Protein Synthesis

Protein synthesis is the process of using a sequence of nucleotides to build polypeptides or proteins in a cell. This is a critical process that occurs in most cell types and is covered extensively in the IB Biology course.

Key Concepts

• Role of DNA: DNA is the code of life, determining cell structure and function by controlling protein production.
• Genes and Polypeptides: Genes are sections of DNA encoding the information to produce polypeptides, the building blocks of proteins.
• Gene Expression: Not all genes are expressed at all times; some remain silenced to control protein synthesis.
• Mutations: Changes in DNA sequence can lead to mutations, affecting protein structure and function, and potentially impacting health and development.

Protection of DNA

• Structural Protections: DNA’s double-stranded structure and nuclear membrane isolation protect it from mutations.
• Histone Proteins: DNA is wrapped around histone proteins for additional protection.
• Repair Mechanisms: These correct changes to DNA sequences and protect against mutagens and carcinogens.

Transcription: Building Messenger RNA (mRNA)

• Transcription Process: Only the gene needed for a specific polypeptide is transcribed into mRNA.
• RNA Polymerase: Catalyzes transcription by binding to DNA, unwinding the helix, and adding RNA nucleotides.
• Base Pairing Rules: RNA synthesis involves pairing adenine with uracil and guanine with cytosine.
• mRNA Modification and Transport: Once transcribed, mRNA is modified and exits the nucleus to the cytoplasm.

Translation: Building a Polypeptide

• Role of RNA Forms:
  • mRNA: Carries genetic information from DNA.
  • rRNA (Ribosomal RNA): A component of the ribosome.
  • tRNA (Transfer RNA): Carries amino acids and matches them to the mRNA codon using its anticodon.
• Ribosome Function:
  • mRNA attaches to the small ribosomal subunit; the large subunit binds to it, facilitating translation.
  • Ribosomes read mRNA in triplets (codons), and tRNA brings the corresponding amino acids.
  • Peptide bonds form between amino acids, elongating the polypeptide chain.
• Completion of Polypeptide:
  • Translation continues until a stop codon is reached, signaling the release of the polypeptide.
  • The free polypeptide can fold into a functional protein or undergo further processing.

Summary

• DNA to Proteins: DNA's sequence of nucleotide bases forms the template for mRNA.
• Transcription: Utilizes DNA as a template to create mRNA via base pairing rules.
• Translation: Occurs in the ribosome, where mRNA codons direct amino acid addition to the growing polypeptide chain, resulting in a complete protein.