Protein Synthesis


 Protein synthesis is the cellular process through which proteins are made. This complex process occurs in two main stages: transcription and translation.

 1. Transcription:

  • Location: Occurs in the nucleus (eukaryotes) or nucleoid (prokaryotes).
  • Initiation: RNA polymerase binds to the promoter region of DNA. The DNA strands separate.
  • Elongation: RNA polymerase reads the DNA template strand and synthesizes a complementary mRNA strand using nucleotides.
  • Termination: RNA polymerase reaches a terminator sequence, and mRNA is released.

 2. mRNA Processing (Eukaryotes):

  • Capping: A 5' cap (methyl guanosine triphosphate) is added to the mRNA's 5' end.
  • Polyadenylation: A poly-A tail (adenylate residues) is added to the mRNA's 3' end.
  • Splicing: Introns (non-coding regions) are removed, and exons (coding regions) are joined together to form mature mRNA.

 3. Translation:

  • Location: Takes place in the cytoplasm on ribosomes.
  •  Initiation: mRNA binds to the small ribosomal subunit. The initiator tRNA with methionine binds to the start codon (AUG). The large ribosomal subunit joins, forming a functional ribosome.
  • Elongation: The ribosome moves along the mRNA, and tRNAs bring amino acids according to the mRNA codons. Peptide bonds form between adjacent amino acids, creating a growing polypeptide chain.
  • Termination: A stop codon (UAA, UAG, or UGA) is reached. Release factors bind to the ribosome, leading to the release of the completed polypeptide.

 4. Post-translational Modifications:

  • Folding: The polypeptide chain folds into its functional three-dimensional structure.
  • Addition of Prosthetic Groups: Some proteins require the addition of non-polypeptide components, like heme in hemoglobin.
  • Cleavage: Enzymes may remove specific amino acids to activate the protein.
  • Chemical Modifications: Phosphorylation, glycosylation, and other chemical changes can occur.

5. Protein Targeting:

  • Signal Sequences: Proteins are tagged with signal sequences that direct them to their correct cellular location, like the endoplasmic reticulum or mitochondria.

6. Chaperone Proteins:

  • Chaperones assist in protein folding, ensuring they attain their correct structure.

 7. Protein Function:

  •  Proteins perform a wide range of functions in the cell, including enzymatic activity, structural support, transport of molecules, immune defense, and signaling.