IB Biology Quiz: Understand Dna Replication
19 questions · exam conditions
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Understand Dna ReplicationQuestion 1 of 19

If a mutation rendered the enzyme primase non-functional in a cell, what would be the immediate effect on DNA replication?

The leading strand would be synthesized, but the lagging strand would not.
The parent DNA strands would separate, but polymerization of new strands would not begin.
Replication would proceed normally, but the Okazaki fragments could not be joined.
The DNA would be replicated, but the new strands would contain segments of RNA.
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IB Biology Quiz

IB Biology Quiz: Understand Dna Replication

Practice Understand Dna Replication in IB Biology with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

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This quiz focuses on Understand Dna Replication, giving you a quick way to practice the rules, question types, and explanations that matter most for IB Biology.

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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.

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Question 1

If a mutation rendered the enzyme primase non-functional in a cell, what would be the immediate effect on DNA replication?

  1. The leading strand would be synthesized, but the lagging strand would not.
  2. The parent DNA strands would separate, but polymerization of new strands would not begin. (correct answer)
  3. Replication would proceed normally, but the Okazaki fragments could not be joined.
  4. The DNA would be replicated, but the new strands would contain segments of RNA.
Explanation: Primase synthesizes short RNA primers that provide a necessary 3'-OH group for DNA polymerase to begin synthesis. Without functional primase, no primers can be laid down on either the leading or the lagging strand. Helicase can still unwind the DNA, but DNA polymerase cannot initiate synthesis of new strands, halting the entire process after unwinding.

Question 2

A scientist is studying a DNA molecule where one strand has the base sequence 5'-AGCTTCG-3'. What will be the sequence of the complementary strand synthesized during replication?

  1. 5'-CGAAGCT-3'
  2. 3'-UCGAAGC-5'
  3. 5'-TCGAAGC-3'
  4. 3'-TCGAAGC-5' (correct answer)
Explanation: DNA strands are antiparallel. The complementary strand will run in the opposite direction (3' to 5'). Base pairing rules are A with T, and G with C. Therefore, the complementary sequence to 5'-AGCTTCG-3' is 3'-TCGAAGC-5'. Choice A is incorrect because it has the wrong directionality (5' to 3') although the bases are complementary. Choice B is incorrect because it contains uracil (U), which is found in RNA, not DNA. Choice C has the correct bases but is written in the wrong direction.

Question 3

A hypothetical drug is developed that allows DNA polymerase to synthesize in both the 5' to 3' and 3' to 5' directions. If this drug were applied to a dividing cell, what would be the most likely outcome for DNA replication? [HL]

  1. Replication would fail to initiate as helicase would be inhibited.
  2. Both the leading and lagging strands would be synthesized continuously. (correct answer)
  3. Replication would be much slower due to the added complexity of bidirectional synthesis.
  4. Okazaki fragments would be produced on both the leading and lagging strands.
Explanation: The entire reason for discontinuous synthesis and Okazaki fragments on the lagging strand is the strict 5' to 3' directionality of DNA polymerase. If the polymerase could synthesize in the 3' to 5' direction as well, it could simply follow the replication fork along both template strands, synthesizing both new strands continuously. This would eliminate the need for Okazaki fragments and the action of DNA ligase.

Question 4

DNA replication is described as semi-conservative because each new DNA molecule formed consists of:

  1. two new strands synthesized from free nucleotides, with the original molecule acting as a template before being degraded.
  2. one original parental strand and one newly synthesized daughter strand. (correct answer)
  3. a mosaic of segments from both original parental strands interspersed with newly synthesized segments.
  4. two original parental strands hydrogen-bonded to each other and two new strands hydrogen-bonded to each other.
Explanation: The term 'semi-conservative' means that half of the original molecule is conserved in each of the two new molecules. Specifically, the parental double helix unwinds, and each of its strands serves as a template for a new complementary strand. The result is two identical DNA molecules, each composed of one old (parental) strand and one new (daughter) strand.

Question 5

Bacteria are grown in a medium containing a heavy isotope of nitrogen (¹⁵N) for many generations. They are then transferred to a medium containing a light isotope (¹⁴N) and allowed to replicate exactly twice. After these two rounds of replication, what percentage of the DNA molecules will contain at least one strand of ¹⁵N?

  1. 0%
  2. 25%
  3. 50% (correct answer)
  4. 100%
Explanation: After the first replication in ¹⁴N medium, all DNA molecules are hybrids (¹⁵N-¹⁴N). After the second replication, these two hybrid molecules replicate. Each produces one new hybrid molecule and one entirely light (¹⁴N-¹⁴N) molecule. This results in a total of four DNA molecules: two are hybrid and two are light. Therefore, 2 out of 4 molecules, or 50%, contain at least one ¹⁵N strand.

Question 6

The 'end-replication problem' in eukaryotes refers to the progressive shortening of linear chromosomes with each cell division. Which statement accurately describes the underlying cause of this problem? [HL]

  1. Helicase cannot unwind the final segment of DNA at the very end of a chromosome.
  2. DNA ligase is unable to join the final Okazaki fragment to the end of the chromosome.
  3. The removal of the final RNA primer on the lagging strand leaves a gap that cannot be filled by DNA polymerase. (correct answer)
  4. The high concentration of histone proteins at the chromosome ends physically blocks the replication machinery.
Explanation: On the lagging strand, synthesis requires RNA primers. When the final primer at the 5' end of the newly synthesized strand is removed, there is no existing 3'-OH group for DNA polymerase to add nucleotides to. Consequently, this gap is not filled, and the chromosome becomes shorter with each round of replication. This shortening occurs at the telomeres.

Question 7

The sequence of events for synthesizing a portion of the lagging strand is: 1. DNA ligase joins fragments. 2. Primase adds an RNA primer. 3. Helicase unwinds DNA. 4. DNA polymerase synthesizes a DNA fragment. What is the correct chronological order of these events? [HL]

  1. 2, 4, 3, 1
  2. 3, 2, 4, 1 (correct answer)
  3. 3, 4, 2, 1
  4. 4, 2, 1, 3
Explanation: The correct sequence starts with the unwinding of the DNA template by helicase (3). Once a single-stranded region is available, primase must synthesize an RNA primer to provide a starting point (2). DNA polymerase then extends this primer, synthesizing a short DNA fragment (an Okazaki fragment) (4). Finally, after multiple fragments are made and primers are replaced, DNA ligase joins the adjacent fragments to create a continuous strand (1).

Question 8

In the process of DNA replication, what is the functional relationship between helicase and DNA polymerase?

  1. Helicase synthesizes the RNA primers that DNA polymerase requires to begin synthesis.
  2. Helicase proofreads the strand synthesized by DNA polymerase and corrects errors.
  3. DNA polymerase signals helicase to begin unwinding at the origin of replication.
  4. Helicase unwinds the DNA, providing single-stranded templates for DNA polymerase to copy. (correct answer)
Explanation: The two enzymes have sequential roles. Helicase moves along the DNA, separating the two strands to create a replication fork. This action exposes the nucleotide bases on each strand, creating single-stranded templates. DNA polymerase then uses these templates to synthesize the new complementary strands. The other options misrepresent the roles: primase makes primers, polymerase itself proofreads, and initiation is controlled by initiator proteins, not polymerase.

Question 9

The energy required to form the phosphodiester bond between an incoming nucleotide and the growing DNA strand is supplied by which of the following?

  1. The hydrolysis of ATP molecules by a separate ATPase enzyme.
  2. The breaking of hydrogen bonds between the template strands by helicase.
  3. The hydrolysis of the two terminal phosphate groups from the incoming deoxynucleoside triphosphate. (correct answer)
  4. The action of DNA ligase creating a high-energy bond that DNA polymerase then utilizes.
Explanation: The building blocks for DNA synthesis are deoxynucleoside triphosphates (e.g., dATP, dGTP). These molecules carry their own energy. When DNA polymerase adds a nucleotide to the growing chain, it cleaves off the two terminal phosphate groups (pyrophosphate). The hydrolysis of this high-energy phosphate bond provides the energy to drive the formation of the phosphodiester bond.

Question 10

A researcher uses a drug that specifically inhibits the function of DNA ligase in a culture of dividing eukaryotic cells. Which of the following would be the most direct consequence for DNA synthesis?

  1. The DNA double helix would fail to unwind, preventing the initiation of replication.
  2. RNA primers would not be synthesized, halting the action of DNA polymerase.
  3. Synthesis of the leading strand would be unaffected, but the lagging strand would consist of unjoined fragments. (correct answer)
  4. DNA polymerase would be unable to add new nucleotides to either the leading or lagging strands.
Explanation: DNA ligase is responsible for joining the Okazaki fragments on the lagging strand by forming phosphodiester bonds. Its inhibition would not affect the unwinding of DNA (helicase), the synthesis of primers (primase), or the continuous synthesis of the leading strand. It would, however, prevent the fragments of the discontinuously synthesized lagging strand from being joined into a single continuous strand.

Question 11

Which statement correctly distinguishes DNA replication in prokaryotes from that in eukaryotes?

  1. Prokaryotic replication involves leading and lagging strands, while eukaryotic replication is continuous on both strands.
  2. Eukaryotic replication requires RNA primers to initiate synthesis, whereas prokaryotic replication does not.
  3. Prokaryotic replication proceeds from a single origin of replication, while eukaryotic replication begins at multiple origins. (correct answer)
  4. Prokaryotic DNA polymerase synthesizes in the 3' to 5' direction, while eukaryotic polymerase synthesizes in the 5' to 3' direction.
Explanation: Prokaryotes typically have a single, circular chromosome and initiate replication from one origin of replication. Eukaryotes have large, linear chromosomes and must initiate replication at multiple origins simultaneously to complete the process in a timely manner. Both systems involve leading/lagging strands, require primers, and have polymerases that synthesize in the 5' to 3' direction.

Question 12

What is the primary role of the proofreading function of DNA polymerase?

  1. To detect and remove incorrectly paired nucleotides immediately after their insertion. (correct answer)
  2. To synthesize RNA primers needed to initiate DNA synthesis.
  3. To unwind the DNA double helix ahead of the replication fork.
  4. To join adjacent Okazaki fragments by forming phosphodiester bonds.
Explanation: The proofreading function, typically an exonuclease activity of the DNA polymerase enzyme itself, acts as a quality control mechanism. It checks each newly added nucleotide against the template. If a mismatch is detected, the polymerase removes the incorrect nucleotide and inserts the correct one, greatly increasing the fidelity of DNA replication.

Question 13

During DNA replication, the leading strand is synthesized continuously while the lagging strand is synthesized discontinuously. Which statement best explains this difference?

  1. The leading strand template allows DNA polymerase to move in the same direction as the replication fork, while the lagging strand template does not. (correct answer)
  2. The two strands of the parent DNA have different chemical compositions, making one easier to replicate continuously.
  3. More primase enzymes are available for the leading strand, allowing for a single, continuous initiation event.
  4. The leading strand is replicated by DNA polymerase III, while the lagging strand is replicated by a different, less efficient enzyme, DNA polymerase I.
Explanation: The core reason is the antiparallel nature of DNA and the unidirectional 5' to 3' synthesis activity of DNA polymerase. The replication fork unwinds in one direction. The leading strand template is oriented such that DNA polymerase can follow the fork continuously. The lagging strand template is oriented in the opposite direction, forcing the polymerase to synthesize away from the fork in short fragments.

Question 14

During which phase of the cell cycle does DNA replication occur in eukaryotes?

  1. S phase of interphase (correct answer)
  2. G1 phase of interphase
  3. Prophase of mitosis
  4. G2 phase of interphase
Explanation: The cell cycle is divided into interphase (G1, S, and G2) and the M phase (mitosis and cytokinesis). DNA replication, or synthesis, occurs specifically during the S phase of interphase. The purpose is to create an identical copy of the genome, resulting in chromosomes composed of two sister chromatids, in preparation for cell division.

Question 15

A cell is exposed to a chemical agent that prevents hydrogen bonds from forming between cytosine and guanine, but not between adenine and thymine. How would this affect DNA replication?

  1. Replication would be completely halted as the DNA helix could not be unwound.
  2. Replication would proceed, but with a very high error rate specifically at G-C base pairs. (correct answer)
  3. Only the leading strand would be synthesized, as the lagging strand requires stable G-C pairs.
  4. The process would be unaffected as phosphodiester bonds, not hydrogen bonds, are key to replication.
Explanation: Hydrogen bonds are crucial for the specificity of base pairing. If C cannot properly bond with G, DNA polymerase might incorrectly insert A or T opposite a G on the template strand, or vice-versa. This would dramatically increase the mutation rate. Replication would still proceed because A-T bonds would form, and the polymerase would still create the phosphodiester backbone, but the fidelity would be severely compromised at G-C sites.

Question 16

Which of the following events in DNA replication occurs first?

  1. Helicase breaks hydrogen bonds between base pairs. (correct answer)
  2. DNA ligase joins Okazaki fragments.
  3. DNA polymerase adds nucleotides to a growing strand.
  4. Primase synthesizes an RNA primer.
Explanation: DNA replication must begin with the separation of the two parent strands. This is accomplished by helicase, which unwinds the double helix by breaking the hydrogen bonds between complementary bases. Only after the strands are separated can primase create a primer (D), and then DNA polymerase can begin adding nucleotides (A). Joining of Okazaki fragments by ligase (B) occurs later in the process on the lagging strand.

Question 17

Okazaki fragments are a feature of DNA replication. Which combination of enzymes is required to produce a continuous strand of DNA from these fragments? [HL]

  1. Helicase and primase
  2. DNA polymerase and DNA ligase (correct answer)
  3. Primase and DNA ligase
  4. Helicase and DNA polymerase
Explanation: After primase creates RNA primers and DNA polymerase synthesizes the short DNA fragments (Okazaki fragments), two steps are needed to create a continuous strand. First, a type of DNA polymerase removes the RNA primers and replaces them with DNA nucleotides. Second, DNA ligase forms the final phosphodiester bond that joins the adjacent fragments together. Therefore, both DNA polymerase (for primer replacement) and DNA ligase (for joining) are essential.

Question 18

Which statement correctly describes the formation of chemical bonds during DNA replication?

  1. Helicase forms hydrogen bonds, while DNA polymerase forms phosphodiester bonds.
  2. DNA ligase breaks hydrogen bonds, and DNA polymerase forms phosphodiester bonds.
  3. Helicase breaks phosphodiester bonds, while DNA ligase forms hydrogen bonds.
  4. DNA polymerase forms phosphodiester bonds, and hydrogen bonds form between complementary bases. (correct answer)
Explanation: During replication, DNA polymerase catalyzes the formation of phosphodiester bonds to create the sugar-phosphate backbone of the new strand. Hydrogen bonds form spontaneously between the complementary base pairs (A-T and G-C) without enzymatic action. Helicase breaks, not forms, hydrogen bonds. DNA ligase forms phosphodiester bonds, not hydrogen bonds.

Question 19

The synthesis of the lagging strand during DNA replication is discontinuous. What is the fundamental reason for this discontinuity? [HL]

  1. The lagging strand template is more tightly wound around histones, making it less accessible to enzymes.
  2. DNA polymerase can only synthesize new DNA in a 5' to 3' direction, which is opposite to the direction of the replication fork's movement for the lagging strand. (correct answer)
  3. There are fewer available primers for the lagging strand, causing DNA polymerase to start and stop frequently.
  4. Helicase unwinds the DNA in short bursts, which directly causes the fragmented synthesis of the lagging strand.
Explanation: DNA polymerase has a strict structural requirement: it can only add nucleotides to the 3' end of a growing strand, meaning synthesis always proceeds in the 5' to 3' direction. As the replication fork unwinds, the leading strand can be synthesized continuously in the same direction. However, the lagging strand template runs in the opposite direction, so it must be synthesized discontinuously in short segments (Okazaki fragments) that are individually made in the 5' to 3' direction.