Q.11 Eukaryotic ribosomes sediment in sucrose gradient with a sedimentation coefficient of:

1. 100 S
2. 90 S
3. 80 S
4. 150 S

   80 S is the sedimentation coefficient for complete eukaryotic ribosomes in sucrose density gradients.

   Question Breakdown

   Ribosome sedimentation coefficients (Svedberg units, S) measure sedimentation rate during ultracentrifugation, reflecting size, shape, and density. Eukaryotic cytoplasmic ribosomes sediment at 80S, comprising 60S large and 40S small subunits. Bacterial ribosomes are 70S (50S + 30S); mitochondrial/plastid ribosomes resemble prokaryotic types.

   Option Analysis

   100 S

   Incorrect. No standard ribosomes sediment here; too high for eukaryotic cytoplasmic ribosomes.

   90 S

   Incorrect. This approximates nucleolar pre-ribosomal particles (90S pre-60S), not mature functional ribosomes.

   80 S

   Correct. Mature eukaryotic ribosomes from cytoplasm sediment at 80S in sucrose gradients. Confirmed across yeast, mammals, plants.

   150 S

   Incorrect. No biological ribosomes match this; far exceeds known values.

   Correct Answer: 80 S – Standard for eukaryotic translation machinery.

   Eukaryotic ribosomes sediment in sucrose gradient at 80S, distinguishing them from prokaryotic 70S ribosomes. This sedimentation coefficient identifies mature cytoplasmic ribosomes essential for protein synthesis in eukaryotes.

   Ribosome Sedimentation Basics

   Svedberg units measure ultracentrifugation behavior:

   • Prokaryotes: 70S (50S large + 30S small).

   • Eukaryotes: 80S (60S + 40S) – Larger rRNAs/proteins.

   • Polysomes (mRNA-bound ribosomes) sediment faster but disassemble to 80S monomers.

   80S value arises from ~4.3 MDa mass, with 25S/18S/5.8S/5S rRNAs in large subunit, 18S in small.

   Experimental Context

   Sucrose gradient centrifugation separates ribosomal subunits:

   text

   Cytoplasm extract → Ultracentrifuge (20-60% sucrose) → 40S, 60S, 80S peaks


   80S peak confirms translationally active ribosomes.

   Option	Sedimentation	Biological Context
   100 S	None	Non-existent
   90 S	Pre-ribosomes	Nucleolar 60S biogenesis
   80 S	Mature ribosomes	Cytoplasmic translation
   150 S	None	Too high