A1.2.4—Bases form the basis of a code
Genetic information is encoded by the order of nitrogenous bases, with triplet codons providing a sequence language for protein synthesis.
- Syllabus
- First assessment 2025
- Objective
- A1.2.4
- Level
- HL
Base sequences and RNA roles
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.
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
- Bases form the basis of a code 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.