AHL: Transcription and translation extensions
IB Biology HLΒ· Theme D.7Β· 15 min read
1. Post-transcriptional modificationβ β βββHL onlyβ± 5 min
Post-transcriptional modification
Set of processing steps that convert precursor mRNA (pre-mRNA) to mature mRNA in eukaryotic nuclei
Example:
Includes 5' capping, 3' polyadenylation, and RNA splicing
Unlike prokaryotes, which transcribe and translate mRNA simultaneously, eukaryotic pre-mRNA requires processing before translation. This increases mRNA stability and enables alternative splicing to generate multiple proteins from one gene.
5' capping: A modified guanine nucleotide is added to the 5' end, protecting mRNA from degradation and aiding ribosome binding.
3' polyadenylation: A 50-200 adenine nucleotide tail is added to the 3' end, increasing stability and aiding cytoplasmic export.
RNA splicing: Non-coding introns are removed, coding exons are spliced together to form mature mRNA.
A eukaryotic gene has 4 exons (150bp, 220bp, 180bp, 90bp) and 3 introns (300bp, 410bp, 250bp). What is the length of mature mRNA excluding the cap and poly(A) tail?
- 1
Only exons are retained after splicing, introns are removed.
- 2
Sum the lengths of all exons: 150 + 220 + 180 + 90 = 640
- 3
The mature mRNA length is 640 base pairs.
2. Nucleosome regulation of transcriptionβ β β ββHL onlyβ± 4 min
Nucleosomes consist of DNA wrapped around histone proteins. Modification of histone tails changes how tightly DNA is packaged, which determines if transcription factors can access promoter regions to initiate transcription.
Histone acetylation
Addition of acetyl groups to histone tails that loosens chromatin structure, activating transcription
Example:
Associated with actively expressed genes in euchromatin
Predict the effect of increased methylation of the promoter region of a tumor suppressor gene. Explain your answer.
- 1
DNA methylation adds methyl groups to cytosine bases in promoter regions, tightening chromatin structure.
- 2
Tighter packing prevents transcription factors from binding to the promoter.
- 3
Outcome: Transcription of the tumor suppressor gene is repressed.
- 4
Reduced tumor suppressor protein production leads to uncontrolled cell division, a common driver of cancer.
3. Translation and protein targetingβ β β ββHL onlyβ± 4 min
All translation initiates on free ribosomes in the cytosol. Proteins destined for secretion, the endomembrane system, or lysosomes have a signal sequence that targets the ribosome to the endoplasmic reticulum (ER) mid-translation.
Signal peptide
Short 15-30 amino acid sequence at the N-terminus of a polypeptide that targets the ribosome to the ER
Outline the path of a ribosome translating a protein destined for secretion from a eukaryotic cell.
- 1
Translation initiates on a free ribosome in the cytosol.
- 2
The first 15-30 amino acids form the signal peptide sequence.
- 3
A signal recognition particle (SRP) binds the signal peptide and moves the ribosome to an ER membrane receptor.
- 4
Translation continues, with the growing polypeptide entering the ER lumen through a channel.
- 5
The signal peptide is cleaved, and the protein is transported to the Golgi for packaging and secretion.
4. Alternative splicingβ β β β βHL onlyβ± 3 min
Alternative splicing is a regulated post-transcriptional process where different combinations of exons are included or excluded from the final mature mRNA. This generates multiple distinct proteins from a single gene, increasing proteome diversity far beyond the number of genes in the genome.
A pre-mRNA has three exons: E1, E2, E3. List three possible mature mRNA products from alternative splicing.
- 1
Alternative splicing can skip one or more exons to produce different combinations.
- 2
Possible products: 1) E1-E2-E3 (all exons included), 2) E1-E3 (E2 skipped), 3) E1-E2 (E3 skipped)
- 3
Each product is translated into a unique polypeptide with a distinct structure and function.
5. Common Pitfalls
Wrong move:
Claiming prokaryotes perform post-transcriptional modification
Why:
Prokaryotes do not have a nucleus and transcribe/translate simultaneously. They do not process pre-mRNA as described in the IB syllabus.
Correct move:
Post-transcriptional modification is exclusively a eukaryotic process for IB Biology exams.
Wrong move:
Including introns in the length of mature mRNA
Why:
Students often mix up the roles of introns and exons in splicing.
Correct move:
Remember the mnemonic: Exons are expressed, Introns are intervening and removed.
Wrong move:
Stating translation of secreted proteins starts on bound ribosomes
Why:
Translation initiation always occurs in the cytosol, targeting to the ER happens mid-translation.
Correct move:
All translation begins on free ribosomes; only proteins with a signal peptide are targeted to the ER.
Wrong move:
Claiming histone acetylation represses transcription
Why:
Students confuse the effect of acetylation and methylation on chromatin structure.
Correct move:
Acetylation loosens chromatin and activates transcription; methylation usually tightens chromatin and represses transcription.
6. Quick Reference Cheatsheet
Process | Location | Key Outcome |
|---|---|---|
5' capping | Eukaryotic nucleus | Protects mRNA, aids ribosome binding |
Poly(A) tail | Eukaryotic nucleus | Increases mRNA stability |
RNA splicing | Eukaryotic nucleus | Removes introns, joins exons |
Histone acetylation | Chromatin | Loosens DNA, activates transcription |
DNA methylation | Promoter | Tightens DNA, represses transcription |
Free ribosome translation | Cytosol | Makes cytosolic/organelle proteins |
Bound ribosome translation | ER | Makes secreted/endomembrane proteins |
Alternative splicing | Eukaryotic nucleus | One gene β multiple proteins |
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2025 Β· 1
Outline RNA splicing
- 2023 Β· 2
Compare prokaryotic/eukaryotic translation
- 2022 Β· 1
Explain post-transcriptional modification
Going deeper
What's Next
This sub-topic extends basic protein synthesis concepts to eukaryotic-specific processes, which are core to understanding cell differentiation, cancer development, and epigenetic inheritance. Nucleosome modification and transcriptional regulation introduced here form the foundation of modern genetics and biotechnology applications. Mastery of these concepts is essential for higher-level questions on gene expression in IB Biology HL exams.