Describe the process of protein synthesis.

Protein synthesis is the biological process by which cells generate new proteins. It occurs in two main stages: transcription and translation.

1. Transcription (Nucleus)

• Takes place in the nucleus of eukaryotic cells.
• A segment of DNA (gene) acts as a template to synthesize messenger RNA (mRNA).
• The enzyme RNA polymerase binds to the promoter region of the DNA, unwinds the double helix, and synthesizes a complementary mRNA strand in the 5'→3' direction.
• Only one strand of DNA (the template strand) is used.
• The newly formed mRNA is complementary to the DNA template but contains uracil (U) instead of thymine (T).

2. mRNA Processing (Nucleus)

• The initial mRNA transcript (pre-mRNA) undergoes modifications:
  • Intron splicing: Non-coding regions (introns) are removed, and coding regions (exons) are joined.
  • 5' cap addition: A modified guanine nucleotide is added to the 5' end to protect mRNA and aid ribosome binding.
  • Poly-A tail addition: A long chain of adenine nucleotides is added to the 3' end to stabilize mRNA and prevent degradation.

3. mRNA Export (Nucleus → Cytoplasm)

• The processed mRNA exits the nucleus through nuclear pores and enters the cytoplasm.

4. Translation (Ribosome)

• Occurs on ribosomes (free in cytoplasm or bound to the rough ER).
• The mRNA binds to a small ribosomal subunit, and the initiation complex forms with the help of initiation factors.
• A start codon (AUG) signals the beginning of translation, where methionine (Met) is the first amino acid.
• Transfer RNA (tRNA) molecules bring amino acids to the ribosome:
  • Each tRNA has an anticodon that pairs with the mRNA codon (complementary base pairing).
  • The large ribosomal subunit joins, and peptide bonds form between amino acids, elongating the polypeptide chain.
• The process continues until a stop codon (UAA, UAG, UGA) is reached, releasing the completed polypeptide.

5. Polypeptide Folding

• The newly synthesized polypeptide chain folds into its tertiary structure (3D shape) due to interactions like hydrogen bonds, ionic bonds, and disulfide bridges.
• Some proteins undergo post-translational modifications (e.g., glycosylation, phosphorylation) before becoming functional.

This coordinated process ensures the production of functional proteins essential for cellular structure, metabolism, and regulation.