H1 Biology 8876 · Nov 2023BiologyPaper 1Most challenging

Paper 1 Q14 — Molecules vs nucleotides with light nitrogen

Semi-conservative replication — count whole molecules, not individual nucleotides

What the examiners said

“Option C… was the option selected most commonly… These candidates had considered the percentage of nucleotides, rather than the percentage of complete double-stranded DNA molecules, that would contain light nitrogen.”

Try the question

DNA fully labelled with heavy nitrogen (¹⁵N) replicates twice in ¹⁴N medium. What percentage of the double-stranded DNA molecules contain at least some light (¹⁴N) nitrogen?

Examiner report: “Candidates counted the percentage of nucleotides/strands rather than of complete molecules.”

Explore — track strands vs molecules

Step through the rounds of semi-conservative replication. Compare the percentage of light strands with the percentage of molecules that contain any light nitrogen — they are different numbers.

Replication rounds: 2

At two rounds: 75% of strands are light, but 100% of molecules contain a light strand. The question’s wording — molecules, not nucleotides — is the whole point.

Original reconstruction inspired by the misconceptions described in the Singapore-Cambridge GCE A-Level H1 Biology 8876 November 2023 Examiner Report. Not the actual examination question. For classroom exploration.

Made by lookang, using Claude Fable 5. More resources: iwant2study.org