Protein synthesis — IB Diploma Biology SL
IB Biology D1.2: transcription, translation, the genetic code, and mutations that change proteins.
Revision notes
4 short notes, in the order of the specification. Each one in short:
Transcription is the synthesis of RNA using a DNA template. RNA polymerase separates the DNA strands and links RNA nucleotides that pair with the template strand, with uracil pairing with adenine. The DNA is not changed, so it can be transcribed throughout a cell's life, and transcription is a key stage at which genes are switched on or off.
Translation is the synthesis of polypeptides from mRNA: the base sequence of mRNA is translated into the amino acid sequence of a polypeptide. mRNA binds to the small ribosomal subunit, tRNA anticodons pair with mRNA codons, and the ribosome moves along the mRNA one codon at a time, linking amino acids by peptide bonds to the growing chain.
The genetic code is the set of rules by which mRNA codons are translated into amino acids. It is a triplet code because 4³ = 64 combinations are enough for 20 amino acids, whereas pairs give only 16. It is degenerate, as most amino acids have several codons, and universal, as almost all organisms use the same codons.
A mutation that changes the base sequence of a gene can change the amino acid sequence of its polypeptide, and so the protein's structure. In sickle-cell anaemia, a base substitution in the haemoglobin beta-chain gene changes the codon GAG to GUG, so valine replaces glutamic acid. Haemoglobin S forms fibres at low oxygen, distorting red blood cells.
Exam questions
5 exam-style questions (15 marks), each with its mark scheme.
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13 cards: flip them, mark what you knew, and practise the rest.
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The whole of molecules (continuity and change) on one page, so you can see where this subtopic fits.
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Quick check questions
Which base on RNA pairs with adenine on the DNA template strand?
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Uracil.
Why must DNA in non-dividing somatic cells stay unchanged by transcription?
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The same sequences must be transcribed throughout the life of the cell, so they must be conserved.
What is meant by the degeneracy of the genetic code?
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More than one codon can code for the same amino acid.
How many tRNAs can bind to the large ribosomal subunit at once?
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Two.
Frequently asked questions
What is the difference between transcription and translation?
Transcription is the synthesis of RNA using a DNA template, carried out by RNA polymerase in the nucleus of eukaryotes. Translation is the synthesis of a polypeptide from mRNA at a ribosome, where tRNA anticodons pair with mRNA codons and amino acids are joined by peptide bonds. Transcription copies the sequence; translation decodes it.
Why is the genetic code a triplet code?
The genetic code is a triplet code because there are four bases and twenty amino acids. Pairs of bases would give only 4² = 16 combinations, too few for twenty amino acids. Triplets give 4³ = 64 codons, enough for every amino acid plus start and stop signals, which is why the code is also degenerate.
How does sickle-cell anaemia change the haemoglobin protein?
Sickle-cell anaemia is caused by a base substitution in the haemoglobin beta-chain gene. The mRNA codon changes from GAG to GUG, so valine replaces glutamic acid at the sixth amino acid. This hydrophobic valine makes haemoglobin S molecules stick together into fibres at low oxygen, distorting red blood cells into sickle shapes.
Written and checked against the IB Biology SL specification · Updated October 2026