Protein synthesis — IB Diploma Biology HL
IB Biology D1.2: transcription, translation, the genetic code, mutations that change proteins, and HL mRNA splicing, initiation and protein modification.
Revision notes
8 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.
Transcription and translation both proceed 5' to 3'. RNA polymerase adds each RNA nucleotide to the 3' end of the growing RNA, moving along the template strand 3' to 5'; ribosomes read mRNA from its 5' end to its 3' end. Transcription starts at the promoter, where transcription factors bind and allow RNA polymerase to attach.
Non-coding sequences are DNA that does not code for polypeptides, such as regulators of gene expression, introns, telomeres and genes for rRNA and tRNA. In eukaryotes, introns are removed from pre-mRNA, exons are spliced together, and a 5' cap and 3' polyA tail stabilise it. Alternative splicing joins different exons, giving several polypeptides from one gene.
Initiation of translation begins when the small ribosomal subunit attaches to the 5' terminal of mRNA and moves to the start codon AUG. The initiator tRNA, carrying methionine, pairs with it and the large subunit attaches. During elongation, each tRNA enters the A site, holds the growing polypeptide in the P site and leaves from the E site.
Many polypeptides must be modified before they can function. Insulin is made in two stages: the signal sequence is removed from pre-proinsulin to give proinsulin, which folds and forms disulfide bonds, then the C-peptide is cut out, leaving two chains. Proteasomes break down proteins into short peptides, so amino acids are recycled and a functional proteome is sustained.
Exam questions
8 exam-style questions (24 marks), each with its mark scheme.
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Quick check questions
Which base on RNA pairs with adenine on the DNA template strand?
Show answer
Uracil.
Why must DNA in non-dividing somatic cells stay unchanged by transcription?
Show answer
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?
Show answer
More than one codon can code for the same amino acid.
How many tRNAs can bind to the large ribosomal subunit at once?
Show answer
Two.
HL only What is removed from proinsulin to produce insulin?
Show answer
The C-peptide.
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.
Why are introns removed from pre-mRNA?
HL only Introns are removed from pre-mRNA because they are non-coding sequences within a gene that would otherwise be translated into the wrong amino acids. In eukaryotes, introns are cut out and exons are spliced together to form mature mRNA. Splicing different combinations of exons, called alternative splicing, lets one gene code for several polypeptides.
What do the A, P and E sites on a ribosome do?
HL only The A, P and E sites are the three tRNA binding sites on the large ribosomal subunit. A tRNA carrying an amino acid enters the A site, the tRNA holding the growing polypeptide sits in the P site, and a tRNA that has given up its polypeptide leaves from the E site as the ribosome moves along.
Written and checked against the IB Biology HL specification · Updated October 2026