CourseCourseQ BankQuestion BankDocsDocuments

A1.2.5—RNA as a polymer

RNA is a usually single-stranded polynucleotide containing ribose and uracil, with different RNA forms supporting protein synthesis in cells in IB Biology exam contexts.

Syllabus
First assessment 2025
Objective
A1.2.5
Level
SL

Base sequences and RNA roles

Base sequence to codon to amino acid pathway.

The genetic code is stored in the order of nitrogenous bases, not in the repeating sugar–phosphate backbone. DNA uses A, T, G and C; RNA uses A, U, G and C. Groups of three bases form codons, and the order of codons carries the instructions for the amino-acid sequence of a polypeptide.

RNA is a single-stranded polynucleotide formed by condensation of nucleotide monomers. Its main types have different roles in protein synthesis:

  • mRNA carries a transcript of the DNA instructions to the ribosome.
  • tRNA carries amino acids to the mRNA sequence.
  • rRNA forms part of the ribosome and provides catalytic activity for joining amino acids.

Answer chain: base sequence → genetic instructions; condensation → RNA polymer; mRNA, tRNA and rRNA → different steps in gene expression.

RNA as a polymer

Assessment in practice

4 marks
How it is assessed

This objective is assessed through structured response, commonly using Draw.

Command terms

Draw

What earns marks

Build the answer around this relationship: RNA as a polymer should be described using precise molecular vocabulary.

Representative question

Question 1

[Maximum number: 4]

Draw labelled diagrams to show the structure of RNA nucleotides and how they are linked together to form a molecule of RNA.

Retrieve The Core Rules

The core chain is: nucleotides have three parts; condensation builds the sugar-phosphate backbone; base order stores information; complementary pairing lets DNA copy and express that information.

  • Nucleotide parts: phosphate, pentose sugar, nitrogenous base.
  • Polymer rule: condensation forms a sugar-phosphate backbone.
  • Code rule: base order stores information and triplet codons specify amino acids.
  • Pairing rule: A-T or A-U, and C-G, enables replication and gene expression.
  • DNA/RNA contrast: DNA uses deoxyribose/T and is usually double-stranded; RNA uses ribose/U and is usually single-stranded.

Concept essentials

  • RNA as a polymer should be described using precise molecular vocabulary.
  • Structure and information must be kept separate: backbones provide continuity while bases carry sequence meaning.
  • Exam answers should match the command term and avoid adding unsupported extra claims.