Read the given passage and answer the following question.
The process of copying genetic information from one strand of the DNA into RNA is termed as transcription. Here also, the principle of complementarity governs the process of transcription, except the adenosine complements now forms base pair with uracil instead of thymine. However, unlike in the process of replication, which once set in, the total DNA of an organism gets duplicated, in transcription only a segment of DNA and only one of the strands is copied into RNA. The promoter and terminator flank the structural gene in a transcription unit. The promoter is said to be located towards 5' -end (upstream) of the structural gene (the reference is made with respect to the polarity of coding strand). It is a DNA sequence that provides binding site for RNA polymerase, and it is the presence of a promoter in a transcription unit that also defines the template and coding strands. By switching its position with terminator, the definition of coding and template strands could be reversed. The terminator is located towards 3' -end (downstream) of the coding strand and it usually defines the end of the process of transcription.
In eukaryotes, the structural genes are split. The sequences that appear in mature or processed RNA are called:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Exons
In eukaryotes, the coding part of a structural gene is not one continuous stretch. It is broken into pieces, so the gene is called a split gene. The whole gene is copied into RNA, and then the non-coding pieces are cut out (splicing) and the coding pieces are joined. Only the coding pieces remain in the mature RNA.
Option (1) Introns — Incorrect. Introns (intervening sequences) lie between the coding pieces. They are removed by splicing, so they do not appear in mature RNA.
Option (2) Exons — Correct. Exons are the coding or expressed sequences. They are joined together after splicing and are the parts that appear in the mature, processed RNA.
Option (3) Cistrons — Incorrect. A cistron is a segment of DNA coding for a polypeptide. The term describes a whole structural gene (monocistronic in eukaryotes, polycistronic in bacteria), not the pieces left in mature RNA.
Option (4) Promoters — Incorrect. The promoter is the DNA sequence where RNA polymerase binds to start transcription. It lies upstream of the structural gene, outside the part that becomes mature RNA.
NCERT Extract:
Transcription Unit and the Gene – “… The 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.”