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.
