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Unit 4: Heredity and continuity of life
Quick questions on DNA structure and semi-conservative replication (QCE Biology Unit 4)
10short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is semi-conservative replication?Show answer
<!-- Diagram: semi-conservative DNA replication | reviewed 2026-05-21 --> <svg class="fig" viewBox="0 0 540 240" role="img" aria-labelledby="semi-t semi-d"> <title id="semi-t">Semi-conservative DNA replication</title> <desc id="semi-d">A parent DNA double helix shown as two strands. After replication, two daughter helices appear, each consisting of one original parental strand (dark) and one newly synthesised strand (lighter).</desc> <defs> <marker id="sc-arr" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="6" markerHeight="6" orient="auto"> <path d="M0 0 L10 5 L0 10 z" fill="var(--accent)"/> </marker> </defs> <line x1="100" y1="60" x2="100" y2="180" stroke="var(--ink)" stroke-width="2.4"/> <line x1="116" y1="60" x2="116" y2="180" stroke="var(--ink)" stroke-width="2.4"/> <text x="108" y="208" text-anchor="middle" font-size="11" font-weight="700">parental</text> <line x1="200" y1="120" x2="280" y2="120" stroke="var(--accent)" stroke-width="1.6" marker-end="url(#sc-arr)"/> <text x="240" y="108" text-anchor="middle" font-size="11" font-weight="700" class="accent">replication</text> <line x1="340" y1="60" x2="340" y2="180" stroke="var(--ink)" stroke-width="2.4"/> <line x1="356" y1="60" x2="356" y2="180" stroke="var(--accent)" stroke-width="2.4"/> <line x1="420" y1="60" x2="420" y2="180" stroke="var(--accent)" stroke-width="2.4"/> <line x1="436" y1="60" x2="436" y2="180" stroke="var(--ink)" stroke-width="2.4"/> <text x="348" y="208" text-anchor="middle" font-size="11" font-weight="700">daughter 1</text> <text x="428" y="208" text-anchor="middle" font-size="11" font-weight="700">daughter 2</text> <text x="270" y="232" text-anchor="middle" font-size="11" class="muted">Each daughter has one parental strand (dark) and one new strand (accent).</text> </svg>
What is the replication fork?Show answer
Replication begins at a specific sequence called the origin of replication. Several enzymes act at the replication fork where the helix opens.
What is helicase?Show answer
Breaks hydrogen bonds between paired bases and unwinds the helix. Single-strand binding proteins keep the separated strands from re-annealing. Topoisomerase relieves the supercoiling tension ahead of the fork.
What is primase?Show answer
Synthesises a short RNA primer (about 5 to 10 ribonucleotides) complementary to the template. The primer provides a free 3 prime hydroxyl group, which is what DNA polymerase needs to extend from.
What is dNA polymerase III?Show answer
The main copying enzyme in prokaryotes (eukaryotes use a family of polymerases). It adds DNA nucleotides to the 3 prime end of the growing strand, reading the template 3 prime to 5 prime and synthesising the new strand 5 prime to 3 prime. Each new nucleotide arrives as a deoxynucleoside triphosphate (dATP, dTTP, dCTP, dGTP); two phosphates are cleaved off, releasing energy for the bond.
What is dNA polymerase I?Show answer
Removes the RNA primers and fills the gaps with DNA.
What is ligase?Show answer
Seals the nicks between Okazaki fragments by forming the final phosphodiester bond, producing one continuous strand.
What is q1?Show answer
Describe the role of helicase, primase, DNA polymerase and ligase in semi-conservative replication. [4 marks]
What is q2?Show answer
A DNA strand has the sequence 5'-AGCTTAGC-3'. Write the complementary strand and the mRNA that would result if the original strand were transcribed. [3 marks]
What is q3?Show answer
Refer to leading and lagging strand synthesis. (a) Explain why one strand is synthesised continuously. (b) Identify the role of Okazaki fragments.