Part 5: Transcription

DNA → RNA

Transcription is the process by which genetic information in DNA is copied into RNA. RNA polymerase reads the template strand and synthesizes a complementary RNA molecule. This is the first step in gene expression.

The Transcription Process

1. Initiation

  • • RNA polymerase binds to promoter region
  • • Prokaryotes: σ factor recognizes -10 and -35 boxes
  • • Eukaryotes: TATA box, general transcription factors (TFIIA-H)
  • • DNA unwinds to form transcription bubble

2. Elongation

  • • RNA polymerase moves along template strand (3' → 5')
  • • RNA synthesized 5' → 3'
  • • No primer required
  • • Rate: ~40-80 nucleotides/second

3. Termination

  • • Prokaryotes: ρ-dependent or ρ-independent (hairpin)
  • • Eukaryotes: polyadenylation signal (AAUAAA)
  • • Release of RNA and polymerase

Eukaryotic RNA Processing

5' Capping

7-methylguanosine cap added co-transcriptionally. Protects from degradation, aids translation.

Splicing

Introns removed, exons joined by spliceosome. Alternative splicing increases diversity.

3' Polyadenylation

Poly(A) tail (~200 A's) added. Stability, nuclear export, translation.

Types of RNA

📜
mRNA
Messenger RNA—Carries genetic code for protein synthesis
🔗
tRNA
Transfer RNA—Brings amino acids to ribosome, anticodon recognition
🏭
rRNA
Ribosomal RNA—Structural and catalytic component of ribosomes
✂️
snRNA
Small nuclear RNA—Splicing, part of spliceosome
🎯
miRNA
MicroRNA—Gene regulation, ~22 nt, targets mRNA
📏
lncRNA
Long non-coding RNA—Diverse regulatory roles
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