During DNA replication, DNA polymerase cannot begin synthesizing a new strand from scratch. What is the correct explanation for this limitation?
ADNA polymerase can only extend an existing strand — it requires a free 3'-OH group to add the first nucleotide
BDNA polymerase only works in the 3'→5' direction and needs a primer to reverse direction
CThe template strand must be fully unwound before synthesis can begin at any point
DATP is not available at the start of replication to power the first nucleotide addition
DNA polymerase adds nucleotides exclusively to the 3'-OH end of an existing strand. It cannot initiate synthesis de novo. The RNA primer synthesized by primase provides the initial 3'-OH group so DNA polymerase can begin extension. The primer is later removed and replaced with DNA.
Question 2 True / False
In semi-conservative replication, each daughter DNA molecule contains a patchwork of old and new nucleotides distributed across both strands.
TTrue
FFalse
Answer: False
Semi-conservative means each daughter molecule retains one complete, intact parental strand paired with one completely new strand — not a mixture within each strand. This was demonstrated by the Meselson-Stahl experiment: after one round of replication in light (¹⁴N) medium, each daughter molecule had exactly one heavy (¹⁵N parental) strand and one light (new) strand.
Question 3 Short Answer
Why is the lagging strand synthesized as discontinuous Okazaki fragments rather than as one continuous strand like the leading strand?
Think about your answer, then reveal below.
Model answer: DNA polymerase can only synthesize DNA in the 5'→3' direction. On the lagging strand template, this means synthesis must proceed away from the replication fork. As helicase unwinds more DNA, new primers must be laid down periodically, generating short Okazaki fragments that are later joined by DNA ligase.
The two template strands are antiparallel. The leading strand template (3'→5' relative to fork movement) allows continuous synthesis toward the fork. The lagging strand template runs 5'→3' toward the fork, so the polymerase must work away from the fork and restart repeatedly as new template is exposed. This is not an inefficiency but an unavoidable consequence of the directionality constraint on DNA polymerase.