Protein Synthesis Detailed Notes

Key Concepts in Protein Synthesis

Overview of DNA and Protein Synthesis

• DNA Structure:

  • Consists of nitrogenous bases: Adenine (A), Cytosine (C), Guanine (G), and Thymine (T).
  • Base pairing rules:
  • A-T and C-G pairing in DNA.
  • In RNA (which has uracil - U instead of thymine), A pairs with U and C pairs with G.
  • DNA is located in the nucleus and serves as a blueprint for proteins.

• Function of Proteins:

  • Proteins are essential for cellular structure and function, acting as enzymes, structural components, and signaling molecules.
  • Proteins are synthesized based on the information encoded in DNA.

Steps in Protein Synthesis

1. Transcription

• Definition: The process of copying a segment of DNA into mRNA (messenger RNA).
• Process:
  • RNA polymerase enzyme binds to the DNA and separates the strands.
  • The DNA strand serves as a template for mRNA synthesis. The base pairing occurs:
  • A-U (in RNA), U-A, C-G, G-C.
  • Result: mRNA molecule is synthesized and moves from the nucleus to the cytoplasm.

2. Translation

• Definition: The process where the mRNA is translated into a polypeptide (protein).

• Key Components:

  • Ribosomes:
  • Composed of ribosomal RNA (rRNA) and proteins.
  • Contains A, P, and E sites:
    • A site: binds new aminoacyl-tRNA.
    • P site: holds the tRNA with the growing polypeptide chain.
    • E site: where tRNA exits after delivering its amino acid.
  • tRNA (transfer RNA):
  • Has an anticodon that pairs with the mRNA codon and carries a specific amino acid.
  • Structure includes a cloverleaf shape with an amino acid attached to the 3' end.

• Process:

  • Initiation:
  • mRNA, ribosomal subunits, and initiator tRNA come together at the start codon (AUG).
  • Elongation:
  • The ribosome facilitates successive tRNA binding to the mRNA, causing peptide bonds to form between amino acids and elongating the polypeptide chain.
  • Codon recognition, peptide bond formation, and translocation occur.
  • Termination:
  • A release factor binds to the A site when a stop codon is reached (UAA, UAG, UGA), leading to the release of the polypeptide chain.

mRNA Code

• Codons:
  • Codons are sequences of three nucleotides (triplets) that correspond to specific amino acids.
  • Redundant nature of the genetic code allows multiple codons to encode the same amino acid (mutation insurance).
  • Important codons include:
  • Start codon: AUG (Methionine)
  • Stop codons: UAA, UAG, UGA

tRNA Loading and Amino Acid Activation

• Amino Acid Activation:
  • An enzyme called aminoacyl-tRNA synthetase attaches the appropriate amino acid to its corresponding tRNA, utilizing ATP.
• Function:
  • Activates the amino acid for incorporation into the growing polypeptide chain at the ribosome.

Summary of Transcription and Translation

• In transcription:
  • mRNA is synthesized in the nucleus and is processed (capping, polyadenylation).
  • Introns (non-coding sequences) are removed, and exons (coding sequences) are joined.
• In translation:
  • mRNA is translated in the cytoplasm into a functional polypeptide, forming the basis for cellular function and structure.