This MCQ module is based on: Genetic Code Translation
TOPIC 22 OF 64
Genetic Code Translation
🎓 Class 12
Biology
CBSE
Theory
Ch 5 – Molecular Basis of Inheritance
⏱ ~14 min
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Genetic Code Translation
6.5 The Genetic Code — Three-Letter Words of Life
The information in mRNA is read in groups of three nucleotides called codons. Each codon specifies one amino acid (or a stop signal). With 4 bases (A, U, G, C) and 3 positions per codon, there are 4³ = 64 possible codons.
6.5.1 Salient Features of the Genetic Code
- Triplet code: Three bases code for one amino acid. (Why not 2? 4²=16, too few. Why not 4? Wasteful.)
- Universal: the same code applies to almost all organisms — bacteria, plants, humans, viruses (with rare exceptions in mitochondria and a few protozoa).
- Degenerate: most amino acids are coded by more than one codon (synonymous codons). E.g., Leucine: UUA, UUG, CUU, CUC, CUA, CUG.
- Non-overlapping: codons are read sequentially without overlap. Each base belongs to only one codon.
- Comma-less: no punctuation between codons; reading proceeds continuously.
- Specific: each codon codes for only ONE amino acid (no ambiguity).
- Has start and stop signals: AUG = start (Met); UAA, UAG, UGA = stop.
6.5.2 Codon Table
| 1st base ↓ | 2nd base → | 3rd ↓ | |||
|---|---|---|---|---|---|
| U | C | A | G | ||
| U | UUU Phe | UCU Ser | UAU Tyr | UGU Cys | U |
| UUC Phe | UCC Ser | UAC Tyr | UGC Cys | C | |
| UUA Leu | UCA Ser | UAA Stop | UGA Stop | A | |
| UUG Leu | UCG Ser | UAG Stop | UGG Trp | G | |
| C | CUU Leu | CCU Pro | CAU His | CGU Arg | U |
| CUC Leu | CCC Pro | CAC His | CGC Arg | C | |
| CUA Leu | CCA Pro | CAA Gln | CGA Arg | A | |
| CUG Leu | CCG Pro | CAG Gln | CGG Arg | G | |
| A | AUU Ile | ACU Thr | AAU Asn | AGU Ser | U |
| AUC Ile | ACC Thr | AAC Asn | AGC Ser | C | |
| AUA Ile | ACA Thr | AAA Lys | AGA Arg | A | |
| AUG Met (start) | ACG Thr | AAG Lys | AGG Arg | G | |
| G | GUU Val | GCU Ala | GAU Asp | GGU Gly | U |
| GUC Val | GCC Ala | GAC Asp | GGC Gly | C | |
| GUA Val | GCA Ala | GAA Glu | GGA Gly | A | |
| GUG Val | GCG Ala | GAG Glu | GGG Gly | G | |
This codon table was painstakingly cracked by Marshall Nirenberg, Har Gobind Khurana (Indian-American), Severo Ochoa and others between 1961–66 — earning Nirenberg, Khurana and Holley the Nobel Prize in 1968.
6.6 Translation — mRNA to Protein
Translation is the process where the message in mRNA is decoded by the ribosome to synthesize a protein. Three components are essential: mRNA, ribosome, and tRNA charged with amino acids.
6.6.1 The tRNA — Adapter Molecule
Transfer RNA acts as the physical link between codon (mRNA) and amino acid. Each tRNA:
- Has a clover-leaf secondary structure (L-shaped in 3D).
- Carries a specific amino acid at its 3' end (CCA tail).
- Has an anticodon — three bases complementary to the mRNA codon — at the bottom loop.
- Is "charged" by enzymes called aminoacyl-tRNA synthetases — one for each amino acid.
6.6.2 The Ribosome
The ribosome is the cellular machine that performs translation. It has two subunits:
| Prokaryotic (70S) | Eukaryotic (80S) | |
|---|---|---|
| Large subunit | 50S (23S + 5S rRNA + 31 proteins) | 60S (28S + 5.8S + 5S rRNA + 49 proteins) |
| Small subunit | 30S (16S rRNA + 21 proteins) | 40S (18S rRNA + 33 proteins) |
| Sites | A (acceptor), P (peptidyl), E (exit) | Same: A, P, E sites |
The peptidyl transferase activity that forms peptide bonds is performed by the rRNA itself — making the ribosome a ribozyme (RNA enzyme).
6.6.3 Steps of Translation
1. Initiation: The small ribosomal subunit binds mRNA at the start codon (AUG). Initiator tRNA (carrying Met) base-pairs with AUG. Large subunit joins to form complete ribosome.
2. Elongation: The cycle repeats:
- A new charged tRNA enters the A site; its anticodon pairs with the next mRNA codon.
- A peptide bond forms between the amino acid in the A site and the growing peptide chain in the P site (catalysed by peptidyl transferase / rRNA).
- Ribosome moves one codon (translocation): the A-site tRNA shifts to P; the empty P-site tRNA moves to E and exits.
3. Termination: When a stop codon (UAA / UAG / UGA) reaches the A site, no tRNA matches. Release factors bind, the polypeptide is released, ribosome dissociates.
🧬 Interactive: Codon → Amino Acid Translator
Enter an mRNA sequence (5'→3') and see the protein it codes for:
📐 Activity 6.3 — Decode the Message
Setup: An mRNA reads: 5'-AUGCUUCAGUUUUAA-3'
Predict: (a) How many codons? (b) What protein does this make? (c) What happens if the third base of the second codon (U) is mutated to G?
(a) Codons: AUG-CUU-CAG-UUU-UAA = 5 codons (one is stop).
(b) Met-Leu-Gln-Phe-STOP = 4 amino acid protein: Met-Leu-Gln-Phe.
(c) CUU → CUG. CUG also codes for Leu! So the protein is unchanged — this is a silent mutation, made possible by the degeneracy of the genetic code.
Worked Examples
Worked Example 1: mRNA → Protein
Translate the mRNA: 5'-AUGGCAUCCUAA-3'.
Read codons starting from AUG:
AUG = Met (Methionine) — start
GCA = Ala (Alanine)
UCC = Ser (Serine)
UAA = STOP
Protein: Met–Ala–Ser (3 amino acids long; STOP is not translated as an amino acid).
AUG = Met (Methionine) — start
GCA = Ala (Alanine)
UCC = Ser (Serine)
UAA = STOP
Protein: Met–Ala–Ser (3 amino acids long; STOP is not translated as an amino acid).
Worked Example 2: Effect of Mutation
The mRNA AUG-GAA-UCG codes for Met-Glu-Ser. What happens if a mutation changes GAA → GUA in the second codon?
Original: AUG-GAA-UCG → Met-Glu-Ser
Mutated: AUG-GUA-UCG → Met-Val-Ser
Type: Missense mutation — one amino acid replaced by another. The mutation A→U at position 2 of the second codon swapped Glu (acidic) for Val (non-polar).
Real-world example: This is exactly the mutation in sickle cell anaemia — Glu→Val substitution on β-globin position 6. A single base mutation has dramatic consequences.
Mutated: AUG-GUA-UCG → Met-Val-Ser
Type: Missense mutation — one amino acid replaced by another. The mutation A→U at position 2 of the second codon swapped Glu (acidic) for Val (non-polar).
Real-world example: This is exactly the mutation in sickle cell anaemia — Glu→Val substitution on β-globin position 6. A single base mutation has dramatic consequences.
Worked Example 3: Reading Frame
Compare the protein from these two mRNAs: (i) AUG-GCA-UUC-UAA and (ii) AUG-GCA-UUC-UAA after a single insertion of "G" between the start codon and second codon (AUG-GGC-AUU-CUA-A). Which kind of mutation is this?
Original: AUG-GCA-UUC-UAA → Met-Ala-Phe-STOP (3 amino acids).
After "G" insertion: AUG-GGC-AUU-CUA-A → Met-Gly-Ile-Leu-... (reading frame entirely changed).
Type: Frameshift mutation. A single base insertion shifts the reading frame for ALL downstream codons. This usually creates a completely different (often non-functional) protein, OR a premature stop codon.
Why it matters: Insertions/deletions of multiples of 3 do NOT cause frameshift; only ±1 or ±2 bases do. Frameshifts often cause genetic disease (e.g., some forms of Duchenne muscular dystrophy).
After "G" insertion: AUG-GGC-AUU-CUA-A → Met-Gly-Ile-Leu-... (reading frame entirely changed).
Type: Frameshift mutation. A single base insertion shifts the reading frame for ALL downstream codons. This usually creates a completely different (often non-functional) protein, OR a premature stop codon.
Why it matters: Insertions/deletions of multiples of 3 do NOT cause frameshift; only ±1 or ±2 bases do. Frameshifts often cause genetic disease (e.g., some forms of Duchenne muscular dystrophy).
🎯 Competency-Based Questions
Q1. Total number of codons in the standard genetic code is:L1 Remember
Answer: (b) 64. 4 bases (A, U, G, C) at each of 3 codon positions = 4³ = 64 codons. Of these, 61 code for 20 amino acids (degeneracy!) and 3 are stop codons (UAA, UAG, UGA).
Q2. Fill in the blank: The codon AUG codes for the amino acid _____ and also serves as the _____ codon. L2 Understand
Answer: Methionine; start. AUG has a dual role: it specifies methionine (Met) when found internally in a gene, and serves as the start codon to initiate translation. Every protein begins with methionine, though it is often removed after translation.
Q3. The mRNA sequence 5'-AUGUCGAAGUAA-3' codes for which protein? L3 Apply
Codons: AUG-UCG-AAG-UAA
AUG → Met (start)
UCG → Ser
AAG → Lys
UAA → STOP
Protein: Met–Ser–Lys (a tripeptide).
AUG → Met (start)
UCG → Ser
AAG → Lys
UAA → STOP
Protein: Met–Ser–Lys (a tripeptide).
Q4. Analyse: Why is the genetic code described as 'degenerate but unambiguous'? Give one biological advantage. L4 Analyse
Degenerate but unambiguous:
- Degenerate: Most amino acids are coded by multiple codons (Leu has 6, Ser has 6). 64 codons → 20 amino acids means redundancy.
- Unambiguous: Each codon codes for exactly ONE amino acid (no codon codes for two different amino acids).
- Mutation buffer: Many point mutations at the third codon position don't change the amino acid (silent mutations). E.g., GCU, GCC, GCA, GCG all code for Alanine.
- This protects the protein from harmful effects of common DNA replication errors.
- Reduces the impact of UV/radiation-induced mutations.
Q5. HOT (Create): Predict the consequences of a mutation that changes a stop codon (UAA) to a coding codon (CAA, glutamine). L6 Create
Effects of stop → coding mutation (read-through mutation):
- Translation continues past the original stop: The ribosome reads the next codon (in 3' UTR) instead of stopping. This produces an extended protein.
- Eventually translation stops at the next in-frame stop codon (in the 3' UTR).
- The extra C-terminal amino acids may:
- Disrupt protein folding → reduced/no function.
- Add a target sequence (e.g., for degradation) → instability.
- Add a signal that mislocalizes the protein.
- mRNA stability may decrease: Read-through can trigger non-stop decay pathway.
- Disease example: α-thalassemia is sometimes caused by stop-codon read-through producing extended haemoglobin chains that don't assemble correctly.
🧠 Assertion–Reason Questions
Choose: (A) Both true, R explains A. (B) Both true, R doesn't explain A. (C) A true, R false. (D) A false, R true.
A: Each amino acid is specified by exactly one codon.
R: The genetic code has 64 codons.
Answer: (D). A is FALSE — most amino acids are coded by MULTIPLE codons (degeneracy). R is TRUE — there are 64 codons total. The codon-to-amino-acid mapping is many-to-one (e.g., 6 codons → Leu).
A: Ribosomes are called ribozymes.
R: The peptidyl transferase activity is catalysed by rRNA, not by protein.
Answer: (A). Both true; R explains A. The catalytic role of rRNA was a major discovery — it shows that RNA can have enzymatic function (supporting the RNA World hypothesis).
A: The genetic code is universal across all life forms.
R: All organisms share a common evolutionary ancestor.
Answer: (A). Both true; R explains A. The fact that AUG codes for Met in bacteria, plants, fungi, and humans alike is powerful evidence for common descent. (A few exceptions exist in mitochondria and certain protozoa, but they are minor variations.)
Frequently Asked Questions - Genetic Code Translation
What is the main concept covered in Genetic Code Translation?
In NCERT Class 12 Biology Chapter on Molecular Basis of Inheritance, "Genetic Code Translation" covers the core biological structures, processes, and pathways students need for board exam success. The MyAiSchool lesson explains the topic with definitions, labelled diagrams, comparison tables, and interactive simulations. Scientific terminology and physiological/genetic significance are highlighted throughout to build conceptual depth aligned with CBSE 2025-26 syllabus.
How is Genetic Code Translation useful in real-life or applied biology?
Real-life applications of "Genetic Code Translation" from NCERT Class 12 Biology Molecular Basis of Inheritance include medical diagnostics, agriculture, biotechnology, public health, evolutionary insights, and ecological monitoring. The MyAiSchool lesson links every biological concept to a tangible application so students see biology as a problem-solving framework for living systems and real-world challenges.
What are the key terms students should memorize for Genetic Code Translation?
Key terms in "Genetic Code Translation" (NCERT Class 12 Biology Molecular Basis of Inheritance) are tabulated in the MyAiSchool key-terms grid. Students should memorize each term with its precise definition, function, and example. Terminology is high-yield in CBSE board exams — 1-mark MCQs and 2-mark short answers test definitions directly. The Summary section provides a printable quick-reference card.
How does this part connect to other parts of the chapter?
NCERT Class 12 Biology Molecular Basis of Inheritance is structured so each part builds biological understanding sequentially. "Genetic Code Translation" connects to neighbouring parts via shared mechanisms, structural hierarchies, and physiological processes. The MyAiSchool lesson cross-references related concepts with internal links so students can navigate the whole chapter as one connected biological story rather than disconnected fragments.
What types of CBSE board questions come from Genetic Code Translation?
CBSE board questions from "Genetic Code Translation" typically include: (1) 1-mark MCQs on definitions and processes, (2) 2-mark short-answer differences/comparisons, (3) 3-mark labelled-diagram questions, (4) 5-mark long-answer essays combining mechanism + diagram + significance. The MyAiSchool lesson tags each Competency-Based Question (CBQ) with Bloom level (L1-L6) so students know how to study for each weight.
How can students use the interactive simulation effectively?
The interactive simulation in the "Genetic Code Translation" lesson allows students to explore biological processes, classifications, or pathways using selectors and sliders, with live visual feedback. To use it effectively: (1) explore each option/state, (2) compare with textbook diagrams, (3) note the function/outcome changes, (4) try the integrated practice quiz. The simulation reinforces visual-spatial understanding that pure text-based study cannot.
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