AQA A Level Biology复习笔记 1.5.5 Ribosomes

• Ribosomes are small organelles that are either free in the cytoplasm (of all cells) or are attached to the rough endoplasmic reticulum (only in eukaryotic cells)
• Ribosomes are the site of protein synthesis (where proteins are made)
  • They ‘read’ RNA to make polypeptides (proteins) in a process known as translation

   

• Ribosomes are themselves formed from RNA and proteins
  • The RNA that forms part of the structure of ribosomes is a specific type of RNA known as ribosomal RNA (rRNA)
  • The rRNA in ribosomes has enzymatic properties that catalyse the formation of peptide bonds between amino acids
  • Each ribosome is a mixture of ribosomal RNA and proteins

   

• Ribosomes in eukaryotic cells are larger than those in prokaryotic cells. In both cell types, ribosomes are composed of a small subunit and a large subunit
  • 80S ribosomes (composed of 60S and 40S subunits) are found in eukaryotic cells
  • 70S  ribosomes (composed of 50S and 30S subunits) are found in prokaryotic cells, as well as in the mitochondria and chloroplasts of eukaryotic cells

   

• The large subunit is the site of translation
  • The rRNA and proteins of the large subunit hold tRNA molecules (with their attached amino acids) in place
  • rRNA can then catalyse the condensation reactions between amino acids

   

• mRNA sits between the two subunits and the ribosome moves along it as it translates it into a polypeptide
• Unlike some organelles, ribosomes are not surrounded by a membrane

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