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2.4 Protein Synthesis

  • Definition: Protein synthesis is the process by which cells build proteins from DNA and RNA.

  • Key Phases: Involves transcription, post-transcriptional modifications, translation, and post-translational modifications.

Understanding DNA and RNA

  • DNA (Deoxyribose Nucleic Acid):

    • Contains genetic instructions.

    • Key discoveries:

      • Griffith (1928): DNA as genetic material.

      • Franklin: X-ray diffraction of DNA.

      • Watson & Crick (1950s): First model of DNA, structured as a double helix.

  • Structure of DNA:

    • Composed of two polynucleotide chains.

    • Made of nucleotides: phosphate, deoxyribose sugar, and nitrogenous bases (A, T, G, C).

    • Base pairing: A-T (2 H-bonds), G-C (3 H-bonds).

    • Chargaff's Rule: Amount of A = T and G = C.

    • Antiparallel strands, width of 20Å, and one turn of helix has ten base pairs.

DNA Replication

  • Definition: Process of copying DNA for inheritance.

  • Steps:

    • Unwinding: DNA strands unzip at origins.

    • Base Addition: New strands synthesized by DNA polymerase.

Ribonucleic Acid (RNA)

  • Definition: RNA contains ribose sugar and is mostly single-stranded.

  • Types of RNA:

    • Messenger RNA (mRNA): Carries genetic information for protein synthesis; makes up 3-5% of total RNA.

    • Transfer RNA (tRNA): Transports amino acids for protein synthesis; makes up 15-17% of total RNA.

      • Cloverleaf structure with anticodon for mRNA.

    • Ribosomal RNA (rRNA): Forms ribosomes; around 100-3000 nucleotides long, constituting 80% of total RNA.

Transcription

  • Definition: Synthesis of RNA from DNA.

  • Steps:

    • Initiation: RNA polymerase binds to the promoter.

    • Elongation: RNA polymerase synthesizes RNA in the 5' to 3' direction.

    • Termination: RNA polymerase stops at the stop signal.

  • Result: Precursor mRNA (pre-mRNA) containing both introns and exons.

Translation

  • Definition: Converting mRNA codons to amino acids, forming proteins.

  • Components: mRNA, ribosomes, tRNA, amino acids.

  • Steps:

    • Initiation: mRNA binds to ribosome, starting at AUG codon.

    • Elongation: Amino acids added sequentially. Peptide bonds form between amino acids.

    • Termination: Occurs when a stop codon is reached, releasing the completed protein.

Differences Between Eukaryotic and Prokaryotic Protein Synthesis

  • Ribosome Size: Prokaryotes (70S) vs. Eukaryotes (80S).

  • Initiation Codon: Multiple start sites in prokaryotes; only AUG in eukaryotes.

  • Starting Amino Acid: N-Formyl methionine in prokaryotes vs. methionine in eukaryotes.

Technological Advances in Genetics

  • Human Genome Project: Sequencing human DNA using high-throughput machines and collaborative technologies, published results in 2006.

  • GRAIL: Early bioinformatics tool for gene identification in DNA.