IB Biology Transcription and Translation: D1.2 Codon Table Example
Transcription copies a DNA sequence into RNA in the 5’→3’ direction, and translation decodes that mRNA into a polypeptide on ribosomes. Together they carry the central dogma from DNA to RNA to protein. At SL you need the core steps and codon use, while HL adds post-transcriptional modifications such as capping, polyadenylation and splicing, as the IB Biology guide sets out.
TL;DR:
- For codon questions, read mRNA 5′ to 3′, begin at the first AUG, group triplets, and stop at UAA, UAG, or UGA.
- SL answers should cover core steps, synthesis direction, and codon decoding; HL answers should explain how capping, polyadenylation, and splicing process eukaryotic mRNA.
- In eukaryotes, transcription occurs in the nucleus and processed mRNA exits before translation; prokaryotes can translate mRNA while it is being transcribed.
- During elongation, charged tRNAs enter the A site, peptide bonds form, and the ribosome shifts one codon as tRNAs pass through A, P, and E sites.
- HL students should explain that introns are removed, exons are joined, and alternative splicing can produce several proteins from one gene.
Table of Contents
- How does transcription work, step by step?
- What happens during translation?
- Which post-transcriptional modifications do HL students need?
- How do prokaryotes and eukaryotes differ?
- How do you use a codon table?
- What should you revise for exam marks?
- Which practice questions test D1.2?
- An examiner-minded view of the common mistakes
- Where can you practise D1.2 with examiner-written content?
- FAQ
- Sources
How does transcription work, step by step?
Transcription makes an RNA copy of one DNA strand using complementary base pairing, and a single gene can be copied many times, which is how cells make protein quickly. As NCBI Bookshelf explains, eukaryotic nuclei contain three RNA polymerases, and RNA polymerase II handles protein-coding genes. Learn these three stages in order so you can annotate any diagram:
- Initiation: RNA polymerase binds the promoter, and the DNA helix opens so the template strand is exposed.
- Elongation: RNA nucleotides are added to the 3’ end of the growing strand, so the mRNA is built 5’→3’ while the template is read 3’→5’.
- Termination: terminator sequences signal the polymerase to stop, and the pre-mRNA is released.
Three types of RNA come out of this process: mRNA carries the code, tRNA brings amino acids, and rRNA forms part of the ribosome. Our D1.2 protein synthesis notes show each stage with labelled diagrams.
What happens during translation?
In translation, the ribosome reads mRNA 5’→3’ and builds the polypeptide from the N-terminus to the C-terminus. NCBI Bookshelf describes ribosomes with P, A and E sites, and the same three-stage pattern applies:
- Initiation: the small and large subunits assemble around the mRNA with the initiator Met-tRNA paired to the start codon, AUG.
- Elongation: a charged tRNA enters the A site, peptidyl transferase forms a peptide bond, and the ribosome shifts along one codon so the tRNA moves A to P to E. GTP-dependent elongation factors drive each step.
- Termination: a stop codon (UAA, UAG or UGA) reaches the A site. Release factors, not tRNA, recognise it and free the polypeptide.
Several ribosomes can read one mRNA at once, forming a polysome, which raises the protein output from each transcript.
Which post-transcriptional modifications do HL students need?
Eukaryotic pre-mRNA is processed before it leaves the nucleus, a point covered in the NCBI account of RNA processing. HL answers should name all three modifications:
- 5’ cap: a modified guanine added to the 5’ end protects the mRNA and helps the ribosome recognise it.
- Poly-A tail: the transcript is cleaved after a signal sequence (AAUAAA), then a string of adenine nucleotides is added. This improves stability and supports translation.
- Splicing: introns are removed and exons joined. Alternative splicing joins exons in different combinations, so one gene can produce several mRNAs and several proteins.
In an exam, say what each step does for the mRNA, not just what it is called.
How do prokaryotes and eukaryotes differ?
Comparison questions reward clean contrasts, so keep each system separate in your head:
- Prokaryotes: transcription and translation are coupled because there is no nucleus, and the mRNA has no cap or tail. Many carry several genes on one mRNA (polycistronic). Ribosomes are 70S, and initiation uses the Shine–Dalgarno sequence, as NCBI Bookshelf notes.
- Eukaryotes: the nucleus separates transcription from translation, so mRNA must be processed and exported first. Ribosomes are 80S, and initiation is cap-dependent with scanning to the first AUG. OpenStax gives the 70S and 80S distinction clearly.
Never write that a bacterial mRNA is spliced and capped. That is the most common mix-up we see.
How do you use a codon table?
The method is the same every time, and checking the frame is where marks are won or lost:
- Read the mRNA 5’→3’ and find the first AUG. That sets the reading frame.
- Split the sequence into codons from the AUG, then look each up in the codon table.
- Write the amino acids from N-terminus to C-terminus and stop at the first stop codon.
Take 5’-GGAUGGCUUACUAAGC-3’. The first AUG begins the frame: AUG GCU UAC UAA. That reads Met, Ala, Tyr, then a stop, so the polypeptide is three amino acids long. The leading GG and trailing GC are not translated.
Pro Tip: Circle the start codon before you split anything. If your answer looks wrong, you have almost certainly picked the wrong frame.
What should you revise for exam marks?
Use this checklist when you review any D1.2 answer:
- State the direction (5’→3’) every time you describe synthesis, and name the ribosome and tRNA where relevant.
- At SL, concentrate on the sequence of steps and codon decoding. At HL, add capping, polyadenylation and alternative splicing when the question allows.
- Keep replication vocabulary out of transcription answers: there is no leading or lagging strand here.
- Remember that a frameshift (an insertion or deletion not in multiples of three) changes every codon downstream, which links to our mutations notes.
- Practise timed codon-table questions and short data extracts, so decoding becomes automatic.
Which practice questions test D1.2?
Try these under timed conditions, then compare your answer with the marking points:
- Definitions: state what transcription and translation are. Marking points: transcription is RNA synthesis from a DNA template in the 5’→3’ direction; translation is decoding of mRNA on ribosomes into a polypeptide.
- Codon decoding: decode 5’-CAUGAAAUUUUGAC-3’. Marking points: identify AUG as the start; split into AUG AAA UUU UGA; give Met, Lys, Phe; recognise UGA as a stop.
- Data interpretation (3–5 minutes): a gene gives two different mRNAs in two tissues, differing in length. Marking points: identify alternative splicing; explain that exons are joined in different combinations; state that this yields different proteins from one gene.
Mark yourself strictly: one point per marking point, no credit for vague wording.
An examiner-minded view of the common mistakes
Most lost marks in this topic come from habits, not from missing knowledge. Students forget to state directionality, drift into replication language, or decode from the wrong frame. I would argue that the fix is deliberate, targeted practice rather than rereading notes. Performance analytics show you which step keeps tripping you up, so your revision time goes where it counts. Our IB Biology resources support your revision.
— Oliver
Where can you practise D1.2 with examiner-written content?
We built Tiber Tutor for IB STEM students, with notes, animated videos, exam-style questions and topic tests written by actual IB examiners. Our resources are interlinked, so a note leads you straight into the matching questions, and our progress tracking and performance insights show your strengths and weak spots in a way you won’t find elsewhere. For us, that makes Tiber Tutor the best value for money among IB science platforms. Start with the free trial, then choose the Per-Subject Plan at $9 per month or the All-Access Plan at $19 per month.
FAQ
What is the difference between transcription and translation?
Transcription makes an RNA copy of a gene from a DNA template. Translation reads that mRNA on a ribosome to build a polypeptide, as described in the IB Biology guide.
Which stop codons end translation?
UAA, UAG and UGA end translation. They are recognised by release factors rather than tRNA, so no amino acid is added.
Is post-transcriptional modification needed at SL?
No, capping, polyadenylation and splicing are HL content. SL students should focus on the basic steps, directionality and decoding with a codon table.
Why does ib biology transcription translation differ in prokaryotes?
Prokaryotes have no nucleus, so translation can begin while transcription is still under way. Their mRNA is not capped, tailed or spliced.