Edexcel A Level (IAL) Biology -2.11-2.12 The Genetic Code- Study Notes- New Syllabus
Edexcel A Level (IAL) Biology -2.11-2.12 The Genetic Code- Study Notes- New syllabus
Edexcel A Level (IAL) Biology -2.11-2.12 The Genetic Code- Study Notes -Edexcel A level Biology – per latest Syllabus.
Key Concepts:
- 2.11 understand the nature of the genetic code (triplet code, non-overlapping and degenerate)
- 2.12 know that a gene is a sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide chain
Nature of the Genetic Code
🌱 Introduction
The genetic code is the set of instructions stored in DNA that determines the sequence of amino acids in a protein.
It acts like a language of life, where three DNA bases code for one amino acid.
Found in all living organisms → showing it is universal.
1. Triplet Code
- DNA has four nitrogen bases: A, T, G, and C.
- A combination of three consecutive bases (a triplet) codes for one amino acid.
- In mRNA, this triplet is called a codon.
🧩 Example:
DNA triplet: A–T–G
mRNA codon: A–U–G
Codes for amino acid Methionine (Start codon)
So, each amino acid = 3 bases → 1 codon → 1 amino acid
2. Non-Overlapping Code
- Each base is read only once in the sequence.
- The codons do not share bases.
- Reading frame moves 3 bases at a time.
🧩 Example:
If the sequence is AUG CCG GAA, it’s read as:
AUG → Methionine
CCG → Proline
GAA → Glutamic acid
❌ Not overlapping like AUG, UGC, GCC, etc.
This ensures accurate reading and prevents confusion during protein synthesis.
3. Degenerate Code
- The code is degenerate because most amino acids are coded by more than one codon.
- There are 64 possible codons but only 20 amino acids.
- This redundancy reduces errors — if one base changes, the amino acid may still remain the same.
👉 Example:
Leucine → coded by CUA, CUG, CUU, CUC (4 codons)
Glycine → coded by GGU, GGC, GGA, GGG
4. Start & Stop Codons
- Start codon (AUG): signals the beginning of protein synthesis (codes for Methionine).
- Stop codons (UAA, UAG, UGA): signal the end of translation; they don’t code for any amino acid.
5. Universal Code
- The same codons represent the same amino acids in all organisms.
- Example: AUG codes for Methionine in bacteria, plants, and humans alike.
This shows the common evolutionary origin of life.
🧠 Quick Recap
| Property | Meaning | Example / Importance |
|---|---|---|
| Triplet Code | 3 bases = 1 amino acid | AUG → Methionine |
| Non-overlapping | Each base read once | AUG CCG GAA → 3 codons |
| Degenerate | Many codons → same amino acid | GGU, GGA, GGC → Glycine |
| Start Codon | Begins protein synthesis | AUG |
| Stop Codon | Ends translation | UAA, UAG, UGA |
| Universal | Same for all organisms | Shows common ancestry |
✳️ Memory Trick:
“TND-SU” → Triplet, Non-overlapping, Degenerate, Start/Stop, Universal.
Just remember TND for the main three key features often asked in exams.
✅ In summary:
The genetic code is triplet, non-overlapping, and degenerate – universal to all life, ensuring accurate protein synthesis and reducing translation errors.
Gene & Its Role in Coding for Proteins
🌱 Introduction
A gene is a specific sequence of DNA bases that carries the instructions to make a particular polypeptide (protein).
Every characteristic of an organism (eye color, enzyme shape, hair texture) ultimately depends on the proteins produced by its genes.
What Is a Gene?
- A gene is a section of DNA that codes for the sequence of amino acids in a polypeptide chain.
- Each polypeptide may function as:
- an enzyme,
- a structural protein, or
- part of a complex (e.g., haemoglobin).
So,
DNA sequence → mRNA → sequence of amino acids → specific protein → specific function.
How the Gene Codes for a Polypeptide
1. DNA Base Sequence
DNA has four bases: A, T, G, C.
These bases form a triplet code → 3 bases = 1 codon = 1 amino acid.
2. Transcription
The DNA sequence of the gene is copied into mRNA in the nucleus.
mRNA then carries this genetic code to the ribosome.
3. Translation
Ribosomes read the mRNA codons.
tRNA molecules bring the matching amino acids.
Amino acids are linked by peptide bonds → forming a polypeptide chain.
4. Folding & Function
The polypeptide folds into a specific 3D shape to become a functional protein.
The shape determines its role (e.g., enzyme active site, structural function).
🧬 Example
Gene for insulin:
Contains the base sequence that codes for the exact amino acid sequence of the insulin polypeptide.
If one base changes (mutation), a different amino acid may form → altering the protein’s function.
🧩 Relationship Between DNA, Genes & Proteins
| Level | Structure | Function |
|---|---|---|
| DNA | Sequence of bases | Stores genetic information |
| Gene | Section of DNA | Codes for one polypeptide |
| mRNA | Copy of gene | Carries code to ribosome |
| Protein | Chain of amino acids | Performs cellular functions |
💡 Key Idea
One gene = one polypeptide
(Some proteins have several polypeptide chains, each coded by a different gene.)
🧠 Quick Recap
| Concept | Explanation |
|---|---|
| Gene | A sequence of DNA bases that codes for a polypeptide |
| Polypeptide | A chain of amino acids joined by peptide bonds |
| Triplet code | 3 DNA bases = 1 amino acid |
| Transcription | DNA → mRNA (in nucleus) |
| Translation | mRNA → protein (in ribosome) |
| Outcome | Protein formed determines cell structure & function |
In short: A gene is a coded message on DNA that tells the cell how to make a specific protein.
The base sequence decides the amino acid sequence, which in turn decides the shape and function of the protein.