4.2.6Additional lessons
Post-Transcriptional Modification — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 35 of 101
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5From Pre-mRNA to Mature mRNA
In eukaryotes the first RNA copy is not ready for translation.
Introns
- Introns These are non-coding sequences inside a gene that are transcribed but removed before translation.
Exons
- Exons These are the coding sequences that remain in the mature mRNA and are expressed as amino acids.
Splicing
- Splicing Enzymes remove introns and join the exons together to form a continuous coding message.
Mature mRNA
- Mature mRNA The finished transcript leaves the nucleus through a nuclear pore for translation in the cytoplasm.
| Feature | Prokaryote | Eukaryote |
|---|---|---|
| Introns in genes | Rare | Common |
| Processing of RNA | Little or none | Splicing before export |
| Transcription and translation | Can be simultaneous | Separated by nuclear envelope |
Part 2 of 5Steps of Processing
1
RNA polymerase produces a long primary transcript, also called pre-mRNA, containing both introns and exons.
2
A protective cap is added to the 5' end and a tail of adenine nucleotides is added to the 3' end, helping stability and export.
3
The spliceosome, made of RNA and protein, cuts out each intron and joins the neighbouring exons.
4
The mature mRNA is checked, then exported to the cytoplasm where ribosomes bind to it.
Part 3 of 5Alternative Splicing
Alternative Splicing
Definition
- Definition Different combinations of exons are joined from the same pre-mRNA, so one gene can give rise to several mRNAs.
Result
- Result Each mRNA can code for a slightly different polypeptide, so a cell can make more proteins than it has genes.
Proteome size
- Proteome size This helps explain why the human proteome is much larger than the roughly twenty thousand human genes.
Control
- Control Different cell types splice the same transcript differently, giving tissue specific forms of a protein.
Part 4 of 5Worked Example
Question
- A pre-mRNA has five exons.
- In one tissue, exon 3 is left out.
- How many exons are in the mature mRNA and why might the protein differ?
- Starting number of exons is 5; omitting one gives 5 − 1 = 4 exons.
- Removing the region coded by exon 3 removes a stretch of amino acids from the polypeptide.
- The shorter polypeptide may fold differently or lack a domain, so its function can change.
Answer
- Four exons;
- the resulting protein isoform has a different structure and possibly different function.
Part 5 of 5Exam Checklist & Tips
4.2.6 – Key Points for the Exam
- Intron Non-coding region removed from pre-mRNA.
- Exon Region of a gene that is retained in mature mRNA and codes for protein.
- Splicing Removal of introns and joining of exons.
- Pre-mRNA The primary transcript before processing.
- Alternative splicing Joining of different exon combinations to make different mRNAs from one gene.
Common Mistakes
- Believing that introns are removed from DNA; they remain in the gene and are removed from the RNA.
- Saying that exons are the parts that are not expressed; the opposite is true.
- Assuming bacteria splice most of their genes, whereas they rarely have introns.
- Stating that alternative splicing changes the DNA sequence;
- it only changes which exons are joined in the RNA.
Last updated:
\n