Explain why RNA is described as a single-stranded molecule and outline how this structural feature relates to the functional diversity of RNA molecules within a cell.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
RNA is a single‑stranded polynucleotide, unlike DNA’s double helix.
Because it is single‑stranded it can fold back on itself, forming localised complementary base pairs that give rise to hairpin loops and complex three‑dimensional shapes.
This folding allows tRNA to adopt a cloverleaf (or L‑shaped) structure, enabling it to attach amino acids and recognise codons during translation.
mRNA remains linear, acting as a template that ribosomes read codon by codon. rRNA, also single‑stranded, folds into structures that provide the ribosome’s catalytic core and structural scaffold. Other RNAs (e.g. miRNA, siRNA) use similar folding to regulate gene expression.
Because it is single‑stranded it can fold back on itself, forming localised complementary base pairs that give rise to hairpin loops and complex three‑dimensional shapes.
This folding allows tRNA to adopt a cloverleaf (or L‑shaped) structure, enabling it to attach amino acids and recognise codons during translation.
mRNA remains linear, acting as a template that ribosomes read codon by codon. rRNA, also single‑stranded, folds into structures that provide the ribosome’s catalytic core and structural scaffold. Other RNAs (e.g. miRNA, siRNA) use similar folding to regulate gene expression.
Examiner tips
- State that RNA is single‑stranded first. Show the folding ability and give at least one example (tRNA, rRNA).
- Use the exact terms: cloverleaf, hairpin, codon, ribosome, catalytic core.
Mark scheme (4 marks)
- RNA consists of a single polynucleotide strand (rather than two complementary strands held together by hydrogen bonds between bases)
- The single strand can fold back on itself, forming localised complementary base pairs / hairpin loops / three-dimensional shapes
- This folding ability allows tRNA to adopt a specific cloverleaf / L-shaped structure that enables it to carry amino acids and recognise codons during translation
- mRNA functions as a linear template / carries a sequence of codons read by ribosomes, and rRNA has a structural and catalytic role in ribosomes — accept any named RNA type with a correct functional link
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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