Question

Difference between prokaryotes and eukaryotes ribosome structure

Answer 1

Eukaryotic ribosomes are larger. They consist of a 60S large subunit and a 40S small subunit, which come together to form an 80S particle having a mass of 4200 kd, compared with 2700 kd for the prokaryotic 70S ribosome. The 40S subunit contains an 18S RNA that is homologous to the prokaryotic 16S RNA.

Answer 2

Prokaryotic Ribosomes

• They found inside bacteria and archaea.
• Small and mass is 27000 kd.
• Sedimentation Coefficient is 70S ( S = Svedberg's unit ).
• Diameter is roughly 200A ( A = Armstrong unit ).
• Made up of 50S and 30S subunits.
• Large subunit ( 50S ) is made up of two rRNA molecules: 23S rRNA and 5S rRNA.
• Constituents are : 60% rRNA and 40% ribosomal proteins.
• Occurs freely in the cytoplasm.
• Small subunit ( 30S ) contain 16S rRNA complexed with roughly 21 proteins.

Eukaryotic Ribosomes

• Found in animals, plants and fungi and other unicellular eukaryotes with a nucleus.
• Large and mass is 42000 kd.
• Sedimentation Coefficient is 80S.
• Diameter is roughly 250 to 300A.
• Made up of 60S and 40S subunits.
• Large subunit ( 60S ) is made up of three rRNA molecules: 28S rRNA, 5.3S rRNA and 5S rRNA.
• Constituents are : 40% rRNA and 60% ribosomal proteins.
• Most are occurred to the outer surface of nucleus and endoplasmic reticulum ( RER ) . Some are attached in mitochondria ( in animal cells ) whereas some are attached in chloroplasts ( in plant cells ).
• Small subunit ( 40S ) contains 18S rRNA complexed with roughly 33 proteins.

What is Svedberg's Unit ?

Svedberg's unit is used as a measuring unit for sedimentation speed in ultracentrifugation which is related to the size, shape and density of the particle.

However, Mitochondria and Chloroplasts in the eukaryotic cells also contain 70S ribosomes ( their own ribosomes ) since these organelles are evolved from endosymbiosis.