Lecture 42: Genes and Proteins

Central Dogma of Molecular Biology

• Previously covered transcription
• Upcoming topics include translation

Types of RNA

1. rRNA (Ribosomal RNA)
   • Structural component of ribosomes
   • Enzymatic role in ribosome structure
2. tRNA (Transfer RNA)
   • Structural role in translation
   • Will be detailed in translation section
3. mRNA (Messenger RNA)
   • Carries genetic blueprint
   • Directs protein synthesis

Genetic Code

• Composed of nucleotides: adenine (A), guanine (G), cytosine (C), and uracil (U) in RNA
• DNA has thymine (T) instead of uracil
• 20 amino acids, not a 1:1 or 2:1 ratio, but 3:1
  • Codons: groups of three nucleotides
  • Redundancy: Multiple codons can encode the same amino acid (degeneracy)
  • Start codon: AUG (also codes for methionine)
  • Stop codons: 3 types; signal the end of translation
• Genetic code is almost universally shared among organisms

Translation

• Conducted by Ribosomes
• Three phases:
  1. Initiation
     • Small ribosomal subunit binds to mRNA
     • tRNA with methionine binds to start codon
     • Large ribosomal subunit joins
  2. Elongation
     • tRNAs enter one by one, matching anticodon to codon
     • Amino acids are linked by peptide bonds
     • Empty tRNA exits ribosome
  3. Termination
     • Stop codon reached, release factor enters
     • Ribosome dissociates, releasing protein and mRNA
• mRNA can be reused for multiple translations

Comparisons

• Prokaryotes: Simultaneous transcription and translation
• Eukaryotes: Transcription (nucleus) and translation (cytoplasm) are separate

Mutations

1. Point Mutations
   • Change in a single base pair
   • Silent Mutation: No change in protein sequence
   • Missense Mutation: Change in one amino acid
   • Nonsense Mutation: Change to stop codon, truncates protein
2. Frameshift Mutations
   • Insertion or deletion not in multiples of three
   • Alters reading frame, changes entire protein sequence downstream

Gene Expression

• Gene Expression: Process from DNA to functional protein
• Genes can be 'on' (expressed) or 'off' (not expressed)
• Different cells express different genes
• Expression responds to environmental changes

Regulation of Gene Expression

• Control gene expression primarily at the transcription stage in prokaryotes
• Transcription Factors: Proteins influencing transcription
  • Repressors: Decrease transcription
  • Activators: Increase transcription
• Eukaryotes regulate at multiple stages: transcription, translation, and post-translation

Next Steps

• Detailed exploration of transcription regulation using repressors and activators in the next lecture.