Lecture Notes: Transcription and Translation

Overview

• Transcription: Process where DNA is converted into mRNA (messenger RNA).
• Translation: Process where information in mRNA is used to synthesize proteins.
• Ribosome: The cellular organelle where proteins are manufactured.

Transcription

Steps of Transcription

1. Initiation

   • RNA polymerase binds to the promoter region of DNA.
   • Promoter Region: A short DNA sequence (TATA box in eukaryotic DNA) located 25 nucleotides upstream of transcription start.
   • RNA polymerase separates the two DNA strands.

2. Elongation

   • RNA polymerase adds nucleotides to the growing mRNA strand.
   • Synthesis direction: 5' to 3', while reading the DNA template in 3' to 5'.
   • Template Strand (Anti-sense Strand): The strand that is used to synthesize mRNA.
   • Non-template Strand (Sense/Coding Strand): Matches RNA sequence except thymine (T) is replaced by uracil (U) in RNA.

3. Termination

   • RNA polymerase, mRNA, and DNA template strand separate.
   • Poly A Polymerase: Adds a poly A tail to 3' end of mRNA for protection.
   • 5' end is also capped for protection from degradation.

Post-Transcriptional Modifications

• Pre-mRNA: Initially created, contains introns and exons.
• Introns: Non-coding sequences, removed via RNA splicing.
• Exons: Coding sequences, retained for protein synthesis.

Practice Problem

• DNA to mRNA Matching
  • G corresponds to C, A to U, T to A, etc.

Translation

Overview

• Converts mRNA sequence to a protein.
• Occurs in the cytosol where mRNA interacts with ribosomes and tRNA.

Structure and Process

• Ribosome: Consists of small and large subunits with active sites E, P, and A.

• Initiation

  • Start codon AUG pairs with tRNA anticodon UAC.
  • tRNA carrying methionine enters P site.

• Elongation

  • New tRNA enters A site.
  • Peptide bonds form between amino acids.
  • tRNA moves from P site to E site, losing amino acids.
  • Growing polypeptide chain forms, extending out of the ribosome.

• Termination

  • Occurs when stop codons (UAA, UAG, UGA) are read.
  • Release factor binds, causing ribosome subunits to disassemble.
  • Protein is released and transported to Golgi body for modification.

Key Concepts

• Codons and Anticodons: Sets of three nucleotides that dictate amino acids.
• Stop Codons (Nonsense Codons): Signal end of translation.
• Polypeptide Chain: Result of translation, later modified into functional proteins.

Final Notes

• This lecture provided an overview of two essential biological processes: transcription and translation.