In prokaryotic genetic code, which is the initial code for N-formyl methionine ?

Question

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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.

Why the other options are wrong — (a) 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.  (b) 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.  (c) 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.

Concept: 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.

State PCS 2025 Q64 — Science & Technology PYQ with Answer

State PCS 2025 Q64 — Science & Technology PYQ with Answer