Lecture Notes: Transcription and Translation

Overview

• Transcription: Conversion of DNA into mRNA (messenger RNA)
• Translation: Using mRNA to build a protein
• Ribosome: Organelle that manufactures proteins in the cell

Transcription

Steps of Transcription

1. Initiation
   • RNA polymerase binds to the promoter region of DNA
   • Promoter Region: Short sequence of DNA (TATA box at 25 nucleotides upstream from transcription start site)
2. Elongation
   • RNA polymerase separates DNA strands
   • Adds nucleotides to the growing mRNA strand (synthesizes mRNA from 5' to 3')
   • Reads DNA template strand from 3' to 5'
   • Template Strand/Anti-sense Strand: Used by RNA polymerase to synthesize mRNA
   • Non-template Strand/Sense Strand/Coding Strand: Not used, matches mRNA sequence with uracil (U) instead of thymine (T)
3. Termination
   • RNA polymerase, mRNA strand, and DNA template separate
   • Poly-A polymerase adds a poly-A tail to the 3' end
   • 5' end is capped for protection from degradation

Pre-mRNA Processing

• Contains introns (non-coding) and exons (coding sequences)
• Introns must be removed via RNA splicing
• Exons remain to synthesize proteins

Practice Problem Example

• DNA sequence read from 3' to 5' to write corresponding mRNA sequence in 5' to 3'
  • G => C, C => G, A => U, T => A
  • Example: If DNA = GCTAT, mRNA = CGAUA

Translation

Process Overview

• mRNA strand leaves nucleus & enters cytosol
• Interacts with ribosome (free or attached to rough ER)
• Involves tRNA (transfer RNA)

Codons and Anticodons

• mRNA nucleotides grouped in sets of three (codons)
• tRNA nucleotides grouped in sets of three (anticodons), each codon matches a specific amino acid

Steps of Translation

1. Initiation
   • Start codon (AUG) corresponds to anticodon (UAC)
   • tRNA molecule with methionine amino acid attaches at P site of ribosome
2. Elongation
   • New tRNA enters at A site and forms covalent bonds with amino acids
   • tRNA in P site moves to E site and exits losing amino acid
   • Amino acid chain grows
3. Termination
   • Stop codons (UAA, UAG, UGA) cause release factors to dismantle ribosomal subunits
   • Protein is released for processing in Golgi body

Post-Translational Modification

• Protein undergoes folding at Golgi body to form specific shapes for function

Summary

• Overview of key processes for protein synthesis: transcription and translation
• Detailed mechanisms, steps, and involved molecules in both processes