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13. DNA, RNA, and DNA Replication — Practice Questions 3
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 component of the nucleotide differs between RNA and DNA?
(A) Phosphate group
(B) Nitrogenous base
(C) Five-carbon sugar
(D) Hydrogen bonds
Answer
(C) — DNA contains deoxyribose, whereas RNA contains ribose.
2. Which base pair is connected by three hydrogen bonds in DNA?
(A) A–T
(B) C–G
(C) A–U
(D) G–U
Answer
(B) — Cytosine and guanine form three hydrogen bonds, making them more stable than A–T pairs.
3. In which direction does the lagging strand form during DNA replication?
(A) 5′ to 3′ in fragments
(B) 3′ to 5′ continuously
(C) 5′ to 3′ continuously
(D) Random orientation
Answer
(A) — The lagging strand is synthesized discontinuously in 5′ to 3′ direction using Okazaki fragments.
4. What is the primary role of primase during DNA replication?
(A) Rejoining DNA segments
(B) Releasing tension from supercoiling
(C) Adding RNA primers
(D) Unzipping DNA strands
Answer
(C) — Primase synthesizes RNA primers to initiate DNA synthesis.
5. Which feature makes RNA more chemically reactive than DNA?
(A) Single-stranded nature
(B) Presence of thymine
(C) Lack of phosphate backbone
(D) Presence of 2′ hydroxyl group
Answer
(D) — The 2′ hydroxyl group in ribose makes RNA more prone to hydrolysis.
6. Which base is unique to RNA and not found in DNA?
(A) Thymine
(B) Uracil
(C) Cytosine
(D) Adenine
Answer
(B) — RNA uses uracil instead of thymine to pair with adenine.
7. What describes the relationship between the two strands of a DNA molecule?
(A) Identical and parallel
(B) Complementary and parallel
(C) Complementary and antiparallel
(D) Randomly arranged and antiparallel
Answer
(C) — DNA strands run in opposite directions (antiparallel) and are complementary in base pairing.
8. Which enzyme relieves supercoiling during DNA replication?
(A) Ligase
(B) Helicase
(C) Topoisomerase
(D) Primase
Answer
(C) — Topoisomerase prevents overwinding ahead of the replication fork.
9. What is the role of DNA polymerase III in replication?
(A) Joining DNA fragments
(B) Synthesizing RNA primers
(C) Removing RNA primers
(D) Extending the new DNA strand
Answer
(D) — DNA polymerase III adds nucleotides to the growing DNA strand during replication.
10. What kind of bond holds nucleotides together in a single strand of DNA?
(A) Hydrogen bond
(B) Ionic bond
(C) Covalent phosphodiester bond
(D) Peptide bond
Answer
(C) — Adjacent nucleotides are joined by covalent phosphodiester bonds in the sugar-phosphate backbone.
11. What process is responsible for transmitting genetic information to mRNA?
(A) Translation
(B) Replication
(C) Transcription
(D) Mutation
Answer
(C) — Transcription copies DNA into RNA, allowing gene expression.
12. Which sugar is found in DNA but not in RNA?
(A) Ribose
(B) Deoxyribose
(C) Glucose
(D) Fructose
Answer
(B) — DNA contains deoxyribose, while RNA contains ribose.
13. Which of the following enzymes forms covalent bonds between DNA fragments?
(A) Helicase
(B) DNA polymerase
(C) Ligase
(D) Primase
Answer
(C) — Ligase connects Okazaki fragments by forming phosphodiester bonds.
14. What is the function of the origin of replication?
(A) Site of transcription
(B) Site where the ribosome binds
(C) Site where DNA replication begins
(D) Site of mutation
Answer
(C) — Replication begins at specific locations on the DNA known as origins of replication.
15. Which RNA type serves as the blueprint for protein synthesis?
(A) rRNA
(B) tRNA
(C) mRNA
(D) siRNA
Answer
(C) — Messenger RNA (mRNA) carries genetic instructions to the ribosome for protein synthesis.
16. Which enzyme opens the DNA double helix during replication?
(A) Ligase
(B) Helicase
(C) DNA polymerase I
(D) Exonuclease
Answer
(B) — Helicase breaks hydrogen bonds between base pairs to unwind the DNA.
17. Which base pair would be found in RNA but not DNA?
(A) A–T
(B) G–C
(C) A–U
(D) T–A
Answer
(C) — RNA contains uracil (U) instead of thymine, so adenine pairs with uracil.
18. Which type of RNA is involved in peptide bond formation at the ribosome?
(A) mRNA
(B) tRNA
(C) rRNA
(D) snRNA
Answer
(C) — Ribosomal RNA (rRNA) is a key structural and catalytic component of the ribosome.
19. What is the first step in DNA replication?
(A) DNA ligase joins fragments
(B) Primase adds an RNA primer
(C) Helicase unwinds the DNA
(D) DNA polymerase adds bases
Answer
(C) — Helicase must unwind the DNA before any synthesis can occur.
20. In DNA, what ensures consistent width of the double helix?
(A) Binding of identical bases
(B) Alternating base positions
(C) Purine–pyrimidine pairing
(D) Deoxyribose spacing
Answer
(C) — Pairing a purine with a pyrimidine keeps the helix diameter uniform.
21. Which enzyme replaces RNA primers with DNA nucleotides?
(A) DNA polymerase I
(B) Primase
(C) Ligase
(D) Helicase
Answer
(A) — DNA polymerase I removes RNA primers and fills in the gaps with DNA.
22. What provides energy for DNA polymerization?
(A) ATP hydrolysis
(B) Hydrolysis of nucleoside triphosphates
(C) Ribosomal activity
(D) DNA helicase spinning
Answer
(B) — Cleavage of phosphate bonds in dNTPs releases energy for DNA synthesis.
23. Which cellular process involves complementary base pairing but does not produce a double helix?
(A) DNA replication
(B) Transcription
(C) Translation
(D) Cell division
Answer
(B) — In transcription, RNA forms via base pairing with DNA but does not form a helix with it.
24. Which feature of DNA allows it to act as a stable long-term information storage molecule?
(A) Use of uracil
(B) Hydrogen bonds
(C) 2′ hydroxyl group
(D) Deoxyribose sugar
Answer
(D) — The lack of a 2′ OH in deoxyribose makes DNA less reactive and more stable than RNA.
25. Which of the following is TRUE about Okazaki fragments?
(A) They are made on the leading strand
(B) They are joined by primase
(C) They result from discontinuous replication
(D) They contain uracil
Answer
(C) — Okazaki fragments form on the lagging strand as replication occurs in short segments.
26. Which part of a nucleotide is responsible for forming the DNA backbone?
(A) Nitrogenous base
(B) Phosphate and sugar
(C) Ribose and base
(D) Phosphate and base
Answer
(B) — The sugar and phosphate form the repeating backbone of DNA through phosphodiester bonds.
27. A virus’s genome is composed of RNA. Which base would be found in this viral genome but not in DNA?
(A) Thymine
(B) Cytosine
(C) Adenine
(D) Uracil
Answer
(D) — RNA contains uracil, while DNA contains thymine.
28. Which of the following statements about DNA polymerase III is correct?
(A) It initiates synthesis by laying down RNA primers
(B) It synthesizes DNA in the 3′ to 5′ direction
(C) It extends DNA from an RNA primer
(D) It removes mismatched nucleotides during translation
Answer
(C) — DNA polymerase III adds nucleotides to the 3′ OH end of the RNA primer in the 5′ to 3′ direction.
29. Which of the following would result if DNA ligase were inactive during replication?
(A) Primers would not form
(B) Leading strand synthesis would fail
(C) Lagging strand would have unjoined fragments
(D) DNA would not unwind
Answer
(C) — Ligase is required to seal nicks between Okazaki fragments on the lagging strand.
30. Which characteristic of DNA allows replication to occur using a template strand?
(A) Double helix structure
(B) Complementary base pairing
(C) Histone packaging
(D) Phosphodiester linkage
Answer
(B) — Complementary base pairing allows new strands to be synthesized using existing ones as templates.
31. Which observation in an experiment would support the semiconservative model of DNA replication?
(A) One DNA strand is replaced with RNA
(B) All new DNA is radioactive after one round
(C) Each DNA molecule has one original and one new strand
(D) Both strands are completely replaced each cycle
Answer
(C) — In semiconservative replication, each new DNA double helix consists of one old and one new strand.
32. In the Meselson-Stahl experiment, how was isotope labeling used to prove the semiconservative model?
(A) By using radioactive phosphorus to tag bases
(B) By tracking DNA replication in viruses
(C) By using nitrogen isotopes to separate heavy and light DNA by density
(D) By using carbon isotopes to measure strand length
Answer
(C) — Nitrogen-15 and nitrogen-14 were used to differentiate old and new DNA via density gradient centrifugation.
33. What would happen if topoisomerase were inhibited during replication?
(A) Replication would proceed more rapidly
(B) DNA would not unzip at the origin
(C) Supercoiling would accumulate, stalling the fork
(D) The lagging strand would not form
Answer
(C) — Topoisomerase prevents supercoiling; without it, tension builds up and halts replication.
34. Which of the following explains why DNA polymerase requires a primer to function?
(A) It can only bind to RNA
(B) It requires a free 3′ OH to add nucleotides
(C) It cannot use DNA as a template
(D) It unwinds the DNA double helix
Answer
(B) — DNA polymerase must have a free 3′ hydroxyl group to begin adding nucleotides.
35. Which process would be directly impacted by a mutation in primase?
(A) Removal of primers
(B) Joining of fragments
(C) Initiation of DNA synthesis
(D) DNA strand separation
Answer
(C) — Without primase, the RNA primers cannot form, so DNA synthesis cannot begin.
36. Explain how the antiparallel structure of DNA affects the way replication occurs on each strand.
Answer
The antiparallel arrangement causes one strand (leading) to be synthesized continuously and the other (lagging) discontinuously using Okazaki fragments.
37. Why does DNA polymerase synthesize new DNA in the 5′ to 3′ direction only?
Answer
Because it requires a free 3′ OH group to attach the incoming nucleotide's phosphate group during phosphodiester bond formation.
38. Describe how hydrogen bonding contributes to DNA’s structure and function.
Answer
Hydrogen bonds between complementary bases hold the two strands together, allowing specific base pairing and easy strand separation during replication and transcription.
39. Identify one key difference between the structure of DNA and RNA that impacts their stability.
Answer
RNA has a hydroxyl group at the 2′ carbon of ribose, making it more reactive and less stable than DNA, which lacks this group.
40. Explain why lagging strand replication requires DNA ligase.
Answer
The lagging strand is synthesized in fragments that must be joined together by ligase to create a continuous DNA strand.
41. What would be the result of a mutation that disables DNA polymerase proofreading activity?
Answer
The mutation rate during replication would increase due to an inability to correct mismatched nucleotides.
42. Describe the role of RNA primers in DNA replication.
Answer
RNA primers provide a starting point with a free 3′ OH group for DNA polymerase to begin synthesizing a new strand.
43. Predict the effect of deleting the origin of replication from a DNA molecule.
Answer
Replication would not initiate, as there would be no recognized site for helicase and other enzymes to bind and begin the process.
44. Explain how semiconservative replication ensures genetic continuity.
Answer
Each new DNA molecule receives one old strand as a template, preserving accurate genetic information across generations.
45. Why do Okazaki fragments form during DNA replication?
Answer
Because DNA polymerase can only add nucleotides in the 5′ to 3′ direction, and the lagging strand runs 5′ to 3′, synthesis must occur in segments away from the fork.
46. What experiment demonstrated the semiconservative nature of DNA replication?
Answer
The Meselson-Stahl experiment used nitrogen isotopes to show that DNA molecules consist of one old and one new strand after replication.
47. How is topoisomerase essential for proper progression of the replication fork?
Answer
It alleviates tension from supercoiling ahead of the replication fork, preventing the DNA from becoming tangled or broken.
48. What distinguishes the leading strand from the lagging strand during replication?
Answer
The leading strand is synthesized continuously in the same direction as the replication fork, while the lagging strand is synthesized in fragments in the opposite direction.
49. Why is DNA ligase not needed on the leading strand?
Answer
Because the leading strand is synthesized continuously without breaks, so there are no gaps to seal between fragments.
50. If DNA replication were conservative instead of semiconservative, what result would you expect after one replication round?
Answer
One molecule would contain both original strands, and the other would consist of entirely new DNA strands.