9.2 Ribosome Structure and Function

Ribosomes are the protein-making factories of cells. These complex structures, made of RNA and proteins, have two subunits that work together to read genetic instructions and build proteins.

The ribosome's job is to translate mRNA into proteins. It does this through a series of steps involving different sites on the ribosome, peptide bond formation, and coordinated movements of tRNAs and mRNA.

Ribosome Structure

Composition of Ribosomal Subunits

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• Ribosomes consist of two main subunits 30S and 50S
• 30S subunit smaller, contains 16S rRNA and about 21 proteins
• 50S subunit larger, contains 5S and 23S rRNA along with approximately 31 proteins
• Subunits combine to form functional 70S ribosome in prokaryotes
• Eukaryotic ribosomes slightly larger, form 80S complex

Role of Ribosomal RNA (rRNA)

• rRNA forms backbone of ribosomal structure
• 16S rRNA in 30S subunit crucial for mRNA binding and translation initiation
• 23S rRNA in 50S subunit contains peptidyl transferase center for peptide bond formation
• 5S rRNA aids in tRNA binding and ribosome stability
• rRNA catalyzes peptide bond formation, classifying ribosomes as ribozymes

Functions of Ribosomal Proteins

• Ribosomal proteins stabilize rRNA structure
• Assist in tRNA and mRNA binding
• Facilitate interactions with translation factors (initiation factors, elongation factors)
• Some ribosomal proteins have regulatory roles in translation
• Contribute to overall ribosome assembly and maintenance

Ribosome Function

Translation Sites on the Ribosome

• A site (aminoacyl-tRNA site) accepts incoming aminoacyl-tRNAs
• P site (peptidyl-tRNA site) holds tRNA with growing peptide chain
• E site (exit site) where deacylated tRNAs leave the ribosome
• Sites work together to facilitate orderly movement of tRNAs during translation
• Precise positioning of tRNAs ensures accurate codon-anticodon pairing

Peptide Bond Formation and Elongation

• Peptidyl transferase center located in 50S subunit
• Catalyzes formation of peptide bonds between amino acids
• Elongation cycle involves tRNA movement from A site to P site to E site
• GTP hydrolysis by elongation factors powers tRNA movement
• Process repeats until stop codon reached, signaling translation termination

Ribosome Dynamics During Translation

• Ribosomes undergo conformational changes during translation
• Intersubunit rotation occurs between elongation steps
• Movement along mRNA facilitated by interactions between ribosome and mRNA
• Translocation involves shift of mRNA-tRNA complex by one codon
• Energy from GTP hydrolysis drives these movements, ensuring unidirectional translation