Biology · Molecular Basis of Inheritance · NEET
Exons are the coding (expressed) sequences that appear in the mature or processed RNA. Introns, also called intervening sequences, do NOT appear in the mature RNA; they are removed during processing. Memory hook: EXon = EXpressed (kept), INtron = IN nucleus (removed).
In eukaryotes the coding sequence of a structural gene is not continuous. It is interrupted: exons are broken up by introns. This interrupted arrangement is called a split gene. NCERT states genes in eukaryotes are split, so split genes are a EUKARYOTIC feature, not prokaryotic.
No. A transcription unit has three parts: promoter, the structural gene, and terminator. The structural gene is only the part that is actually transcribed into RNA. So the gene (structural gene) is one part of the transcription unit, not the whole unit.
A cistron is a segment of DNA coding for one polypeptide. A monocistronic structural gene codes for one polypeptide (mostly in eukaryotes). A polycistronic gene codes for many polypeptides (mostly in bacteria/prokaryotes). Split genes with exons and introns are seen in the monocistronic eukaryotic genes.
No. Introns are removed during RNA processing (splicing), so they are absent from the mature mRNA. Only exons remain and are joined together. This is a common NEET trap: students wrongly think introns are copied into the final message.
A cistron is defined as a segment of DNA that codes for a polypeptide. It is the molecular equivalent of a structural gene. Muton and recon are units of mutation and recombination, and an operon is a group of genes regulated together, so they are different terms.
The equivalent of a structural gene is
Identify the correct statement.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Split genes are found in eukaryotes. NCERT states the genes in eukaryotes are split, with exons interrupted by introns. Prokaryotic mRNA does not require processing.
Only exons (the expressed coding sequences) appear in the mature or processed RNA. Introns are removed and do not appear in the final mRNA.
Because a gene can code for a polypeptide, or for tRNA/rRNA, and because eukaryotic genes are split (exons and introns), plus inheritance is also affected by promoter and regulatory sequences. So a gene is not simply one continuous DNA segment.
Eukaryotic structural genes are mostly monocistronic (one gene, one polypeptide). Bacterial/prokaryotic genes are mostly polycistronic (many polypeptides from one unit).
Promoter and regulatory sequences of a structural gene affect inheritance of a character, so these regulatory sequences are sometimes loosely called regulatory genes, even though they do not code for a product themselves.
