M03.03.08 Ribosomal RNA (rRNA) and Ribosome Construction

Ribosomal RNA (rRNA) plays a crucial role in the formation of ribosomes, the cellular machinery responsible for protein synthesis. This document outlines the transcription and assembly of rRNA in eukaryotes and prokaryotes.

Eukaryotic rRNA

Transcription and Processing

• Eukaryotic ribosomal RNA is synthesized in the nucleolus by RNA polymerase I.
• The initial transcript is a single 45S RNA molecule, which is cleaved into:
  • 28S rRNA
  • 18S rRNA
  • 5.8S rRNA
• RNA polymerase III transcribes the 5S rRNA from a separate gene.

Assembly of Ribosomal Subunits

• Ribosomal subunits are formed in the nucleolus, where rRNA molecules combine with ribosomal proteins.
• The resulting eukaryotic ribosomal subunits are:
  • 60S (large subunit)
  • 40S (small subunit)
• These subunits combine during protein synthesis to form the complete 80S ribosome.

Prokaryotic rRNA

Structure and Function

• In prokaryotes, ribosomal RNA is also essential for ribosome formation.
• The ribosomal subunits in prokaryotes consist of:
  • 50S (large subunit)
  • 30S (small subunit)
• The complete prokaryotic ribosome is referred to as a 70S particle.

Important Notes

• The S values (Svedberg units) reflect the sedimentation rates of the ribosomal particles in an ultracentrifuge. These values are influenced by both size and shape and are not additive.

Points to Remember

• Eukaryotic ribosomes are composed of 80S subunits, while prokaryotic ribosomes are 70S.
• rRNA is essential for ribosome assembly and function in both eukaryotes and prokaryotes.
• The transcription and processing of rRNA differ between eukaryotes (45S precursor) and prokaryotes.

Bibliography

• Lodish, H., Berk, A., Kaiser, C. A., et al. (2008). Molecular Cell Biology. W.H. Freeman and Company.
• Alberts, B., Johnson, A., Lewis, J., et al. (2002). Molecular Biology of the Cell. Garland Science.
• Brown, T. A. (2016). Gene Cloning and DNA Analysis: An Introduction. Wiley-Blackwell.