IB DP Biology- A1.2 Nucleic acids- IB Style Questions For HL Paper 2 -FA 2025
▶️ Answer/Explanation
(a)
The nucleic acid shown is DNA. It is double-stranded and contains the base thymine (\(T\)), which is only present in DNA.
(b)
The diagram represents DNA replication. This is because both strands are being copied and thymine (\(T\)) is incorporated. If the diagram showed transcription, uracil (\(U\)) would replace thymine when pairing with adenine (\(A\)).
(c)
Base \(X\) is paired with guanine (\(G\)). According to base pairing rules, guanine pairs with cytosine (\(C\)).
(d)
The terminal labelled \(Y\) represents the \(5’\) end of the DNA strand, where a phosphate group is attached to the \(5’\) carbon of deoxyribose.
(e)(i)
Bond \(Z\) is a phosphodiester bond, a covalent linkage connecting the phosphate of one nucleotide to the \(3’\) carbon of the next sugar.
(e)(ii)
Two enzymes catalyse the formation of phosphodiester bonds during replication:
- DNA polymerase: Adds nucleotides to the growing strand by forming phosphodiester bonds at the \(3’\) end, synthesizing the new DNA strand.
- DNA ligase: Joins Okazaki fragments on the lagging strand by sealing gaps and completing the phosphodiester backbone.