Lecture on Protein Synthesis - D 1.2

Overview

• Protein synthesis occurs in two major steps: Transcription and Translation.
• This lecture focuses on the details of these steps and their significance in gene expression.

Transcription

• Definition: The process of using DNA as a template to create RNA.
• Key Enzyme: RNA polymerase.
  • Separates DNA strands (similar to helicase in DNA replication).
  • Synthesizes a strand of mRNA by following the rules of complementary base pairing.
• Complementary Base Pairing in RNA:
  • Adenine (A) pairs with Uracil (U) instead of Thymine (T).
  • Cytosine (C) pairs with Guanine (G).
• Sense and Antisense Strands:
  • Sense strand contains genetic information but is not used as a template.
  • Antisense strand is used as a template for transcription.
• Stability:
  • DNA's double-stranded form is stable, resulting in fewer mutations.
  • Mutations are more likely when strands are separated.
• Gene Expression:
  • Involves transcription followed by translation.
  • Not all genes are expressed in every cell; expression depends on cell type and function.

Translation

• Definition: Process of synthesizing polypeptides from mRNA.
• mRNA:
  • Acts as a messenger carrying genetic information to ribosomes.
• Ribosome Structure:
  • Composed of large and small subunits.
  • mRNA binds to the small subunit.
  • Large subunit contains sites for tRNA binding and peptide bond formation.
• tRNA:
  • Transfers specific amino acids to the ribosome.
  • Has an anticodon that pairs with mRNA codon.
• Codons and Anticodons:
  • mRNA codons are read in triplets.
  • tRNA anticodons pair complementarily with mRNA codons.
• Genetic Code:
  • Universal and degenerate (multiple codons can code for the same amino acid).
  • Important for understanding genetic mutations.

Translation Process

• Initiation: mRNA attaches to the small subunit of the ribosome, and tRNA binds to the start codon.
• Elongation:
  • Ribosome moves along mRNA, and polypeptide chain elongates as amino acids are added.
  • Peptide bonds form between amino acids.
• Termination:
  • Process ends when a stop codon is reached.

Genetic Code Features

• Triplet Code: Each amino acid is coded by a sequence of three bases.
• Universal: Same codons produce the same amino acids across all organisms and viruses.
• Degenerate: One amino acid can be coded by multiple codons, providing resilience against mutations.

Mutations

• Base Substitution:
  • Change in one base can potentially alter the amino acid sequence.
  • May or may not affect protein function (depends on whether the altered codon still codes for the same amino acid).
  • Can cause conditions like sickle cell anemia.

Conclusion

• Continuity and Change: Genetic information continuity is ensured by complementary base pairing, while mutations introduce changes.
• Understanding transcription and translation is vital for comprehending gene expression and genetic mutations.