14. Transcription. RNA polymerases. Stages of transcription.

Question 1

Principle of transcription

Answer 1

General Characteristics:

-Direction: 5’ to 3’ (template strand is 3’ to 5’).
-Stages: Initiation, elongation, termination.

Differences from Replication:
-Continuity: Transcription is continuous.
-Template: Runs on a specific DNA segment (structural gene).

Requirements:
-Components: DNA template, ATP, GTP, CTP, UTP, RNA polymerase, specific proteins, topoisomerases.

Transcription Unit:
-Definition: DNA segment transcribed into RNA.
-Regions: Promoter, RNA-coding region, terminator.
-Gene Content: May contain multiple genes.
-Control: Single promoter can control several structural genes.

Question 2

DNA dependent RNA polymerase.

Answer 2

Activities:
-Separates DNA strands
-Uses one strand as a template
-Catalyzes covalent bond formation
-Regulates gene activity
Eukaryotic RNA Polymerases:
-RNA Polymerase I: Transcribes rRNA genes
-RNA Polymerase II: Transcribes mRNA and small nuclear RNAs
-RNA Polymerase III: Transcribes tRNA and 5S rRNA genes

Prokaryotic RNA Polymerase:
-Holoenzyme: Multi-subunit complex, binds to promoter, initiates transcription with the first phosphodiester bond

Eukaryotic RNA Polymerase:
-Core Enzyme: Performs catalytic activity but cannot recognize the promoter
-Transcription Factors: Bind to promoter and recruit RNA polymerase for elongation

Question 3

Stages of transciption

Answer 3

Initiation:

1. Promoter Recognition:
   -Prokaryotes: RNA polymerase recognizes promoter.
   -Eukaryotes: Transcription factors help recognition.
   -Note: No primer needed.
2. Binding and Opening:
   -RNA polymerase binds, opens DNA, and forms first phosphodiester bond.
3. Start Point: First transcribed nucleotide.

Elongation:
1. Addition: RNA polymerase adds nucleotides.
2. Energy: From nucleoside triphosphates (NTPs).
3. DNA Reformation: DNA re-forms helix after transcription.

Termination:
Prokaryotes:
-Recognizes terminator, forms “hairpin” structure, dissociates complex.

Eukaryotes:
-mRNA precursors cleaved at 3′-untranslated region, followed by polyadenylation.