Edexcel A Level (IAL) Biology -2.13 Transcription & Translation- Study Notes- New Syllabus
Edexcel A Level (IAL) Biology -2.13 Transcription & Translation- Study Notes- New syllabus
Edexcel A Level (IAL) Biology -2.13 Transcription & Translation- Study Notes -Edexcel A level Biology – per latest Syllabus.
Key Concepts:
- 2.13 (i) understand the process of protein synthesis (transcription and translation), including the role of RNA polymerase, translation, messenger RNA, transfer RNA, ribosomes and the role of start and stop codons
(ii) understand the roles of the DNA template (antisense) strand in transcription, codons on messenger RNA and anticodons on transfer RNA
Protein Synthesis – Transcription & Translation
🌱 Introduction
Protein synthesis is the process by which cells make proteins using the genetic information stored in DNA.
It happens in two main stages:
- Transcription – making an mRNA copy of the DNA code.
- Translation – using that mRNA code to build a polypeptide chain.
🧩 Transcription – Making mRNA from DNA
🔹 Where it occurs: Nucleus
🔹 Main idea: DNA → mRNA
🧠 Step-by-Step Process
- DNA unzips: RNA polymerase binds to a specific region (promoter) and unwinds the DNA helix.
- Template strand used: Only the antisense strand is copied. The sense strand has the same sequence as mRNA (T → U).
- mRNA formation: Free RNA nucleotides line up complementary to DNA antisense strand:
- A → U
- T → A
- C → G
- G → C
- Joining of nucleotides: RNA polymerase links nucleotides → single-stranded mRNA.
- Completion: Transcription ends at stop codon. mRNA detaches → exits nucleus to ribosome.
🧬 Key Terms in Transcription
| Term | Description |
|---|---|
| RNA polymerase | Enzyme that catalyzes mRNA formation |
| Antisense strand | DNA strand used as template for mRNA |
| Sense strand | Complementary DNA strand, same sequence as mRNA |
| mRNA | Messenger RNA carrying code to ribosome |
| Start codon (AUG) | Signals start of translation; codes for methionine |
| Stop codon (UAA, UAG, UGA) | Signals end of translation |
⚙️ Translation – Making Polypeptide from mRNA
🔹 Where it occurs: Cytoplasm (ribosomes)
🔹 Main idea: mRNA → amino acid chain (protein)
🧠 Step-by-Step Process
- mRNA attaches to ribosome: Ribosome reads codons (3-base sequences).
- tRNA brings amino acids: Each tRNA has a specific amino acid and an anticodon complementary to mRNA codon.
- Codon–anticodon pairing: Anticodon pairs with mRNA codon (A-U, G-C).
- Peptide bond formation: Ribosome links amino acids → growing polypeptide chain.
- Movement along mRNA: Ribosome moves codon by codon until stop codon is reached.
- Termination: Polypeptide released → folds into 3D structure → functional protein.
🧩 Codons & Anticodons – Key Relationships
| Molecule | Base Triplet | Function |
|---|---|---|
| DNA (template/antisense) | TAC | Used to make mRNA |
| mRNA codon | AUG | Codes for amino acid |
| tRNA anticodon | UAC | Pairs with mRNA codon |
| Amino acid | Methionine | Added to polypeptide chain |
🧠 Start & Stop Codons
| Codon | Type | Function |
|---|---|---|
| AUG | Start codon | Begins translation, codes for methionine |
| UAA, UAG, UGA | Stop codons | End translation, release polypeptide |
💡 Summary Table
| Stage | Location | Key Enzyme / Molecule | Product |
|---|---|---|---|
| Transcription | Nucleus | RNA polymerase | mRNA |
| Translation | Cytoplasm (ribosome) | Ribosome, tRNA | Polypeptide (protein) |
📦 Quick Recap
| Concept | Key Point |
|---|---|
| Transcription | DNA → mRNA using RNA polymerase |
| Antisense strand | Template for mRNA synthesis |
| Translation | mRNA → amino acid chain at ribosomes |
| mRNA | Carries genetic code from DNA |
| tRNA | Brings amino acids to ribosome |
| Codon | 3-base triplet on mRNA |
| Anticodon | 3-base triplet on tRNA complementary to codon |
| Start codon | AUG (signals where translation begins) |
| Stop codon | UAA, UAG, UGA (signals translation ends) |
In short: DNA acts as the master code. Through transcription, it makes mRNA; through translation, mRNA is decoded into a protein.
This flow from gene → protein is called the central dogma of molecular biology.