Question

Mention the function of promoter gene in transcription.

Answer 1

Transcription cannot start by accident, but must start exactly from the beginning of the gene. The signal that initiates transcription appears in the promoter sequence located immediately above the sequence of the transcribed gene. The promoter contains a specific DNA sequence that serves as the binding point for RNA polymerase.

Complete answer:
Transcription is the process of synthesizing ribonucleic acid (RNA) from DNA. The word gene refers to a transcribed functional unit of DNA.
Transcription involves three steps :-
-Initiation
-Elongation
-Termination

Role of promoter in initiation process:
Promoter: A promoter is a regulatory region of DNA located towards the $5' -$ region (Upstream) of a gene and is a checkpoint for the transcription of a regulated gene. There are specific DNA sequences present in the Promoter region that are recognized by proteins called transcription factors. These factors bind to the promoter sequence and mobilize RNA polymerase, an enzyme that synthesizes RNA in the coding region of a gene .
In prokaryotes, recognition sequence is present upstream for RNA polymerase binding. E. coli is recognized by RNA polymerase and contains two contiguous elements known as sequence $- 10$ or Pribnow box and $- 35$ sequence.
In eukaryotes , TATA box or Hogness box located $20$ bp upstream to the start point , another sequence is CAAT box present between $- 70$ and $- 80$ bp.

Note:
TATA Box: Changes in transcription rate often regulate gene expression. The interaction between RNA polymerase II and the main transcription factor forms a transcription initiation complex (TIC) in the TATA box. About $25$ to $30$ bases are located upstream of the start of transcription and consist of an $8$ base pairs consensus sequence. It consists only of the A = T base pair (TATAAA), but is surrounded on both sides by a region rich in G = C. Mutations in the TATA plane can reduce transcription or change the origin.
CAAT Box: Many promoters contain different components and have a consistent sequence, such as GGCCAATC, which is in the $70 - 80$ bp range. in their hometown and can function in the $5' - 3'$ or $3' - 5'$ direction. Analysis of mutations shows that the CAAT box plays the greatest role in determining the effect of the promoter.
GC box: Another factor that is common to some of the promoter regions, called GC boxes, is consensus.
Shine Dalgarno sequence: It is $5' -$ AGGAGGU$- 3'$ sequence at $5' -$ end near the initiation codon in prokaryotes .It helps in binding of $30$ A subunit of ribosome on it.