Q.Why is the genetic code necessarily a triplet code, and not a code based on single bases or on pairs of bases?
Concept understanding — The Genetic Code and Its Discovery
The Genetic Code and Its Discovery
The genetic code is the set of rules by which the base sequence of mRNA is translated into the amino-acid sequence of a protein. It is read in triplets (codons) of three nucleotides; with four bases there are 43=64 codons specifying 20 amino acids plus start/stop signals. The code is degenerate, non-overlapping, comma-less, nearly universal and has a defined start (AUG) and stop codons.
Its elucidation involved several scientists, frequently matched in examinations:
- George Gamow — proposed that the code is made of three nucleotides.
- Marshall Nirenberg (with Matthaei) — cracked the code using a cell-free system for protein synthesis.
- Har Gobind Khorana — chemically synthesised RNA with defined repeating sequences to assign codons.
- Severo Ochoa — discovered polynucleotide phosphorylase, the enzyme used to make synthetic RNA.
Pairing each worker with their specific contribution to the genetic code is the usual requirement.
The genetic code and the scientists who deciphered it are covered in the NCERT Class 12 Biology chapter on Molecular Basis of Inheritance, and "genetic code properties and discovery" along with "Nirenberg Khorana Ochoa important questions" are heavily searched ahead of CBSE boards and NEET biology. Pairing each scientist with their specific contribution, as explained above, is one of the most frequently repeated NEET molecular biology questions grounded in this NCERT Class 12 chapter.
Single bases give only 4 combinations and pairs give only 16 — both too few for 20 amino acids; triplets give 4x4x4 = 64 combinations, comfortably enough.
A triplet code is necessary because only groups of three bases (64 combinations) provide more than enough distinct codes to uniquely specify all 20 amino acids, whereas single bases (4) or pairs (16) do not.
Step 1. There are only 4 different nucleotide bases available in mRNA, but 20 different amino acids must each be uniquely specified.
Step 2. A code based on single bases could specify only 4 possible combinations — far too few for 20 amino acids.
Step 3. A code based on pairs of bases could specify 4×4 = 16 possible combinations — still short of 20.
Step 4. A code based on triplets (groups of three consecutive bases) gives 4×4×4 = 64 possible combinations — comfortably more than the 20 amino acids that must be encoded, with room left over for redundancy and stop signals.
The genetic code must be a triplet code because only 4^3 = 64 combinations give enough distinct codes to specify all 20 amino acids; 4^1=4 and 4^2=16 are both insufficient.
Work through the simple arithmetic (4^1, 4^2, 4^3) to see exactly why triplets are the minimum group size that clears 20.
- Forgetting to actually compute 4×4×4=64 and just asserting 'triplets are enough' without showing why smaller groupings fail.
Showing the 12 most recent of 18 on this concept.
- CBSE 2025Set ANNUAL1 markQ.Write the name of amino acid coded by genetic code "UUU".
›Reveal solutionSolution
In the standard genetic code (deciphered by Nirenberg and colleagues), the mRNA codon UUU specifies the amino acid phenylalanine.
The genetic code is a triplet code where each set of three mRNA bases (a codon) specifies one amino acid. Early work using synthetic RNA (e.g., poly-U, made only of uracil) showed that a poly-U mRNA directed the synthesis of a polypeptide made entirely of phenylalanine, establishing that the codon UUU codes for phenylalanine — one of the first codons to be deciphered.
✓Final answerPhenylalanine.
- CBSE 2025Set ANNUAL1 markQ.Write the anticodon of 5' AUG 3'.
›Reveal solutionSolution
The anticodon of the codon 5'-AUG-3' is 3'-UAC-5' (i.e.
Codon 5'-AUG-3' pairs with anticodon 3'-UAC-5'.
During translation, the anticodon loop of a tRNA molecule base-pairs with the mRNA codon in an antiparallel orientation (A pairs with U, G pairs with C, following Watson–Crick complementarity, with the codon read 5'→3' and the anticodon read 3'→5' against it). For codon 5'-A U G-3', the complementary antiparallel anticodon is 3'-U A C-5'. This tRNA (carrying methionine, since AUG is the start/methionine codon) is the initiator tRNA that begins every polypeptide chain in translation.
✓Final answer3'-UAC-5' (equivalently CAU written 5'→3').
- CBSE 2025Set ANNUAL1 markMCQQ.Initiation codon is :(a) AUG(b) UUU(c) UAG(d) UGA
›Reveal solutionSolution
AUG is the universal start codon (coding for methionine) that initiates translation; UAG and UGA are stop codons, and UUU codes for phenylalanine.
Working
During translation, the ribosome must begin reading the mRNA in the correct frame, and it does so by recognising a specific initiation codon:
- AUG: this is the near-universal start codon. It codes for the amino acid methionine in eukaryotes (or a modified form, N-formylmethionine, in bacteria), and it is recognised by a special initiator tRNA that, together with initiation factors and the small ribosomal subunit, sets up the correct reading frame at the very first codon of the coding sequence.
- UAA, UAG, UGA: these three codons do not code for any amino acid; instead, they are the three stop (termination) codons, which signal the ribosome to release the completed polypeptide and stop translation. UAG (nicknamed 'amber') and UGA (nicknamed 'opal') among the options given are both termination signals, not initiation signals.
- UUU: this codon codes for the amino acid phenylalanine during elongation; it plays no special role in initiation.
So, of the codons listed, AUG is the one that functions as the initiation codon.
✓Final answerThe correct option is (a): AUG
- CBSE 2025Set ANNUAL1 markMCQQ.The first codon to be deciphered was ________ which codes for ________.(a) UUU, Phenylalanine(b) AAA, Proline(c) TTT, Arginine(d) GGG, Alanine
›Reveal solutionSolution
The genetic code was first cracked using synthetic poly-U mRNA, showing that the codon UUU specifies phenylalanine.
In 1961, Marshall Nirenberg and Heinrich Matthaei used a cell-free (in vitro) protein-synthesising system and added a synthetic RNA made of only uracil bases (poly-U, i.e., a repeating sequence of UUU codons). This directed the synthesis of a polypeptide made entirely of the amino acid phenylalanine, revealing that the codon UUU codes for phenylalanine -- the very first codon assignment in the genetic code to be experimentally established. This landmark experiment opened the way for Nirenberg, Khorana, and others to decipher the rest of the 64 codons of the genetic code over the following years.
✓Final answerThe correct option is a) UUU, Phenylalanine -- the first codon deciphered, using synthetic poly-U mRNA.
- CBSE 2024Set 57/1/11 markMCQQ.You know that there are twenty different types of naturally occurring amino acids and four different types of bases in the DNA. A combination of 3 such bases code for a specific amino acid. If instead there are 96 different amino acids and 12 different bases in the DNA, then the minimum number of combination of bases required to form a codon is: (A) 6 (B) 8 (C) 2 (D) 4
›Reveal solutionSolution
To code for 96 different amino acids using 12 types of bases, the minimum number of bases required in a codon is 2, because 12² = 144 unique codons are possible, which is sufficient.
Concept and Intuition
In genetics, a "codon" is a sequence of bases (nucleotides) in DNA or RNA that codes for a specific amino acid. The fundamental idea is that each unique amino acid must have at least one unique codon assigned to it. This means the total number of possible unique codons must be greater than or equal to the total number of different amino acids that need to be coded for.
Imagine you have B different types of bases. If a codon is formed by a sequence of n bases, then for each position in the codon, you have B choices.
For example, if a codon has 1 base (n=1), you can form B¹ = B unique codons.
If a codon has 2 bases (n=2), you have B choices for the first position and B choices for the second position, leading to B times B = B² unique codons.
In general, for a codon of n bases, there are B^n possible unique combinations.
The number of unique codons possible from B types of bases, with each codon having n bases, is given by:
Number of codons = B^n
We need to find the smallest integer n such that B^n is sufficient to code for all given amino acids. This translates to the inequality:
B^n ge Number of Amino Acids
Step-by-step Solution
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Understand the current biological context (given information):
The problem first provides context from naturally occurring DNA:
- Number of different types of amino acids = 20
- Number of different types of bases in DNA = 4 (A, T, C, G)
- Number of bases in a codon = 3
Let's verify this with our concept. If there are 4 bases and 3 bases per codon, the total number of unique codons possible is 4³ = 4 times 4 times 4 = 64. Since 64 ge 20, this number of codons is indeed sufficient to code for all 20 naturally occurring amino acids. This confirms our understanding of the relationship between bases, codon length, and amino acids.
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Identify the parameters for the new scenario:
The problem then asks us to consider a hypothetical situation:
- New number of different types of amino acids = 96
- New number of different types of bases in DNA = 12
- We need to find the minimum number of bases required to form a codon, let's call this n.
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Set up the inequality:
Using the formula from the concept, the number of unique codons possible with 12 bases and n bases per codon is 12^n. This number must be at least equal to the number of amino acids that need to be coded for.
Therefore, we must have:
12^n ge 96
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Solve for the minimum value of n:
We need to find the smallest integer n that satisfies the inequality 12^n ge 96.
- Try n=1: 12¹ = 12. Since 12 < 96, a codon with only 1 base is not sufficient to code for 96 different amino acids.
- Try n=2: 12² = 12 times 12 = 144. Since 144 ge 96, a codon with 2 bases is sufficient to code for 96 different amino acids.
Since n=1 is not sufficient and n=2 is sufficient, the minimum number of bases required to form a codon is 2.
✓Final answerThe minimum number of combination of bases required to form a codon is 2.
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- CBSE 2024Set D1 markMCQQ.In sickle cell anaemia glutamic acid is replaced by valine. Which one of the following triplet code is for valine?(a) GGG(b) AAG(c) GAA(d) GUG
›Reveal solutionSolution
The valine codon involved in sickle-cell haemoglobin is GUG, arising from a single-base substitution (A→U) in the glutamic acid codon GAG.
Haemoglobin is built from two α and two β globin polypeptide chains. Normal adult haemoglobin (HbA) has glutamic acid (Glu) as the sixth amino acid of the β-chain, coded by the mRNA codon GAG (or GAA).
In sickle-cell anaemia, a point mutation in the β-globin gene changes a single DNA base pair, which changes the mRNA codon from GAG to GUG. Since GUG codes for valine, glutamic acid (a polar, negatively charged amino acid) is replaced by valine (a non-polar, hydrophobic amino acid) at the sixth position of the β-chain.
This single amino acid substitution changes the way haemoglobin molecules associate at low oxygen tension — the abnormal HbS polymerises into long fibres, distorting the red blood cell into a sickle shape.
Among the options: GGG codes for glycine, AAG codes for lysine, GAA codes for glutamic acid itself (the original, unmutated codon), and GUG codes for valine.
✓Final answer(d) GUG — the codon for valine; the mutation GAG→GUG (Glu→Val) causes sickle-cell anaemia.
- CBSE 2024Set ANNUAL1 markMCQQ.Which of the following Nonsense codon is called 'ochre'?(a) AUG(b) UAG(c) UAA(d) UGA
›Reveal solutionSolution
Of the three stop codons, UAA is called 'ochre', UAG is 'amber', and UGA is 'opal'.
In the genetic code, 61 of the 64 codons specify amino acids (sense codons), while three codons — UAA, UAG and UGA — code for no amino acid and instead signal the ribosome to terminate translation. These are called nonsense/stop/termination codons and have informal historical names: UAG is 'amber', UGA is 'opal' (or 'umber'), and UAA is 'ochre'. AUG, by contrast, is a sense codon — it codes for methionine and also serves as the start/initiation codon in eukaryotes.
✓Final answer(c) UAA. (a) AUG is the start codon (methionine), (b) UAG is 'amber', and (d) UGA is 'opal', not ochre.
- CBSE 2023Set ANNUAL1 markMCQQ.The genetic code that codes for Methionine is -(a) UUU(b) AUG(c) ACG(d) AAA
›Reveal solutionSolution
In the genetic code table, the codon AUG specifies methionine and doubles as the universal start codon.
The genetic code is a triplet code where a sequence of three nucleotides (a codon) on mRNA specifies one amino acid. Reading the standard codon table:
- UUU codes for Phenylalanine.
- AUG codes for Methionine, and is also the start codon that initiates translation in almost all organisms.
- ACG codes for Threonine.
- AAA codes for Lysine.
Since AUG is the only codon among the options that specifies methionine, it is the correct answer.
✓Final answer(b) AUG - codes for methionine and functions as the start codon.
- CBSE 2023Set ANNUAL1 markMCQQ.Which of the following Codon codes for methionine ?(a) UUU(b) AUG(c) AUC(d) UAA
›Reveal solutionSolution
AUG is the universal start codon and the only codon that specifies methionine.
In the genetic code, AUG serves a dual role: it is the initiation (start) codon that signals the ribosome to begin translation, and it is the only codon that specifies the amino acid methionine (formyl-methionine in bacteria). Checking the distractors: option (a) UUU codes for phenylalanine; option (c) AUC codes for isoleucine (the codons AUU, AUC and AUA all specify isoleucine); and option (d) UAA is one of the three stop (nonsense) codons (with UAG and UGA) that terminate translation and code for no amino acid. Hence AUG is the only codon among the choices that codes for methionine.
[!ANSWER]
The correct option is b) AUG.
- CBSE 2022Set HE2201 markQ.Write the answer in one word/sentence: Which codon is AUG?
›Reveal solutionSolution
AUG is unique among codons: it is both the universal start codon and the codon for methionine.
In the genetic code table, 61 of the 64 codons specify amino acids and 3 are stop codons. Among the sense codons, AUG plays a dual role — it is the initiator (start) codon that signals the ribosome where to begin translation, and it also codes for the amino acid methionine (in prokaryotes, the first AUG codes for a modified form, N-formylmethionine). Any AUG occurring within the coding sequence (not at the start) simply codes for methionine at that position.
✓Final answerAUG = start codon, coding for methionine (N-formylmethionine as the first amino acid in prokaryotes).
- CBSE 2022Set ANNUAL1 markMCQQ.Initiation codon is :(a) AUG(b) UUU(c) UAG(d) UGA
›Reveal solutionSolution
AUG is the codon that signals the ribosome to start translation, coding for methionine.
During translation, the small ribosomal subunit, together with initiator tRNA carrying methionine (charged as Met-tRNAi in eukaryotes, or fMet-tRNAi in bacteria), scans the mRNA and assembles at the first AUG codon in an appropriate context (such as the Kozak sequence in eukaryotes), establishing the reading frame for the rest of the message; this is why AUG is called the initiation or start codon. Because AUG also happens to be the codon for methionine whenever it occurs internally within a coding sequence, methionine is always the first amino acid incorporated into a nascent polypeptide (it may later be enzymatically removed). This start function is distinct from UUU, which simply codes for the amino acid phenylalanine wherever it appears in the reading frame, and from UAG and UGA, which are two of the three stop (termination) codons (along with UAA) that carry no amino acid and instead signal release factors to terminate translation and release the completed polypeptide.
✓Final answerThe initiation codon is AUG.
- CBSE 2021Set TERM11 markMCQQ.Stop Codon is :(a) AUG(b) UAA(c) UGA(d) Both(b) and (c)
›Reveal solutionSolution
The genetic code has three stop codons -- UAA, UAG and UGA -- that signal the ribosome to end translation; among the given options, both UAA and UGA qualify.
Of the 64 possible codons, 61 code for amino acids and 3 are stop (termination/nonsense) codons: UAA, UAG, and UGA. These do not code for any amino acid; instead, they signal release factors to terminate translation and release the polypeptide chain.
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(a) AUG is actually the start codon (codes for methionine and initiates translation), not a stop codon.
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(b) UAA and (c) UGA are both genuine stop codons, so the correct choice combines them.
✓Final answer(d) Both (b) and (c).
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