Lecture 42: Genes and Proteins Chapter

Central Dogma of Molecular Biology

• Previously discussed Transcription
• Upcoming discussion on Translation

Three Main Types of RNA

1. rRNA (Ribosomal RNA)
   • Structural component of ribosomes
   • Not a blueprint or message
   • Plays a physical and enzymatic role
2. tRNA (Transfer RNA)
   • Plays a structural role in translation
   • Will be discussed further during translation
3. mRNA (Messenger RNA)
   • Acts as a blueprint for protein synthesis
   • Contains information that directs protein synthesis

Genetic Code

• Nucleotides in DNA: Adenine, Guanine, Thymine, Cytosine
• Nucleotides in RNA: Adenine, Guanine, Cytosine, Uracil
• Amino Acids: 20 different kinds
• Genetic Code: Based on sets of three nucleotides known as codons
  • 64 possible combinations of codons (4^3)
  • Redundancy: Multiple codons may encode the same amino acid (termed 'degeneracy')
  • Special codons:
    • Start Codon (AUG): Signals start of protein synthesis; encodes Methionine
    • Stop Codons (UAA, UAG, UGA): Signal end of protein synthesis; do not encode amino acids
• Universality: Genetic code is almost universal across living organisms

Translation

• Ribosomes: Enzyme structures responsible for translation
• Phases of Translation:
  1. Initiation
     • Small ribosomal subunit binds to mRNA
     • Initiator tRNA carrying Methionine binds to start codon
     • Large ribosomal subunit binds
  2. Elongation
     • tRNAs enter ribosome according to mRNA sequence
     • Formation of peptide bonds between amino acids
     • Empty tRNAs exit and next tRNA enters
  3. Termination
     • Stop codon signals release factor to dissociate ribosome and release protein
     • mRNA can be translated multiple times

Differences in Prokaryotes and Eukaryotes

• Prokaryotes: Simultaneous transcription and translation
• Eukaryotes: Transcription in nucleus, translation in cytoplasm

Mutations

• Point Mutations: Changes in a single base pair
  1. Silent: No change in amino acid sequence
  2. Missense: Changes one amino acid; can affect protein function
  3. Nonsense: Changes amino acid to stop codon; results in shorter protein
• Frame Shift Mutations: Addition or removal of nucleotides; alters reading frame

Gene Expression

• Definition: Process of transcribing DNA to mRNA and translating to proteins
• Genes can be "on" (expressed) or "off" (not expressed)
• Differential Expression: Different cell types express different genes for specific functions
• Regulation: Essential for responding to environmental changes

Regulation of Gene Expression

• Prokaryotes: Predominantly control at transcriptional level
  • Use transcription factors: repressors and activators
• Eukaryotes: Regulation can occur at transcriptional, translational, and post-translational stages

Next Steps: Continued exploration of gene expression regulation in next lecture.