Lecture Notes: Transcription and Translation

Overview

• Transcription: Process by which DNA is converted into mRNA (messenger RNA).
• Translation: Process of converting information in mRNA into a protein.
• Ribosome: Site where proteins are manufactured within the cell.

Transcription

Three Steps of Transcription

1. Initiation:

   • RNA polymerase binds to the promoter region of DNA.
   • Promoter region: Short DNA sequence, in eukaryotic DNA, is TATAAA (TATA box).
   • Located 25 nucleotides upstream of transcription start site.

2. Elongation:

   • RNA polymerase separates DNA strands.
   • Adds nucleotides to the growing mRNA strand (5' to 3' direction).
   • Reads DNA template strand in 3' to 5' direction.
   • Template Strand: Also called anti-sense strand, used for mRNA synthesis.
   • Non-template Strand: Also called sense strand or coding strand (sequence matches mRNA with uracil instead of thymine).

3. Termination:

   • RNA polymerase and mRNA strand separate from DNA template.
   • Poly-A polymerase adds a poly-A tail to 3' end of mRNA.
   • 5' end is capped to protect mRNA from degradation.
   • Results in pre-mRNA with introns and exons.

RNA Splicing

• Introns: Non-coding sequences, removed during splicing.
• Exons: Coding sequences, remain in the mRNA to synthesize proteins.

Practice Problem

• Constructing mRNA from a DNA template:
  • G pairs with C, C pairs with G.
  • A pairs with U (in RNA), T pairs with A.
  • Example: DNA 3' to 5' read results in mRNA 5' to 3'.

Translation

Process Overview

• mRNA synthesized in nucleus, enters cytosol, interacts with ribosome.
• Ribosome: Composed of small and large subunits, with sites E, P, and A.
• tRNA (transfer RNA): Interacts with mRNA within ribosome.
• Codons: Sets of three nucleotides on mRNA, match with tRNA anticodons.
• Each codon corresponds to a specific amino acid.

Three Steps of Translation

1. Initiation:

   • Start codon (AUG) matches anticodon (UAC) on tRNA with methionine amino acid.
   • tRNA enters ribosome at P site.

2. Elongation:

   • New tRNA enters A site.
   • Covalent bond forms between amino acids (peptide bond formation).
   • tRNA in P site moves to E site and exits, losing amino acid.
   • Polypeptide chain grows out of ribosome.

3. Termination:

   • Stop codons: UAA, UAG, UGA (nonsense codons).
   • Stop codons do not code for tRNA but cause release factor entry.
   • Ribosomal subunits disassemble, protein leaves ribosome.

Protein Processing

• Proteins move to Golgi body for further processing and modification.
• Undergo folding to form specific shapes for specific functions.

Conclusion

• This lecture provided an overview of transcription and translation processes.