Cistron and Structural Gene Definition — NEET Biology
✅ Asked in NEET 2016The DNA sequence coding for tRNA or rRNA molecule also define a gene. However by defining a cistron as a segment of DNA coding for a polypeptide, the structural gene in a transcription unit could be said as monocistronic (mostly in eukaryotes) or polycistronic (mostly in bacteria or prokaryotes). In eukaryotes, the monocistronic structural genes have interrupted coding sequences – the genes in eukaryotes are split. The coding sequences or expressed sequences are defined as exons. Exons are said to be those sequence that appear in mature or processed RNA. The exons are interrupted by introns. Introns or intervening sequences do not appear in mature or processed RNA. The split-gene arrangement further complicates the definition of a gene in terms of a DNA segment.
NTA tests the definition of a cistron: a DNA segment coding for a single polypeptide chain, which is essentially a structural gene. Students often confuse cistrons with genes in prokaryotes vs eukaryotes—remember that cistrons are monocistronic in eukaryotes (one gene = one polypeptide) but polycistronic in prokaryotes (one gene = multiple polypeptides). The key trap: thinking exons and introns change the cistron definition. They don't—introns are removed during processing, so only exons (expressed sequences) code for the polypeptide. To ace this: a cistron always equals the DNA that ultimately produces one functional polypeptide, regardless of whether it contains introns or not.
The equivalent of a structural gene is: (NEET 2016 Phase 2)
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