15.2 Ribosome structure and function

Ribosomes are the protein-making factories of cells, composed of RNA and proteins. They have two subunits that work together to read genetic instructions and build proteins. Understanding their structure is key to grasping how cells translate genetic information into functional molecules.

The ribosome's intricate design allows it to accurately decode mRNA and link amino acids into proteins. Its binding sites and catalytic centers ensure precise translation, while interactions with other cellular components facilitate the complex process of protein synthesis.

Ribosome Structure and Composition

Ribosome composition and structure

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• Ribosomes composed of ribosomal RNA (rRNA) and ribosomal proteins
  • Prokaryotic ribosomes: 70S ribosomes
    • Consist of 30S small subunit and 50S large subunit
    • 30S subunit contains 16S rRNA, 50S subunit contains 23S and 5S rRNA
  • Eukaryotic ribosomes: 80S ribosomes
    • Consist of 40S small subunit and 60S large subunit
    • 40S subunit contains 18S rRNA, 60S subunit contains 28S, 5.8S, and 5S rRNA
• Ribosomes have roughly spherical shape with three distinct sites
  • A site (aminoacyl site) binds incoming aminoacyl-tRNA (tRNA charged with amino acid)
  • P site (peptidyl site) holds tRNA carrying growing polypeptide chain
  • E site (exit site) allows release of deacylated tRNA after peptide bond formation (tRNA without amino acid)

Role of rRNA and proteins

• rRNA plays crucial role in ribosome function and catalysis
  • Forms structural and functional core of ribosome
  • Catalyzes peptide bond formation in peptidyl transferase center (PTC) of large subunit
  • Participates in decoding process by interacting with mRNA and tRNA (ensures correct base pairing)
• Ribosomal proteins support and stabilize rRNA structure
  • Assist in assembly and maintenance of ribosome's three-dimensional structure
  • Help in binding and positioning of mRNA, tRNA, and translation factors (initiation, elongation, termination)

Ribosomal Subunits and Translation

Significance of ribosomal subunits

• Small ribosomal subunit (30S in prokaryotes, 40S in eukaryotes)
  • Responsible for mRNA binding and decoding
  • Contains decoding center which ensures correct base pairing between mRNA codons and tRNA anticodons
  • Monitors fidelity of translation by proofreading codon-anticodon interactions (prevents errors)
• Large ribosomal subunit (50S in prokaryotes, 60S in eukaryotes)
  • Contains peptidyl transferase center (PTC) which catalyzes peptide bond formation (links amino acids)
  • Provides exit tunnel for growing polypeptide chain
  • Participates in translocation of tRNAs and mRNA during elongation (moves ribosome along mRNA)

Importance of ribosome binding sites

• Ribosome binding sites (RBS) on mRNA essential for translation initiation
  • In prokaryotes, RBS known as Shine-Dalgarno sequence
    • Located upstream of start codon (usually AUG)
    • Complementary to portion of 16S rRNA in 30S ribosomal subunit
    • Facilitates recruitment and proper positioning of ribosome on mRNA
  • In eukaryotes, 5' cap and poly(A) tail of mRNA play role in ribosome recruitment
    • 40S ribosomal subunit, along with initiation factors, binds near 5' cap and scans mRNA until it reaches start codon
• Proper interaction between ribosome and RBS ensures accurate translation initiation and reading frame maintenance (prevents frameshift mutations)