Lecture on Protein Synthesis in Prokaryotic Cells

Key Concepts

• Protein synthesis is essential in both procaryotic and eukaryotic cells.
• The processes are largely similar, but the ribosomes and enzymes differ slightly.

Players in Protein Synthesis

1. DNA
   • Contains all codes for protein synthesis.
2. Messenger RNA (mRNA)
   • Single-stranded copy of DNA's gene.
   • Shorter than DNA.
3. Ribosome
   • Decoding assembly station for proteins.
   • Made of proteins and ribosomal RNA (rRNA).
4. Transfer RNA (tRNA)
   • Carries amino acids to build polypeptides.

Differences in Ribosomes

• Procaryotic Ribosome: 70S (Small subunit: 30S, Large subunit: 50S)
• Eukaryotic Ribosome: 80S (Small subunit: 40S, Large subunit: 60S)
• Sedimentation coefficient (S) explains why 50 + 30 ≠ 70.

Protein Synthesis Process

1. Transcription

   • DNA is too large and important, so a copy is made as mRNA.
   • Involves unwinding DNA and creating a complementary mRNA strand.
   • Enzymes:
     • Helicase: Unwinds DNA.
     • RNA Polymerase: Synthesizes mRNA.

2. Translation

   • mRNA is translated into a functional protein.
   • Involves reading mRNA in codons (sets of three nucleotides).

Steps of Transcription

1. Initiation
   • Helicase unwinds DNA, exposing promoter region (TATA box).
   • RNA polymerase binds and starts synthesizing mRNA.
2. Elongation
   • RNA polymerase moves along DNA, building mRNA.
   • Corresponding nucleotides: C-G, G-C, A-U, T-A.
3. Termination
   • Stop sequences on DNA signal end of transcription.

Steps of Translation

1. Initiation
   • mRNA binds to ribosome.
   • Start codon (AUG) recognized, methionine is first amino acid.
2. Elongation
   • TRNA brings amino acids matching mRNA codons, forming peptide bonds.
   • Amino acids added sequentially to growing polypeptide chain.
3. Termination
   • Stop codons signal end of translation.
   • Newly formed protein is released from ribosome.

Additional Notes

• In Prokaryotic Cells: No introns/exons, no nucleus.
• In Eukaryotic Cells: Introns and exons present, transcription occurs in nucleus.
• Codon Table
  • 20 amino acids, multiple codons can code for each.
  • Start codon: AUG (methionine), common stop codons: UAA, UAG, UGA.

Gene Expression and Regulation

• Gene expression determines when proteins are synthesized.
• Additional resources such as videos can help visualize these processes.