Lecture Notes: Transcription and Initiation Phase

Central Dogma Overview

• First step is the conversion from DNA to RNA, termed transcription.
• Occurs in the nucleus in eukaryotes because DNA does not leave the nucleus.
• In prokaryotes, transcription occurs in the cytoplasm.

Phases of Transcription

1. Initiation Phase
2. Elongation Phase
3. Termination Phase

Key Focus: Initiation Phase

• Initiation is a critical and widely tested aspect of transcription.
• Process involves the DNA template coding for RNA, primarily protein synthesis.

Promoter Region

• Begins the initiation process.
• Contains a promoter sequence, often a consensus sequence or TATA box (thymine-adenine-thymine-adenine sequence).
• Present in promoters of over 25% of eukaryotic genes.

Transcription Factors

• Proteins that bind specific DNA sequences, recognizable by the naming convention "TF" followed by a number and letter (e.g., TF2A).
• Bind to the promoter to form an initiation complex.

Initiation Complex

• Transcription Factors gather at the promoter and attract RNA polymerase.
• RNA polymerase unzips double-stranded DNA to form a transcription bubble.
• Involves two DNA strands:
  • Plus/Sense/Coding Strand: Resembles RNA code.
  • Minus/Antisense/Template Strand: Used by polymerase to produce RNA.

DNA Reading Direction

• Read from 3' to 5' to produce RNA strand from 5' to 3'.
• Downstream region (closer to 5') is the coding region for RNA transcription.

Regulatory Regions

• Activators and Repressors:
  • Located near the promoter.
  • Activators: Increase likelihood of transcription.
  • Repressors: Decrease likelihood.
• Enhancers:
  • Further upstream, enhance initiation complex formation.
  • Control gene expression levels.
  • Presence of proteins binding these regions modulates expression.

Importance

• Gene expression levels are regulated through these mechanisms.
• Allows for differential expression based on protein needs.

Transition to Elongation Phase

• Key event in initiation is forming the initiation complex.
• RNA polymerase joins, forms the transcription bubble, and prepares for elongation.