Page 11 - CH6 (1)

Transcriptional Unit

● A transcriptional unit has primarily three regions:
○ Promoter − Marks the beginning of transcription; RNA

polymerase binds here
○ Structural gene − Part of the DNA that is actually transcribed

○ Terminator − Marks the end of transcription

Template Strand and Coding Strand

● Enzyme involved in transcription, RNA polymerase (DNA dependent
RNA polymerase), catalyses in only one direction i.e., 5′ to 3′.

● Therefore, the strand with polarity 3′ ​​′ acts as a template

5
→
(Template Strand).
3

● The strand with polarity 5′ ​​′ acts as coding strand (which is a
→
misnomer since it does not code for anything). Coding strand has
sequence similar to RNA formed after transcription except for the
change that thymine is present instead of uracil.

Gene

● The DNA sequence which codes for tRNA or rRNA molecule defines a
gene.

● Cistron − Segment of DNA that contains the genetic code for a single
polypeptide
● The structural genes could be of two types:

○ Monocistronic (mostly in eukaryotes)
○ Polycistronic (mostly in prokaryotes)

● Monocistronic genes have two parts:
○ Exon − Sequences that code for a particular character and is
expressed in a matured and processed mRNA

○ Intron − Interrupting sequences that do not appear in a mature