The trap that costs marks in transcription-translation questions: confusing start and stop codons. Students list UGA as a start codon — it is a stop codon. This single term-confusion error hands away a free mark under negative marking.
Central dogma (NCERT Class 12 Biology Chapter 5, page 106): genetic information flows DNA → RNA → Protein. Transcription copies one DNA strand (template strand, read 3'→5') into mRNA (synthesised 5'→3') by RNA polymerase. Translation decodes mRNA into a polypeptide at ribosomes.
Transcription essentials:
Translation essentials:
Watch-out for NEET: When a stem mentions "initiation codon," the answer is always AUG. When it asks "which is NOT a stop codon," eliminate UAA, UAG, UGA — the odd one out is correct. Confusing UGA with AUG is a common negative-marking trap.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
The start codon in both prokaryotic and eukaryotic translation is:
Answer: A. AUG is the universal initiation codon, coding for methionine (fMet in prokaryotes). NCERT Class 12 Biology Chapter 5, page 106.
Why B is wrong: UAG is the amber stop codon — confusing it with the start codon is a term-confusion trap.
Why C is wrong: UGA is the opal/umber stop codon. A common mistake is listing UGA as a start codon (trap: start-stop codon confusion).
Why D is wrong: UAA is the ochre stop codon, not an initiation signal.
Which of the following is NOT a stop codon?
Answer: C. AUG is the start codon (methionine). The three stop codons are UAA, UAG, and UGA. NCERT Class 12 Biology Chapter 5, page 106.
Why A is wrong: UAA (ochre) is indeed a stop codon — selecting it means you confused it for a sense codon.
Why B is wrong: UAG (amber) is a stop codon — this option is a distractor for students who know only one stop codon.
Why D is wrong: UGA (opal) is a stop codon — choosing this indicates confusing UGA's role with the start codon (trap: start-stop confusion).
In eukaryotes, mRNA is synthesised by:
Answer: B. RNA Polymerase II transcribes precursor mRNA (hnRNA) in eukaryotes. NCERT Class 12 Biology Chapter 5, page 106.
Why A is wrong: RNA Pol I synthesises rRNA (28S, 18S, 5.8S), not mRNA — a similar-term confusion trap.
Why C is wrong: RNA Pol III makes tRNA and 5S rRNA — students who remember 'III for transfer' but flip the assignment pick this.
Why D is wrong: DNA Polymerase III replicates DNA, not RNA — different process entirely (replication vs transcription).
The template strand of a DNA segment reads 3'-TACGGA-5'. What is the sequence of the mRNA transcribed from it?
Answer: C. mRNA is synthesised 5'→3' using the template strand (3'→5') as guide. Complementary base pairing: T→A, A→U, C→G, G→C. Template 3'-TACGGA-5' gives mRNA 5'-AUGCCU-3'. NCERT Class 12 Biology Chapter 5.
Why A is wrong: Correct bases but wrong polarity labelling (3'→5' instead of 5'→3'). mRNA is always written 5'→3'.
Why B is wrong: This copies the template strand's own bases, swapping T for U, instead of pairing with them; the mRNA must be complementary to the template.
Why D is wrong: This copies the template strand's bases unchanged (in DNA, with T) and only relabels the ends. The mRNA is complementary to the template and contains U.
During translation, a charged tRNA carrying leucine has the anticodon 3'-GAC-5'. What is the corresponding mRNA codon?
Answer: D. Anticodon 3'-GAC-5' pairs with mRNA codon 5'-CUG-3' by complementary antiparallel base pairing (G-C, A-U, C-G). CUG is a leucine codon. NCERT Class 12 Biology Chapter 5.
Why A is wrong: CTG uses thymine — mRNA contains uracil, not thymine. This is a DNA sequence written by mistake.
Why B is wrong: GAC would be the anticodon sequence read as if it were the codon — this ignores complementary pairing.
Why C is wrong: GUC is the correct codon read backwards: pairing gives 5'-CUG-3', and writing that sequence 3'→5' as if it were 5'→3' gives GUC.
A eukaryotic hnRNA has 5 exons and 4 introns. After splicing, how many exons will the mature mRNA contain?
Answer: B. Splicing removes introns and joins exons. All 5 exons are retained in the mature mRNA. NCERT Class 12 Biology Chapter 5, page 106.
Why A is wrong: Choosing 4 confuses exon count with intron count — introns are removed, exons stay.
Why C is wrong: 9 is the total number of segments (exons + introns) — but introns are excised during processing.
Why D is wrong: 1 would imply exons are also removed, leaving a single fragment — this misunderstands the purpose of splicing.
A point mutation changes the codon UAC to UAG in the middle of an mRNA. What is the most likely consequence during translation?
Answer: A. UAG is a stop codon (amber). Changing a sense codon to a stop codon produces a truncated polypeptide (nonsense mutation). NCERT Class 12 Biology Chapter 5.
Why B is wrong: A missense mutation would incorporate a different amino acid — but UAG codes no amino acid; it is a termination signal.
Why C is wrong: Reading frame shifts occur with insertions/deletions (indels), not single-base substitutions. The reading frame is preserved here.
Why D is wrong: A silent mutation would have no effect, but UAC→UAG changes tyrosine codon to a stop codon — the polypeptide is definitely altered (truncated).
An mRNA sequence reads 5'-AUGGCUUACUGAUAA-3'. How many amino acids will the translated polypeptide contain?
Answer: C. C is correct. Read in triplets from the start codon: AUG (Met) – GCU (Ala) – UAC (Tyr) – UGA (stop) – UAA. Translation terminates at the first stop codon, UGA, so the polypeptide is Met–Ala–Tyr: three amino acids. The second stop codon, UAA, is never reached (NCERT Class 12 Biology Chapter 5).
Why A is wrong: A is wrong because 5 counts every triplet, including the two stop codons UGA and UAA. Stop codons are termination signals; no tRNA carries an amino acid for them.
Why B is wrong: B is wrong because 4 counts the first stop codon, UGA, as if it coded an amino acid. Translation ends when the ribosome reaches UGA. (trap: start-stop codon confusion)
Why D is wrong: D is wrong because 2 leaves out the initiating methionine. AUG is both the start signal and a codon for Met, and that Met is the first amino acid of the chain.
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Given
An mRNA molecule has the sequence: 5'-AUGAAACCCUAGUUU-3'. You are asked to determine: (a) the number of amino acids in the translated polypeptide, and (b) identify the stop codon.
Required
Number of amino acids coded before termination; identity of the stop codon in this sequence.
Concept
Translation begins at AUG and proceeds codon-by-codon (5'→3') until a stop codon (UAA, UAG, or UGA) is encountered. The stop codon does not code any amino acid.
Formula/Rule
Read mRNA in non-overlapping triplets starting from AUG. Count sense codons before the first stop codon.
Substitution
Divide: AUG | AAA | CCC | UAG | UUU• AUG = Met (start)• AAA = Lys• CCC = Pro• UAG = stop (amber)• UUU = not translated (downstream of stop)
Calculation
Amino acids before stop: Met – Lys – Pro = 3 amino acids.
Stop codon identity: UAG.
Final answer
The polypeptide contains 3 amino acids (Met-Lys-Pro). The stop codon is UAG (amber).
Note: The integers 3 and 15 (nucleotide count) are exact counting numbers and do not involve significant-figure considerations.
Common trap
Confusing UAG with a sense codon (similar-term confusion with AUG) or counting the stop codon as an amino acid position. Students who mistake UGA for a start codon may similarly misread UAG.
Similar NEET-style question
"An mRNA has 600 nucleotides with AUG at position 1 and UAA at position 598-600. How many amino acids does the polypeptide contain?"
Answer approach: (600 ÷ 3) = 200 codons; subtract 1 stop codon = 199 amino acids.
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Transcription: DNA → mRNA via RNA polymerase. Eukaryotic: RNA pol I (rRNA), II (mRNA), III (tRNA). Splicing: GU-AG rule; mature mRNA has 5' cap + 3' poly(A) tail. Translation: ribosome reads codons; tRNA delivers amino acids; start codon AUG (Met), stop UAA/UAG/UGA. Wobble in 3rd codon position.
-- NCERT Class 12 Biology, Ch. 5, p. 95These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Root cause: term confusion
Start codon: AUG (Met / fMet). Stop codons: UAA (ochre), UAG (amber), UGA (opal/umber).
More in Genetics and Evolution: 20 exam traps and mistakes · 4 formulas · 1 question pattern from its other lessons.
15 questions from NEET 2020, 2021, 2022, 2023, 2024, 2025, 2026. Answers verified against NTA official keys.
Which of the following enzymes synthesizes precursor mRNA ?
Which factor is important for termination of transcription?
What is the role of RNA polymerase III in the process of transcription in Eukaryotes?
The process of translation of mRNA to proteins begins as soon as :
What is the role of RNA polymerase III in the process of transcription in eukaryotes?
Identify the correct statement.
Which of the following RNAs is not required for the synthesis of protein?
Name the enzyme that facilitates opening of DNA helix during transcription.
The first phase of translation is :
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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