Schematic Structure of a Transcription Unit: It is a sequence of nucleotides in DNA that codes for a single RNA molecule, along with the sequences necessary for its transcription; normally contains a promoter, an RNA-coding sequence, and a terminator.

Transcription

• Transcription is the process of copying genetic information from one strand of the DNA into RNA. The principle of complementarity governs the process of transcription, except the adenosine now forms base pair with uracil instead of l thymine.
• In transcription, only a segment of DNA is duplicated and on Iv one of the strands is copied into RNA. Both the strands are not copied.
• A transcription unit in DNA is defined by three regions in the DNA which are as follows:
  (a) A promoter: It is the building size for RNA polymerase for initiation of transcription.
  (b) The structural gene: It codes for enzyme or protein for structural functions.
  (c) A terminator: It is the region where transcription ends.
• The two strands of DNA have opposite polarity and the DNA-dependent RNA polymerase also catalyse the polymerisation in only one direction that is 5′ -» 3′.
• The strand that has the polarity 3′-» 5′ acts as a template and is referred to as template strand. The other strand which has the polarity (5′ -> 3′) and the sequence same as RNA (T at the place of U) is displaced during transcription. This strand is called as coding strand.
• The promoter and terminator flank the structural gene in a transcription unit.
• The promoter is located towards 5′ end (upstream) of the structural gene.
• It is the DNA sequence that provides binding site for RNA polymerase and the presence of promoter 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 3f-end (downstream) of the coding strand and it usually defines the end of the process of transcription.
• There are additional regulatory sequences that may be present further upstream or downstream to the promoter.

Schematic Structure of a Transcription Unit

• A gene can be defined as the functional unit of inheritance.
• A cistron is 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).
• The coding sequences or expressed sequences are defined as exons. Exons appear in mature or processed RNA. The exons are interrupted by introns.
• Introns or intervening sequences do not appear in mature or processed RNA.
• Sometimes, the regulatory sequences are loosely defined as regulate even though these sequences do not code for any RNA or protein.

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By Team Learning Mantras