201. Which enzyme is primarily responsible for synthesizing a new DNA strand during replication?
ⓐ. DNA-dependent DNA polymerase
ⓑ. RNA polymerase
ⓒ. DNA ligase
ⓓ. reverse transcriptase
Correct Answer: DNA-dependent DNA polymerase
Explanation: DNA replication requires an enzyme that adds deoxyribonucleotides to a growing DNA chain using an existing DNA strand as template. That central role is performed by DNA-dependent DNA polymerase. It ensures that the new strand is built according to base-pairing rules. This enzyme is therefore the main catalyst of DNA synthesis during replication.
202. The immediate substrates used by DNA polymerase during replication are
ⓐ. ribonucleoside monophosphates
ⓑ. amino acids
ⓒ. deoxyribonucleoside triphosphates
ⓓ. free nitrogenous bases only
Correct Answer: deoxyribonucleoside triphosphates
Explanation: DNA polymerase does not add free bases or amino acids during replication. It uses deoxyribonucleoside triphosphates, which provide both the nucleotide units and the energy needed for chain elongation. These activated building blocks are matched to the template strand by complementarity. Their triphosphate form is important because bond formation is coupled with energy release.
203. Why are deoxyribonucleoside triphosphates especially suitable for DNA replication?
ⓐ. They contain only purine bases.
ⓑ. They can bind histones before polymerization.
ⓒ. They are needed only for repair, not replication.
ⓓ. They act as substrates and also provide energy for chain formation.
Correct Answer: They act as substrates and also provide energy for chain formation.
Explanation: The incoming deoxyribonucleoside triphosphates serve a dual purpose in replication. They supply the actual nucleotide components of the new strand, and their high-energy phosphate bonds drive polymerization. This makes DNA synthesis efficient and chemically favorable. The same molecule therefore contributes both structure and energy.
204. DNA polymerase can add nucleotides to a growing DNA strand only in the
ⓐ. 3'→5' direction
ⓑ. 5'→3' direction
ⓒ. both 5'→3' and 3'→5' directions equally
ⓓ. random direction depending on base sequence
Correct Answer: 5'→3' direction
Explanation: DNA polymerase extends a DNA chain only by adding nucleotides to the free 3' end, so synthesis proceeds in the 5'→3' direction. This directional restriction is a fundamental rule of DNA replication. It also explains why the two strands at the replication fork cannot be synthesized in exactly the same way. Many later features of replication follow directly from this polarity requirement.
205. A small region where the DNA double helix opens during replication is called the
ⓐ. nucleosome
ⓑ. operon
ⓒ. replication fork
ⓓ. cistron
Correct Answer: replication fork
Explanation: Replication does not begin by opening the entire DNA molecule at once. Instead, synthesis is organized around a limited open region called the replication fork. Here the parental strands separate and serve as templates for new DNA synthesis. This localized opening allows replication to proceed in a controlled manner.
206. At a replication fork, continuous DNA synthesis occurs on the strand whose template is oriented
ⓐ. 3'→5' toward the fork
ⓑ. 5'→3' toward the fork
ⓒ. 5'→5' toward the fork
ⓓ. 3'→3' toward the fork
Correct Answer: 3'→5' toward the fork
Explanation: Because DNA polymerase synthesizes only in the 5'→3' direction, it can move continuously along a template that runs 3'→5' toward the replication fork. This allows uninterrupted addition of nucleotides as the fork opens. The opposite template orientation creates a different problem and leads to discontinuous synthesis. Strand polarity is therefore the reason the two daughter strands are made differently.
207. The strand synthesized discontinuously during DNA replication is produced as
ⓐ. a protein-associated chromatin segment
ⓑ. a single unbroken DNA helix
ⓒ. an RNA-only intermediate chain
ⓓ. short DNA fragments that are later joined
Correct Answer: short DNA fragments that are later joined
Explanation: On one template strand, the direction of fork opening does not match the only allowed direction of DNA synthesis. As a result, DNA is made in short segments rather than continuously. These short pieces are synthesized separately and later connected into a single strand. This is the basis of discontinuous replication.
208. Which enzyme joins discontinuously synthesized DNA fragments into a continuous strand?
ⓐ. DNA polymerase
ⓑ. helicase
ⓒ. DNA ligase
ⓓ. RNase
Correct Answer: DNA ligase
Explanation: The short DNA fragments produced on the discontinuously synthesized strand must be sealed into one continuous chain. DNA ligase performs this joining function by forming the necessary covalent bonds in the backbone. Without ligase, the fragments would remain separate pieces. Its role is therefore essential for completing replication properly.
209. At a replication fork, template strand I runs 3'→5' toward the fork, whereas template strand II runs 5'→3' toward the fork. Which statement is correct?
ⓐ. DNA synthesis is discontinuous on both templates.
ⓑ. DNA synthesis is continuous on template I and discontinuous on template II.
ⓒ. DNA synthesis is continuous on template II and discontinuous on template I.
ⓓ. DNA synthesis occurs 3'→5' on one template and 5'→3' on the other.
Correct Answer: DNA synthesis is continuous on template I and discontinuous on template II.
Explanation: DNA polymerase can synthesize DNA only in the 5'→3' direction. A template running 3'→5' toward the fork can therefore be copied continuously as the fork opens. The opposite template must be copied in short fragments because its orientation does not match continuous synthesis. This is the basis of leading and lagging strand formation.
210. Which statement is correct if DNA ligase is absent during replication but DNA polymerase remains functional?
ⓐ. Both new strands will be synthesized continuously.
ⓑ. The leading strand cannot begin synthesis.
ⓒ. Replication will stop before the replication fork opens.
ⓓ. Discontinuous fragments on one new strand will not be joined into a continuous chain.
Correct Answer: Discontinuous fragments on one new strand will not be joined into a continuous chain.
Explanation: DNA ligase is required to seal the gaps between short DNA fragments produced on the discontinuously synthesized strand. DNA polymerase can still synthesize those fragments, but without ligase they remain separate pieces. The leading strand is not affected in the same way because it is synthesized continuously. This question distinguishes synthesis from final joining.
211. Which statement correctly compares the two strands synthesized at a replication fork?
ⓐ. One strand is synthesized continuously, while the other is synthesized discontinuously.
ⓑ. Both strands are synthesized continuously because both templates are identical.
ⓒ. Both strands are synthesized discontinuously because the fork opens irregularly.
ⓓ. One strand is synthesized as RNA, while the other is synthesized as DNA.
Correct Answer: One strand is synthesized continuously, while the other is synthesized discontinuously.
Explanation: The two template strands are antiparallel, but DNA polymerase can work only in the 5'→3' direction. Because of this, one new strand can be extended continuously, whereas the other must be assembled in short segments. This difference is not due to different enzymes or different template sequences, but to opposite strand orientation. The replication fork therefore produces two patterns of synthesis.
212. Why is high accuracy especially important during DNA replication?
ⓐ. It increases the amount of ATP stored in chromosomes.
ⓑ. It helps convert DNA directly into proteins after replication.
ⓒ. It preserves hereditary information with minimal error across generations.
ⓓ. It allows RNA to replace DNA in most organisms.
Correct Answer: It preserves hereditary information with minimal error across generations.
Explanation: DNA carries the genetic instructions of the cell, so replication errors can alter inherited information. High accuracy is therefore essential to maintain continuity of traits and cellular functions. Although mutations can contribute to evolution, most copying events must remain faithful for stability of the organism. Replication is thus both rapid and remarkably precise.
213. Which statement best describes the origin of replication in a DNA molecule?
ⓐ. It is a specific site where DNA replication begins.
ⓑ. It is the region where ribosomes attach during translation.
ⓒ. It is the terminal region where proteins are degraded.
ⓓ. It is a site where RNA replaces DNA permanently.
Correct Answer: It is a specific site where DNA replication begins.
Explanation: DNA replication does not start randomly along the molecule. It begins at a defined sequence or region called the origin of replication. From this point, the replication machinery assembles and DNA synthesis starts. The presence of an origin helps ensure orderly and controlled duplication of genetic material.
214. Why is DNA replication said to be highly accurate?
ⓐ. Because DNA polymerase can synthesize in both directions equally.
ⓑ. Because all mutations are completely prevented during replication.
ⓒ. Because hereditary information must be copied with very few errors.
ⓓ. Because DNA is made without using any template strand.
Correct Answer: Because hereditary information must be copied with very few errors.
Explanation: The sequence of DNA carries essential genetic instructions, so replication errors can alter important traits or cellular functions. For this reason, the replication process must operate with very high fidelity. Accuracy does not mean absolute absence of mutation, but it does mean that mistakes are rare. This balance allows stability of inheritance while still permitting occasional variation.
215. In E. coli, the rate of DNA replication mentioned in this chapter is approximately
ⓐ. 20 bp per second
ⓑ. 200 bp per second
ⓒ. 20000 bp per second
ⓓ. 2000 bp per second
Correct Answer: 2000 bp per second
Explanation: The chapter notes that DNA replication in E. coli proceeds at a very high speed, about 2000 base pairs per second. This rapid synthesis is necessary because the bacterial genome must be copied within a short generation time. High speed, however, is accompanied by high accuracy. Together, these features show the remarkable efficiency of the replication machinery.
216. About how long does replication of the E. coli genome take according to the values given in this chapter?
ⓐ. about 2 minutes
ⓑ. about 18 minutes
ⓒ. about 180 minutes
ⓓ. about 1 hour
Correct Answer: about 18 minutes
Explanation: The E. coli genome is stated here to be about $4.6 \times 10^6$ base pairs long, and replication proceeds at roughly 2000 base pairs per second. At that rate, the genome can be copied in about 18 minutes. This rapid completion is consistent with the fast growth of bacteria. The example shows how speed is important in prokaryotic DNA replication.
217. Which statement correctly explains why one newly formed DNA strand is continuous and the other is discontinuous?
ⓐ. DNA polymerase works only in the 5'→3' direction, while the two template strands are antiparallel.
ⓑ. The two parental strands contain different kinds of sugars.
ⓒ. Ligase functions only on one half of the DNA molecule.
ⓓ. One strand is copied from RNA, whereas the other is copied from DNA.
Correct Answer: DNA polymerase works only in the 5'→3' direction, while the two template strands are antiparallel.
Explanation: The two parental DNA strands run in opposite directions, but DNA polymerase can extend a new strand only in the 5'→3' direction. Because of this, one new strand can be made smoothly in the same direction as fork movement, whereas the other must be assembled in pieces. The difference is therefore caused by strand polarity and enzyme limitation together. This is one of the key structural consequences of antiparallel DNA.
218. A small DNA fragment is introduced into a host cell for recombinant work, but it fails to replicate. Which missing feature most directly explains this failure?
ⓐ. terminator sequence
ⓑ. stop codon
ⓒ. operator sequence
ⓓ. origin of replication
Correct Answer: origin of replication
Explanation: For a DNA piece to be copied inside a cell, the replication machinery must know where to begin. That starting point is provided by an origin of replication. If the DNA fragment lacks such a sequence, it may enter the cell but will not be propagated efficiently. This is why vectors used in recombinant procedures must carry an origin.
219. Why is the origin of replication important in recombinant DNA procedures?
ⓐ. It ensures that inserted DNA is translated into protein immediately.
ⓑ. It allows histones to bind tightly to foreign DNA.
ⓒ. It permits the introduced DNA to be copied and maintained in the host cell.
ⓓ. It prevents any mutation from occurring in the cloned fragment.
Correct Answer: It permits the introduced DNA to be copied and maintained in the host cell.
Explanation: Recombinant DNA technology often depends on carrying a DNA fragment inside a vector and allowing it to multiply in a host. That multiplication requires an origin of replication. Without an origin, the inserted DNA would not be reliably propagated during cell division. The origin therefore gives practical value to the DNA construct by making replication possible.
220. Which component of a cloning vector is most directly required for its propagation inside a host cell?
ⓐ. promoter
ⓑ. origin of replication
ⓒ. anticodon loop
ⓓ. histone octamer
Correct Answer: origin of replication
Explanation: A cloning vector must be able to duplicate inside the host cell if it is to maintain and amplify inserted DNA. The sequence that enables this is the origin of replication. Other elements may help in expression or selection, but replication itself depends on the origin. That is why origin-containing vectors are essential in recombinant DNA work.
