MPSC 2025 Prelims Paper-I · Question 64 of 100

Correct — option (4), AUG. AUG is the universal start codon of the genetic code: in every organism, it is the codon that the ribosome's initiator tRNA recognises to begin translation, and it also happens to be the codon for the amino acid methionine wherever it appears mid-sequence. What differs between prokaryotes and eukaryotes is not the codon but what gets attached to the initiator tRNA that reads it. In bacteria (and in mitochondria and chloroplasts, which retain a bacterial-style translation system), the initiator tRNA carries methionine that is then enzymatically formylated — a formyl group is added to the amino end by the enzyme methionyl-tRNA formyltransferase, using N10-formyltetrahydrofolate as the formyl donor — producing N-formylmethionine (fMet). This formylated methionine is what actually begins every prokaryotic polypeptide chain (the formyl group is often removed later by deformylase enzymes). In eukaryotic cytoplasmic translation, by contrast, the same AUG codon is read by a different initiator tRNA that carries plain, unformylated methionine. So the question's phrase 'initial code for N-formyl methionine' is really asking which codon prokaryotic ribosomes read as the start signal, and the answer is the same AUG that codes for methionine everywhere else in the code — it is the formylation of the amino acid, not a different codon, that marks the prokaryotic start.

• (1)TTG — TTG (read as UUG in the mRNA) is a genuine but minor alternative start codon in bacteria — used to initiate roughly 3-8% of E. coli genes — but it is not the initial code the question is asking about. Even where UUG does serve as a start signal, it is still recognised there by the same initiator N-formylmethionyl-tRNA that normally reads AUG, and the protein still begins with N-formylmethionine; UUG does not code for methionine on its own merit, and it is a rare exception rather than 'the' initial code for fMet. AUG remains overwhelmingly the dominant and standard start codon in prokaryotic translation.
• (2)CUG — CUG is the standard codon for the amino acid leucine in the near-universal genetic code and plays no established role as a start codon in normal bacterial translation. It is sometimes cited as an unusual, minor alternative initiation codon in specific eukaryotic or viral contexts, but it is not associated with N-formylmethionine or with prokaryotic translation initiation, and including it here tests whether a candidate can distinguish an internal, amino-acid-coding codon from a genuine start signal.
• (3)GGC — GGC codes for the amino acid glycine and has no initiator role in any organism's translation system. It shares no functional or sequence relationship with the start-codon machinery at all, making it the most straightforward distractor in the set — included mainly to test whether a candidate is guessing from unfamiliar-looking triplets rather than recalling which codon actually initiates translation.

Translation in all organisms begins at a start codon, almost always AUG, read by a special initiator tRNA rather than the tRNA that reads internal AUG codons for methionine. The distinguishing feature between bacteria and eukaryotes is what is loaded onto that initiator tRNA: bacteria formylate the methionine to produce N-formylmethionine (fMet), while eukaryotic cytoplasmic ribosomes use plain methionine. This bacterial-style formylated initiation is preserved in mitochondria and chloroplasts because both organelles evolved from free-living bacteria (the endosymbiotic origin of eukaryotic organelles), and their own translation machinery still resembles a bacterial system. After translation, a deformylase enzyme in bacteria typically removes the formyl group, and in many proteins the initiator methionine itself is later cleaved off by methionine aminopeptidase, so the mature protein frequently does not retain either the formyl group or the initial methionine.

MPSC's molecular biology questions frequently probe this prokaryote-versus-eukaryote translation-initiation distinction because it is a compact, high-yield fact that rewards precise recall: the start codon itself (AUG) does not change between kingdoms, but the chemistry of the amino acid it carries does. Questions built around this point often present codon triplets as distractors to see whether a candidate genuinely knows the genetic code table or is just recognising 'AUG' by shape; TTG (as UUG) is a particularly effective distractor because it really is used as a rare bacterial start codon, testing whether the candidate can distinguish 'a start codon that exists' from 'the standard start codon associated with fMet'.

• AUG is the near-universal start codon of the genetic code, read by translation machinery in prokaryotes and eukaryotes alike.
• In bacteria, the initiator tRNA's methionine is formylated to N-formylmethionine (fMet) by methionyl-tRNA formyltransferase.
• In eukaryotic cytoplasmic translation, the same AUG start codon initiates with plain, unformylated methionine.
• Mitochondria and chloroplasts use bacterial-style formylated initiation, consistent with their endosymbiotic bacterial origin.
• GUG and UUG can serve as rare alternative bacterial start codons, but AUG remains the dominant, standard initiator codon.

• Assuming a different, exotic-looking codon must be the 'special' prokaryotic start code, when the codon (AUG) is actually the same across kingdoms
• Confusing a rare alternative bacterial start codon (like UUG) with the standard, defining start codon for N-formylmethionine
• Forgetting that it is the formylation of methionine, not a different codon, that distinguishes prokaryotic from eukaryotic initiation
• Mistaking an internal, amino-acid-coding codon (like CUG for leucine or GGC for glycine) for a codon with any initiator function

MPSC science questions on the genetic code often test start-codon knowledge by asking specifically about the prokaryotic fMet-initiation system, using codon-table distractors — some genuinely rare alternative start codons, some ordinary internal codons — to separate candidates who know the underlying biochemistry from those pattern-matching on the letters AUG alone.

No directly related past PYQ was found.

• practice — not a real PYQ

  What distinguishes the start of translation in bacteria from that in eukaryotic cytoplasmic translation, even though both typically use the codon AUG ?

  • (a)Bacteria use a different start codon altogether
  • (b)Bacterial initiator methionine is formylated to N-formylmethionine, while eukaryotic initiator methionine is not
  • (c)Bacteria do not require a start codon
  • (d)Eukaryotes use two start codons simultaneously

  Answer(b) Bacterial initiator methionine is formylated to N-formylmethionine, while eukaryotic initiator methionine is not — the start codon (AUG) is the same in both, but bacterial translation attaches a formyl group to the initiating methionine, a step absent in eukaryotic cytoplasmic translation.

• practice — not a real PYQ

  Which cell organelles use a bacterial-style, formylated-methionine translation-initiation system, consistent with their evolutionary origin ?

  • (a)Golgi apparatus and lysosomes
  • (b)Mitochondria and chloroplasts
  • (c)Nucleus and nucleolus
  • (d)Endoplasmic reticulum and peroxisomes

  Answer(b) Mitochondria and chloroplasts — both organelles are believed to have evolved from free-living bacteria via endosymbiosis, and they retain bacterial-style ribosomes and formylated-methionine translation initiation as a result.