Process of Transcription - Practice Questions & MCQ

• 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 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 DNA strand which is copied is called the template strand (3’ to 5’). The other strand is called the non-template strand or coding strand (5’ to 3’).
• The newly synthesized RNA is complementary to the template strand and almost identical to the non-template strand.
• The reasons why both the strands of DNA are not transcribed are:
  • If both strands act as a template, they would code for RNA molecules with different sequences, and in turn, if they code for proteins, the sequence of amino acids in the proteins would be different. Hence, one segment of the DNA would be coding for two different proteins, and this would complicate the genetic information transfer machinery.
  • The two RNA molecules if produced simultaneously would be complementary to each other, hence would form a double-stranded RNA. This would prevent RNA from being translated into protein and the exercise of transcription would become a futile one.

Transcription Unit:

• The segment of DNA that takes part in transcription is called transcription unit.
• It is defined primarily by the three regions in the DNA:
  • A Promoter: A promoter is a DNA sequence onto which the transcription machinery binds and initiates transcription.
  • The Structural gene
  • A Terminator
• The nucleotide pair in the DNA double helix that corresponds to the site from which the first 5' mRNA nucleotide is transcribed is called the +1 site, or the initiation site.
• Nucleotides preceding the initiation site are given negative numbers and are designated upstream.
• nucleotides following the initiation site are denoted with positive numbering and are called downstream nucleotides.
• The promoter is said to be located towards 5' -end (upstream) of the structural gene.
• The terminator is located towards 3' -end (downstream) of the coding strand and it usually defines the end of the process of transcription.