Questions: DNA Replication: Leading and Lagging Strands

Question 1

Why does the lagging strand require a new RNA primer for each Okazaki fragment, while the leading strand needs only a single primer for the entire strand?

• A. The lagging strand polymerase moves faster than the leading strand polymerase and needs primers to pace itself
• B. Because the lagging strand template runs 5'→3' in the direction of fork movement, each newly exposed segment requires a fresh primer so polymerase can start a new fragment moving away from the fork in the permitted 5'→3' direction
• C. RNA primers protect newly synthesized DNA from nuclease degradation on the lagging strand only
• D. The lagging strand replicates in a different subcellular compartment where primers are continuously required

Answer: (B)

The lagging strand template runs 5'→3' in the direction of fork movement. DNA polymerase can only extend a strand 5'→3', which on this template means moving AWAY from the fork. Each time helicase exposes new template, polymerase cannot extend the existing fragment toward the new region — it is already moving the wrong way. So primase must lay a fresh RNA primer on the newly exposed template, and polymerase extends a new Okazaki fragment away from the fork. The leading strand template runs 3'→5' in the fork direction, so polymerase can follow helicase continuously from a single primer.

Question 2

Imagine a cell where DNA polymerase could synthesize DNA in both 5'→3' and 3'→5' directions. How would this change lagging strand synthesis?

• A. Okazaki fragments would be longer because polymerase wouldn't need to restart as often
• B. The lagging strand could be synthesized continuously in the 3'→5' direction following the fork, eliminating the need for Okazaki fragments and multiple primers
• C. The leading strand would still require multiple primers because it always needs a free 3' OH to extend
• D. Nothing would change — the antiparallel nature of DNA forces discontinuous synthesis regardless of polymerase direction

Answer: (B)

The entire reason for Okazaki fragments is that DNA polymerase cannot synthesize 3'→5'. If it could, the lagging strand polymerase could simply follow the fork in the 3'→5' direction, synthesizing a continuous strand just as the leading strand is synthesized — requiring only one primer. The discontinuity of lagging strand synthesis is entirely a consequence of the unidirectional polymerase constraint combined with the antiparallel template orientation.

Question 3

Okazaki fragments remain as short single-stranded gaps in the final mature DNA molecule, repaired later by DNA ligase.

• T. True
• F. False

Answer: False

Okazaki fragments are transient intermediates, not permanent features of mature DNA. After each fragment is synthesized, DNA polymerase I (in prokaryotes) removes the RNA primer at the 5' end of the next fragment and fills in the gap with DNA. DNA ligase then seals the remaining nick between adjacent fragments. The final lagging strand is a continuous DNA strand with no RNA, no gaps, and no remnant of the fragmented synthesis process.

Question 4

Both the leading strand and lagging strand DNA polymerases add nucleotides exclusively in the 5'→3' direction.

• T. True
• F. False

Answer: True

This is the invariant rule: all DNA polymerases synthesize DNA 5'→3', reading the template strand 3'→5'. This applies to both leading and lagging strand synthesis. The difference between the strands is not polymerase direction — both are 5'→3' — but rather how this constraint interacts with the antiparallel template geometry and fork movement direction.

Question 5

Explain why the lagging strand must be synthesized discontinuously, using the antiparallel nature of DNA and the directionality constraint of DNA polymerase.

Model answer: DNA's two strands are antiparallel: one runs 5'→3' in the direction of fork movement, the other runs 3'→5'. DNA polymerase can only synthesize 5'→3' (reading the template 3'→5'). The leading strand template runs 3'→5' in the fork direction — perfectly aligned for continuous 5'→3' synthesis following the fork. The lagging strand template runs 5'→3' in the fork direction, meaning polymerase would need to synthesize 3'→5' to follow the fork — which it cannot do. Instead, each time helicase exposes new lagging strand template, primase primes it and polymerase synthesizes a short Okazaki fragment 5'→3' (away from the fork). This produces a series of disconnected fragments that are later joined into a continuous strand.

The antiparallel constraint and unidirectional polymerase are both necessary to understand the asymmetry. Either constraint alone would not force discontinuous synthesis — it is their combination that makes Okazaki fragments necessary. This also explains why the lagging strand requires more enzymatic machinery (primase for multiple primers, pol I for primer removal, ligase for joining) than the leading strand.