Notes on Protein Synthesis Lecture

Overview of Cells

• Basic unit of all living tissue.
• Structure called nucleus contains the genome.
• Genome split into 23 pairs of chromosomes.
• Chromosomes consist of DNA wrapped around proteins called histones.
• Sections of DNA known as genes contain instructions for making proteins.

Transcription Process

1. Activation of RNA polymerase:

   • When a gene is activated, RNA polymerase attaches to the start of the gene.
   • Moves along DNA to create a strand of messenger RNA (mRNA).
   • The DNA code determines the sequence of bases in mRNA.

2. Processing of mRNA:

   • Involves removing and adding sections of RNA.
   • Modified mRNA exits nucleus to enter cytoplasm.

Translation Process

• Occurs in the cytoplasm at ribosomes which read mRNA to produce proteins.
• Amino acids: There are 20 different types; brought to ribosomes by transfer RNA (tRNA).

Stages of Translation

1. Initiation Stage:

   • Small ribosomal subunit binds to mRNA strand.
   • Recognizes AUG codon (start codon).
   • Met-tRNA pairs with AUG codon.
   • Large ribosomal subunit combines with small subunit to form initiation complex.

2. Elongation Stage:

   • New tRNA carrying an amino acid enters the A site of the ribosome.
   • A bond forms between amino acids.
   • Ribosome moves along mRNA, shifting tRNA to E site (exit site).
   • Process repeats to form an amino acid chain.

3. Termination Stage:

   • Occurs when stop codon (UAG) reaches A site.
   • Amino acid chain is released as a protein.
   • Ribosomal subunits dissociate.

Gene Expression

• Instructions to make proteins are contained within DNA.
• Gene expression involves:
  • Transcription: Occurs in the nucleus, where RNA polymerase synthesizes mRNA.
  • Translation: Occurs in cytoplasm, where mRNA is translated into polypeptides.

Transcription Stages

1. Initiation:
   • RNA polymerase binds to DNA at the promoter.
   • DNA unwinds.
2. Elongation:
   • RNA polymerase synthesizes mRNA by linking nucleotides.
3. Termination:
   • RNA polymerase reaches the terminator, releasing mRNA.

mRNA Processing

• Involves removal of introns (non-coding sections) and addition of 5' cap and 3' poly-A tail.
• Performed by spliceosome to produce mature mRNA.

Codons and Genetic Code

• Codons are three-letter codes; 64 codons total.
• Four special codons: 1 start, 3 stop.
• Translation begins with binding of mRNA to ribosome; tRNA brings amino acids.

Ribosome Structure

• Composed of large and small subunits.
• Contains three sites: A site, P site, E site.
• Amino acids are added in a cycle involving codon-anticodon matching.

Final Steps of Translation

• Release factor binds at stop codon, releasing the polypeptide.
• Ribosomal subunits and tRNA detach from mRNA.

Importance of Proteins

• Proteins are essential for physiological functions (e.g., digestion).
• mRNA carries genetic information from nucleus to ribosomes for protein synthesis.

Summary

• Protein synthesis involves transcription and translation.
• The process ensures the accurate production of proteins necessary for life.