7.3.2 Ribosomes

• Ribosomes are found in cells
  • Either freely in the cytoplasm (of all cells)
  • Or bound to the endoplasmic reticulum (ER) to form rough ER (only in eukaryotic cells)
• Ribosomes are the site of protein synthesis
• They consist of a large and a small subunit composed of protein and ribosomal RNA (rRNA)
  • Protein provides structure to the ribosome
  • rRNA facilitates the binding of mRNA and tRNA and catalyses the formation of peptide bonds between amino acids
• Ribosomes have three tRNA binding sites and one mRNA binding site
• mRNA sits in a groove between the two subunits and the ribosome moves along, forming a polypeptide as it travels

A diagram of a ribosome, showing the small and large subunits

• In eukaryotic cells, protein synthesis commonly occurs at free ribosomes in the cytoplasm
• Free ribosomes can move within the cytoplasm and synthesise proteins for use primarily within the cell
  • As opposed to proteins destined to be secreted extracellularly
  • Proteins synthesised on free ribosomes are destined for use within the cytosol (the fluid part of the cytoplasm)
    • And within large organelles such as mitochondria and chloroplasts

• Eukaryotic cells make thousands of proteins that need to be delivered to the correct location, sometimes in different tissues/organs altogether
• When free ribosomes make proteins destined for lysosomes, or secretion from the cell, the ribosome becomes bound to the endoplasmic reticulum (ER)
• Signal sequences in the growing polypeptide chain dictate whether the free ribosome needs to move to the ER
  • The signal sequence occurs at the beginning polypeptide
  • Signal recognition proteins bind to the polypeptide, pausing translation
  • The free ribosome binds to a receptor on the ER, forming rough ER
  • Translation is re-initiated and the polypeptide chain moves inside the ER
• The synthesised protein can be carried via a vesicle to the Golgi apparatus before being secreted out of the cell

Proteins destined for lysosomes or secretion out of the cell are synthesised by ribosomes bound to the endoplasmic reticulum