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The below mentioned article provides a note on the process of transcription.
Transcription is the process of copying genetic information from one strand of the DNA into RNA. In transcription, only a segment of DNA or only one out of the two stands is copied into RNA. Unlike replication, which once set in, the total length of DNA of organisms gets duplicated. In transcription only a segment of DNA or only one of the strands is copied into RNA.
Reasons why both the strand are not copied at the same time during transcription:
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(i) If both the strands code for RNA, two complementary (different) RNA molecules and two different proteins would be formed hence, the genetic information transfer machinery would become complicated.
(ii) Since, the two RNA molecules produced would be complementary to each other, they would bind together to form a double-stranded RNA without carrying out translation.
Transcription Unit:
A transcription unit in DNA is defined by three regions:
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(i) A promoter
(ii) The structural gene
(iii) A terminator
The two strands of DNA have opposite polarity and the enzyme DNA-dependent RNA polymerase catalyses the polymerisation in only one direction (5’→ 3′ direction), the other strand with 3′ → 5′ polarity acts as a template and is known as template strand. The strand with 5’→ 3′ polarity has the same sequence as RNA (except thymine at the place of uracil) is displaced during transcription and is known as coding strand.
(i) Promoter is a DNA sequence that provides binding site for RNA polymerase. It is located at 5′ end (upstream) of structural gene. Its presence defines the template and coding strands.
(ii) Structural gene in a transcription unit is flanked by the promoter and terminator. The definition of coding strand and template strand could be reversed by switching the positions of promoter and terminator.
(iii) Terminator is located at the 3′ end (downstream) of the coding strand. It defines the end of transcription process.
Transcription Unit and the Gene:
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(i) Gene is the functional unit of inheritance. The DNA sequence coding for tRNA or rRNA molecule also define a gene.
(ii) Cistron is a segment of DNA that codes for a polypeptide.
(iii) In a transcription unit, the structural gene could be:
(a) Monocistronic (mostly in eukaryotes).
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(b) Polycistronic (mostly in bacteria or prokaryotes).
(iv) Exons Monocistronic genes in eukaryotes have interrupted coding sequences or expressed sequences as that appear in mature or processed RNA called as exons. Introns or intervening sequences do not appear in mature or processed RNA. They only interrupt exons.
(v) The promoter and regulatory sequences of a structural gene also affect the inheritance of a character. That is why, sometimes the regulatory sequences are loosely defined as regulatory genes.
