Lecture on Transcription and Translation

Introduction

• Transcription: DNA is converted into mRNA (messenger RNA).
• Translation: Information in mRNA is used to synthesize proteins.
• Ribosome: The site of protein synthesis in the cell.

Transcription

Overview

• Occurs in three steps: Initiation, Elongation, and Termination.

Steps

1. Initiation:

   • RNA polymerase binds to the promoter region of DNA (TATA box).
     • Promoter region in eukaryotic DNA: TATA box (TATAAA), located 25 nucleotides upstream of transcription start site.
   • RNA polymerase causes DNA strands to separate.

2. Elongation:

   • RNA polymerase adds nucleotides to the growing mRNA strand (5' to 3').
   • RNA polymerase reads DNA strand (3' to 5').
   • Template strand (anti-sense strand): Used to synthesize mRNA.
     • Non-template strand (sense strand): Not used in transcription; sequence matches mRNA except for uracil (U) instead of thymine (T).

3. Termination:

   • RNA polymerase and mRNA strand separate from DNA template strand.
   • Poly-A polymerase enzyme: Adds poly-A tail to 3' end of mRNA.
   • 5' end of mRNA is capped to protect against degradation.
   • Introns (non-coding sequences) are removed via RNA splicing.
   • Exons (coding sequences) remain for protein synthesis.

Practice Problem

• Example of transcribing DNA to mRNA:
  • DNA 3' to 5': G → C, C → G, A → U, T → A (no T in RNA).
  • mRNA 5' to 3': U C A U A U G C

Translation

Overview

• Converts mRNA information into protein.
• Occurs in ribosome (free or attached to rough ER).

Steps

1. Initiation:

   • Start codon (AUG) matches with anticodon (UAC) on tRNA carrying methionine.
   • tRNA enters P site (peptidyl site) of ribosome.

2. Elongation:

   • Incoming tRNA with amino acid enters A site (aminoacyl site).
   • Peptide bond forms between amino acids.
   • tRNA in P site moves to E site (exit site), loses amino acid.
   • Growing polypeptide chain extends out of ribosome.

3. Termination:

   • Stop codons (UAA, UAG, UGA) don’t code for any tRNA.
   • Stop codons cause release factor to enter A site, triggering ribosome disassembly.

Post-Translation

• Completed protein moves to Golgi body for further processing and modification.
• Proteins undergo folding to achieve specific shape and function.

Conclusion

• Overview of transcription and translation processes.

• Thanks for watching!