Book a tutor
SearchResourcesRevision notesFlashcardsMind mapsExam questionsTopic testsPracticalsGCSE Biology coursesAQA GCSE Biology (8461)AQA GCSE Combined Science: Trilogy (8464)Edexcel GCSE Biology (1BI0)Edexcel GCSE Combined Science (1SC0)IGCSE Biology coursesCambridge IGCSE Biology (0610)Cambridge IGCSE Combined Science (0653)Edexcel International GCSE Biology (4BI1)Edexcel International GCSE Science (Double Award) (4SD0)IB Diploma Biology coursesIB Diploma Biology SLIB Diploma Biology HLMoreAboutContact
Book a tutor

Directionality and initiation at the promoter

Molecules (Continuity and change) · Protein synthesis · note 5 of 8

Spec D1.2.12, D1.2.13
HL only (what this means)HL only: additional Higher Level content, only for HL students. SL students can skip it. What the labels mean
Download all Protein synthesis notes (PDF)Download all notes (PDF)11 pages

Directionality and initiation at the promoterSpec D1.2.12, D1.2.13

In short

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.

5' to 3' transcription

RNA polymerase, like DNA polymerase, adds the 5' end of each new RNA nucleotide to the 3' end of the growing RNA strand. RNA is therefore synthesised in the 5' → 3' direction. Because the template strand is antiparallel to the RNA, RNA polymerase moves along the template strand in its 3' → 5' direction.

5' to 3' translation

Ribosomes bind near the 5' end of mRNA and move towards the 3' end, reading codons in order. Translation is therefore 5' → 3' along the mRNA. The first amino acid added (from the start codon) forms the amine (N) terminal of the polypeptide, and the last forms the carboxyl (C) terminal.

The promoter

The promoter is a base sequence in DNA just before the start of a gene. It is not itself transcribed. Transcription is initiated when proteins called transcription factors bind to the promoter. They allow RNA polymerase to bind at the correct site and start transcribing in the right direction along the right strand. Whether transcription factors bind is one way in which the expression of a gene is switched on or off.

A gene as double-stranded DNA, coding strand 5′ to 3′ and template strand 3′ to 5′, with transcription factors bound to the promoter and RNA polymerase bound next to it; RNA leaves the polymerase with its 5′ free end outside and its 3′ growing end inside; below, a ribosome moves along mRNA from 5′ to 3′ with the N-terminal end of the polypeptide leading. (opens full size in a new tab)
Transcription factors at the promoter let RNA polymerase start; RNA is made 5′ → 3′ and translated 5′ → 3′.

Written and checked against the IB Biology HL specification · Updated October 2026

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.

All 5 questions on Protein synthesis