Class 12 Biology Molecular Basis of Inheritance Transcription


  • The process of copying genetic information from one strand of the DNA into RNA is termed as transcription.
  • In transcription only a segment of DNA and only one of the strands is copied into RNA because
  • if both strands act a template, they would code for RNA molecule with different sequences and the sequences of amino acids in the coded protein would be different.
  • the two RNA molecules would be complementary to each other and would form a double stranded RNA which would prevent translation.


Transcription unit

A  transcription unit consists of

  • A Promoter
  • The Structural gene
  • A Terminator


    A transcription unit (source link- self drawn)


  • The two strands of the DNA in the structural gene of a transcription unit is termed as template strand and coding strand.
  • The strand that has the polarity 3'→5' acts as a template, and is referred as template strand.
  • The other strand which has the polarity (5'→3') is referred 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 which provides binding site for RNA polymerase.
  • The terminator is located towards 3'-end (downstream) of the coding strand which defines the end of the process of transcription.

Structure of a gene

  • A gene is defined as the functional unit of inheritance.
  • A gene also referred as a cistron can be defined 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.
  • Exons are the coding sequences or expressed sequences that appear in mature or processed RNA.
  • Introns are the intervening sequences which interrupt exons and do not appear in mature or processed RNA.


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