📝 Transcription and RNA Processing

Overview

Gene expression: DNA → RNA → Protein

Transcription: DNA → RNA (this topic) Translation: RNA → Protein (next topic)

Transcription Process

Purpose: Synthesize RNA from DNA template

Key enzyme: RNA polymerase

• Does NOT need primer (unlike DNA polymerase)
• Synthesizes RNA 5'→3' direction
• Reads template strand 3'→5'

Three Stages

1. Initiation

• Promoter: DNA sequence where RNA polymerase binds
• TATA box: common promoter element in eukaryotes (~25 bp upstream)
• Transcription factors help RNA polymerase bind (eukaryotes)
• RNA polymerase unwinds DNA

2. Elongation

• RNA polymerase moves along DNA (3'→5' on template)
• Adds RNA nucleotides (5'→3')
• Coding strand (non-template) has same sequence as RNA (except T→U)
• Template strand (antisense) used to make RNA

3. Termination

• Prokaryotes: terminator sequence causes hairpin loop
• Eukaryotes: cleavage signal, polyadenylation signal
• RNA polymerase releases
• RNA transcript complete

Prokaryotic vs. Eukaryotic Transcription

| Feature | Prokaryotes | Eukaryotes | |---------|-------------|------------| | RNA polymerase | One type | Three types (I, II, III) | | Promoter | -10, -35 boxes | TATA box, others | | Processing | None | Extensive | | Location | Cytoplasm | Nucleus | | Coupling | Transcription + translation | Separated |

RNA Processing (Eukaryotes Only)

Primary transcript (pre-mRNA) must be processed before translation

1. 5' Cap

• 7-methylguanosine cap added to 5' end
• Functions:
  • Protects from degradation
  • Helps ribosome recognize mRNA
  • Aids in export from nucleus

2. 3' Poly-A Tail

• ~50-250 adenine nucleotides added to 3' end
• Functions:
  • Protects from degradation
  • Aids in export from nucleus
  • Helps ribosome locate start codon

3. RNA Splicing

• Introns (non-coding) removed
• Exons (coding) joined together
• Carried out by spliceosome (snRNPs + proteins)

Alternative splicing:

• Different combinations of exons
• One gene → multiple proteins
• Increases protein diversity
• ~95% of human genes alternatively spliced

Gene Structure (Eukaryotes)

Gene organization:

• Promoter
• 5' UTR (untranslated region)
• Exons (expressed sequences)
• Introns (intervening sequences)
• 3' UTR
• Terminator

Types of RNA

1. mRNA (messenger RNA)

• Carries genetic information DNA → ribosome
• Translated into protein
• ~5% of total RNA

2. rRNA (ribosomal RNA)

• Structural and catalytic component of ribosomes
• Most abundant RNA (~80%)

3. tRNA (transfer RNA)

• Brings amino acids to ribosome
• Has anticodon that pairs with mRNA codon
• ~15% of total RNA

4. Other RNAs

• snRNA: splicing (in snRNPs)
• miRNA: gene regulation (microRNA)
• siRNA: gene silencing (small interfering RNA)

Key Concepts

1. RNA polymerase synthesizes RNA 5'→3', reads DNA 3'→5'
2. Promoter is where transcription starts
3. Template strand is copied; coding strand has same sequence as RNA
4. Eukaryotic processing: 5' cap, poly-A tail, splicing
5. Introns removed, exons joined
6. Alternative splicing increases protein diversity
7. Three main RNAs: mRNA (message), tRNA (transfer), rRNA (ribosomal)