Genetic Code

8 MCQs9-step worked example
Source: NCERT Genetics and EvolutionPYQ coverage: NEET 2025Official key: NTA-verifiedLast updated: 25 Sep 2026

Genetic Code, explained for NEET

Students remember that 64 codons exist and forget three of them are stop signals — a distractor NEET plants by offering "64" where "61" is correct. Of the 64 possible triplet codons, 61 code for amino acids; UAA, UAG and UGA code for none and act as stop (terminator) codons. AUG carries a second job: it codes for Methionine and also serves as the initiator codon.

The code's deciphering needed physicists as much as biochemists. George Gamow argued on mathematical grounds that a two-base code (4×4 = 16) could not specify 20 amino acids, so the code had to be a triplet — a proposal that generated 64 codons, more than the 20 needed. Har Gobind Khorana chemically synthesised RNA of defined, repeating base sequences; Marshall Nirenberg's cell-free protein-synthesising system read what those sequences coded for; Severo Ochoa's enzyme polymerised RNA of defined sequence without a template. Between them, the codon table (NCERT Class 12 Biology, Chapter 5, Table 5.1) was filled in.

Six salient features follow from that table: the codon is a triplet; the code is degenerate (several codons can specify one amino acid — this is not the same as ambiguous, since each codon still specifies only one amino acid); codons are read contiguously, with no punctuation between them; the code is nearly universal (UUU means Phenylalanine from bacteria to humans, with known mitochondrial and protozoan exceptions); AUG is dual-function; and UAA/UAG/UGA are the three stop codons.

Frameshift mutations connect this table to real consequences: NCERT's own "RAM HAS RED CAP" exercise shows that inserting one letter shifts every downstream triplet, but inserting three letters together leaves the triplets after that point untouched — because insertion or deletion of three bases, or any multiple of three, only adds or removes whole codons.

Watch out: "degenerate" and "ambiguous" describe opposite properties. Confusing them is the single likeliest slip in this topic.

Can you answer these Genetic Code MCQs?

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

Of the 64 possible triplet codons, how many function as stop codons that code for no amino acid?

Show answer and why every option is right or wrong

Answer: B. UAA, UAG and UGA are the three stop (terminator) codons; the other 61 code for amino acids (NCERT Class 12 Biology, Chapter 5, page 96).

Why A is wrong: A is wrong because 61 is the count of codons that DO code for amino acids, not the stop-codon count the stem asks for.

Why C is wrong: C is wrong because it treats all 64 codons as non-coding stop signals, ignoring that 61 of them code for amino acids.

Why D is wrong: D is wrong because it undercounts the stop codons; only AUG is dual-function, which is a separate feature from the stop-codon count.

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MCQ 2Easy RecallPractice

Which scientist argued on purely mathematical grounds that the genetic code had to be a triplet, since a two-base code (4 × 4 = 16 combinations) could not specify 20 amino acids?

Show answer and why every option is right or wrong

Answer: A. George Gamow, a physicist, made the combinatorial argument for a triplet code before any experimental proof existed (NCERT Class 12 Biology, Chapter 5, page 95).

Why B is wrong: B is wrong because Khorana's contribution was chemically synthesising RNA with defined base combinations — experimental proof, not the original mathematical proposition.

Why C is wrong: C is wrong because Nirenberg's cell-free protein-synthesising system helped decipher which codon specifies which amino acid, a later step than Gamow's proposal.

Why D is wrong: D is wrong because Ochoa's enzyme polymerised RNA of defined sequence without a template, aiding decoding rather than proposing the triplet logic.

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MCQ 3Easy RecallPractice

The genetic code is described as "degenerate" because:

Show answer and why every option is right or wrong

Answer: C. Degeneracy means several codons can code for the same amino acid (NCERT Class 12 Biology, Chapter 5, page 96).

Why A is wrong: A is wrong because one codon specifying more than one amino acid would make the code ambiguous, and the genetic code is explicitly NOT ambiguous.

Why B is wrong: B is wrong because AUG's dual role as initiator is a separate salient feature, unrelated to degeneracy.

Why D is wrong: D is wrong because it contradicts the fact that the codon is read in mRNA in a contiguous fashion, with no punctuation between codons.

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MCQ 4Direct ApplicationPractice

Which statement about the codon AUG is correct?

Show answer and why every option is right or wrong

Answer: D. AUG has dual functions: it codes for Methionine and also acts as the initiator codon (NCERT Class 12 Biology, Chapter 5, page 96).

Why A is wrong: A is wrong because the three stop codons are UAA, UAG and UGA; AUG is not among them.

Why B is wrong: B is wrong because it denies AUG's second, well-established role as the initiator codon.

Why C is wrong: C is wrong because a codon serving two roles is dual function, not ambiguity — the code stays unambiguous since each codon in context still specifies only one amino acid.

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MCQ 5Direct ApplicationPractice

In a structural gene, 2 bases are deleted from within the coding sequence. What happens to translation from that point onward?

Show answer and why every option is right or wrong

Answer: B. Only deletion of three bases, or a multiple of three, leaves the downstream reading frame unaltered; deleting one or two bases shifts the frame from that point on (NCERT Class 12 Biology, Chapter 5, page 97-98).

Why A is wrong: A is wrong because the frame is preserved only when the deleted count is a multiple of three; 2 is not a multiple of three.

Why C is wrong: C is wrong because NCERT's own letter-deletion exercise shows the phrase keeps reading in shifted triplets, not that reading halts.

Why D is wrong: D is wrong because nothing in the deletion process resets the frame; it stays shifted from the point of deletion onward.

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MCQ 6Direct ApplicationPractice

9 extra bases are inserted together within a structural gene's coding sequence. What happens to the reading frame downstream of the insertion?

Show answer and why every option is right or wrong

Answer: A. Insertion of three bases or a multiple of three adds whole codons (amino acids) and leaves the reading frame downstream unaltered (NCERT Class 12 Biology, Chapter 5, page 98).

Why B is wrong: B is wrong because NCERT's 'RAM HAS BIG RED CAP' result shows inserting a whole triplet together leaves the rest of the sentence's triplets unchanged.

Why C is wrong: C is wrong because whether the frame shifts depends only on whether the inserted base count is a multiple of three, not on which bases were inserted.

Why D is wrong: D is wrong because the rule doesn't depend on position in the gene; a multiple-of-three insertion anywhere leaves the frame unaltered downstream of it.

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MCQ 7Concept TrapPractice

Francis Crick reasoned that reading the genetic code required an adapter molecule because:

Show answer and why every option is right or wrong

Answer: D. Amino acids have no structural specialities to read the code uniquely, so an adapter was needed to link codon to amino acid (NCERT Class 12 Biology, Chapter 5, page 98).

Why A is wrong: A is wrong because ribosome binding is not the gap Crick identified; the gap was between the nucleotide code and amino acid identity.

Why B is wrong: B is wrong because NCERT states amino acids have no structural specialities to read the code at all, regardless of time.

Why C is wrong: C is wrong because tRNA (then called sRNA) was known before the genetic code was even postulated; only its adapter role was assigned later.

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MCQ 8Concept TrapPractice

An mRNA is read strictly codon-by-codon from a fixed starting point, with no gaps or markers between triplets. This describes which salient feature of the genetic code?

Show answer and why every option is right or wrong

Answer: C. The codon is read in mRNA in a contiguous fashion with no punctuation between codons (NCERT Class 12 Biology, Chapter 5, page 96).

Why A is wrong: A is wrong because degeneracy concerns multiple codons coding for one amino acid, not how reading proceeds along the mRNA.

Why B is wrong: B is wrong because universality concerns the same codon meaning the same amino acid across organisms, not reading continuity.

Why D is wrong: D is wrong because AUG's dual role as coding/initiator codon is unrelated to how codons are read along the mRNA.

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How do you solve a Genetic Code question? A worked example

  1. 1

    Given

    A gene segment is represented, letter-for-letter, by the code-phrase "RAM HAS RED CAP" — each three-letter block standing in for one codon, exactly as NCERT's own exercise sets it up.

  2. 2

    Required

    Compare what happens to the phrase (i) when one letter, B, is inserted between HAS and RED, versus (ii) when three letters, BIG, are inserted together at the same place.

  3. 3

    Concept

    Insertion or deletion of one or two bases shifts the reading frame from the point of insertion onward (frameshift mutation). Insertion or deletion of three bases, or any multiple of three, adds or removes whole codons and leaves the reading frame unaltered from that point onward.

  4. 4

    Formula

    No numeric formula applies; the governing rule is: frame shifts unless (bases inserted or deleted) mod 3 = 0.

  5. 5

    Substitution

    Insert B between HAS and RED, then re-break into triplets: RAM HAS BRE DCA P. Every triplet from BRE onward is a new, shifted codon.

  6. 6

    Calculation

    Insert BIG (three letters) at the same place instead, then re-break into triplets: RAM HAS BIG RED CAP. RED and CAP — the codons after the insertion point — read exactly as they did before the insertion.

  7. 7

    Final answer

    A single-letter insertion frameshifts every codon after HAS (RAM HAS BRE DCA P); a three-letter insertion adds one whole new codon (BIG) and leaves RED CAP downstream unchanged, because 3 mod 3 = 0.

  8. 8

    Common trap

    Assuming any insertion or deletion in a gene causes a frameshift. It only does so when the number of bases added or removed is not a multiple of three.

  9. 9

    Similar NEET-style question

    A different structural gene has 6 bases deleted together from within its coding sequence. Does the reading frame downstream of the deletion shift? (Answer: No — check 6 mod 3 = 0, so the deletion removes exactly two whole codons; every codon after the deletion point keeps its original reading frame, just as removing R, E and D one triplet's worth at a time — "RAM HAS RED CAP" → "RAM HAS CAP" — left CAP unshifted once all three letters of RED were gone.)

What to remember before solving Genetic Code questions

Definition

tRNA - the Adapter Molecule

Francis Crick postulated the presence of an adapter molecule that would on one hand read the code and on the other hand bind to specific amino acids, since amino acids have no structural specialities to read the code uniquely. tRNA, previously called sRNA (soluble RNA), was known before the genetic code was postulated, but its role as an adapter molecule was assigned much later. tRNA has an anticodon loop with bases complementary to the code, and an amino acid acceptor end to which it binds amino acids; tRNAs are specific for each amino acid. There is a separate initiator tRNA for initiation, and there are no tRNAs for stop codons.

-- NCERT Class 12 Biology, Ch. 5, p. 98
Key Fact

Genetic Code

George Gamow, a physicist, argued that since there are only 4 bases and they must code for 20 amino acids, the code should be a combination of bases, and proposed that the code is made of three nucleotides (a triplet), since a permutation of 4^3 (4x4x4) generates 64 codons, more than the 20 required. Har Gobind Khorana's chemical method synthesised RNA molecules with defined base combinations (homopolymers and copolymers); Marshall Nirenberg's cell-free protein synthesis system helped decipher the code; Severo Ochoa's enzyme (polynucleotide phosphorylase) helped polymerise RNA with defined sequences in a template-independent manner.

-- NCERT Class 12 Biology, Ch. 5, p. 95
Key Fact

Salient Features of the Genetic Code

The codon is a triplet: 61 codons code for amino acids and 3 codons do not code for any amino acid, functioning as stop codons. Some amino acids are coded by more than one codon, so the code is degenerate. The codon is read in mRNA in a contiguous fashion with no punctuations. The code is nearly universal (e.g. UUU codes for Phenylalanine from bacteria to human), with exceptions found in mitochondrial codons and in some protozoans. AUG has dual functions: it codes for Methionine and also acts as the initiator codon. UAA, UAG and UGA are stop (terminator) codons.

-- NCERT Class 12 Biology, Ch. 5, p. 96
Key Fact

Mutations and Genetic Code

A classical example of point mutation is a change of a single base pair in the gene for the beta globin chain, changing the amino acid residue glutamate to valine, resulting in sickle cell anaemia. Insertion or deletion of one or two bases in a structural gene changes the reading frame from the point of insertion or deletion, referred to as frameshift insertion or deletion mutations. Insertion or deletion of three bases, or a multiple of three, inserts or deletes one or multiple codons (hence amino acids), and the reading frame remains unaltered from that point onwards, illustrated with the word-triplet example 'RAM HAS RED CAP'.

-- NCERT Class 12 Biology, Ch. 5, p. 97

Where do students lose marks on Genetic Code?

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

64 vs 61 vs 3 codon count confusion

Category: Overthinking

Students misstate that all 64 codons code for amino acids, forgetting 3 are stop codons, or forget AUG's dual role.

When it triggers

A question asks how many codons code for amino acids and offers 64 as a distractor instead of 61, or asks what AUG codes for and omits its role as the initiator codon.

How to avoid

Of 64 possible triplet codons, 61 code for amino acids and 3 (UAA, UAG, UGA) are stop/terminator codons that code for no amino acid; AUG both codes for Methionine and acts as the initiator codon.

Trap

Frameshift mutation base-count exception

Category: Inorganic Exception

Students assume any insertion/deletion causes a frameshift, missing that insertions/deletions of three bases (or multiples of three) leave the reading frame unaltered.

When it triggers

A question states 3 bases (or a multiple of 3) were inserted or deleted and asks whether the reading frame shifts, and a distractor claims a frameshift always occurs.

How to avoid

Insertion or deletion of one or two bases shifts the reading frame from that point (frameshift mutation); insertion or deletion of three bases, or a multiple of three, only adds/removes whole codons (amino acids) and the reading frame remains unaltered thereafter, as shown by the 'RAM HAS RED CAP' example.

Mistake

Students confuse 'degenerate' (one amino acid coded by multiple codons) with 'ambiguous' (one codon coding for multiple amino acids), which the genetic code is not.

Root cause: term confusion

Correction

The genetic code is degenerate because some amino acids are coded by more than one codon; it is unambiguous because each codon is read in mRNA in a contiguous fashion specifying only one amino acid, with no punctuations between codons.

Wrong option pattern

labels the code as 'ambiguous' or claims one codon can specify multiple amino acids

Mistake

Under time pressure, students mark any insertion/deletion of bases as a frameshift mutation without checking whether the number of bases is a multiple of three.

Root cause: rushed under time pressure

Correction

Only insertion or deletion of one or two bases (not a multiple of three) shifts the reading frame from that point onward; insertion or deletion of three bases or multiples of three only adds or removes whole codons, leaving the downstream reading frame unaltered.

Wrong option pattern

marks a 3-base or 6-base insertion/deletion as causing a frameshift

More in Genetics and Evolution: 17 exam traps and mistakes · 4 formulas · 1 question pattern from its other lessons.

Genetic Code questions from past NEET papers

1 question from NEET 2025. Answers verified against NTA official keys.

All 82 past-paper questions from Genetics and Evolution →

Sources

Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.

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