DNA strand serves as a template for a new complementary strand.
23. Which of the following best describes transcription?
(A) Copying RNA into DNA
(B) Making RNA from a DNA template
(C) Linking amino acids to form proteins
(D) Synthesizing DNA using Okazaki fragments
Answer
(B) — Transcription is the process of synthesizing RNA from a DNA template.
24. What does DNA polymerase require to initiate synthesis?
(A) DNA strand
(B) 3′ OH group
(C) 5′ cap
(D) Helicase
Answer
(B) — DNA polymerase requires a 3′ hydroxyl group to begin elongation.
25. What is the role of the 5′ phosphate group in DNA structure?
(A) It binds the nitrogen base
(B) It links to the 3′ OH of the previous nucleotide
(C) It stabilizes hydrogen bonds
(D) It adds RNA primers
Answer
(B) — The 5′ phosphate forms a phosphodiester bond with the 3′ OH of the previous nucleotide.
26. Which of the following best explains why DNA synthesis is considered semiconservative?
(A) Each new DNA molecule contains both new strands
(B) Each new DNA molecule contains one new and one old strand
(C) DNA strands are partially conserved
(D) DNA replication occurs in both directions
Answer
(B) — Semiconservative replication means each new DNA molecule contains one parental and one newly synthesized strand.
27. In PCR, why is a thermostable DNA polymerase (like Taq polymerase) necessary?
(A) It enhances transcription
(B) It initiates translation
(C) It can withstand high temperatures used in denaturation
(D) It helps ligate Okazaki fragments
Answer
(C) — Taq polymerase remains active during repeated heating cycles that denature DNA strands.
28. Which of the following is true about the direction of replication forks?
(A) They always move in the same direction
(B) They only move from 5′ to 3′ on both strands
(C) They form at origins and move bidirectionally
(D) They move randomly across the genome
Answer
(C) — Replication forks proceed in both directions away from the origin of replication.
29. A scientist mutates the gene for ligase. Which outcome is most likely?
(A) DNA cannot unwind
(B) RNA primers cannot be synthesized
(C) Okazaki fragments remain unjoined
(D) Ribosomes are inhibited
Answer
(C) — Ligase is responsible for sealing the nicks between Okazaki fragments on the lagging strand.
30. Which of the following pairs is mismatched in DNA replication?
(A) Primase — synthesizes RNA primer
(B) Helicase — unwinds DNA
(C) Ligase — unwinds DNA
(D) DNA polymerase — adds nucleotides
Answer
(C) — Ligase joins DNA fragments but does not unwind DNA.
31. What would happen if the RNA primers were not removed after replication?
(A) The DNA would be unstable
(B) Proteins could not bind
(C) Mutations would be corrected
(D) Replication would be faster
Answer
(A) — RNA is less stable and lacks proofreading, so its retention in DNA would compromise stability and integrity.
32. What is the role of exonuclease activity in DNA polymerase I?
(A) Unwinds the double helix
(B) Removes RNA primers
(C) Joins DNA fragments
(D) Initiates replication at the origin
Answer
(B) — DNA polymerase I removes RNA primers via 5′ to 3′ exonuclease activity and replaces them with DNA.
33. What part of DNA contributes to its negative charge?
(A) Nitrogenous base
(B) Ribose sugar
(C) Phosphate group
(D) Thymine
Answer
(C) — The phosphate groups in the sugar-phosphate backbone are negatively charged.
34. Which component is NOT directly involved in the synthesis of the leading strand?
(A) Helicase
(B) Ligase
(C) Primase
(D) DNA polymerase III
Answer
(B) — Ligase is primarily used to seal Okazaki fragments on the lagging strand.
35. During transcription, which DNA strand serves as the template?
(A) Sense strand
(B) Coding strand
(C) Antisense strand
(D) Leading strand
Answer
(C) — The antisense strand is used as the template to make mRNA during transcription.
36. Explain why DNA is more stable than RNA at the molecular level.
Answer
DNA lacks the 2′ hydroxyl group that RNA has, making it less prone to hydrolysis and therefore more stable.
37. Describe the direction in which DNA polymerase synthesizes new DNA strands.
Answer
DNA polymerase synthesizes new strands in the 5′ to 3′ direction by adding nucleotides to the 3′ OH end.
38. Justify why uracil is used in RNA instead of thymine.
Answer
Uracil is energetically cheaper to produce, and since RNA is short-lived, thymine’s stability is unnecessary.
39. Identify the function of RNA primers in DNA replication.
Answer
RNA primers provide a free 3′ OH group for DNA polymerase to begin synthesizing a new DNA strand.
40. Predict what would happen to replication if helicase were nonfunctional.
Answer
DNA strands would not be separated, so replication could not proceed.
41. Explain the role of topoisomerase in DNA replication.
Answer
Topoisomerase prevents supercoiling ahead of the replication fork by cutting and resealing the DNA backbone.
42. Describe one similarity and one difference between DNA and RNA nucleotides.
Answer
Both have phosphate groups and nitrogenous bases, but DNA contains deoxyribose and thymine, while RNA contains ribose and uracil.
43. What is the primary role of DNA ligase during replication?
Answer
Ligase joins the Okazaki fragments by forming phosphodiester bonds between nucleotides on the lagging strand.
44. Predict the effect of a mutation that disables DNA polymerase proofreading ability.
Answer
The mutation rate would increase due to the accumulation of replication errors.
45. Explain why leading strand replication is continuous.
Answer
The leading strand is synthesized in the same direction as the replication fork opens, allowing continuous elongation.
46. Describe the role of complementary base pairing in DNA replication.
Answer
Complementary base pairing ensures accurate copying of the genetic code, with A pairing to T and G to C.
47. How does DNA polymerase recognize where to start replication?
Answer
It begins replication at the 3′ end of an RNA primer laid down by primase.
48. What is the function of the replication fork?
Answer
The replication fork is the Y-shaped structure where the DNA is unwound and replication occurs.
49. Why is it essential that DNA strands are antiparallel?
Answer
Antiparallel orientation allows base pairs to align properly and enables enzymes like DNA polymerase to function.
50. Predict the consequence of having a mutation that prevents synthesis of RNA primers.
Answer
DNA replication would not initiate, since DNA polymerase requires a primer to begin synthesis.