70S ribosomes, found in prokaryotes, consist of 50S and 30S subunits that associate during protein synthesis. The correct answer is option 1.

Introduction

The 70S ribosome is made up of distinct subunits critical for bacterial protein translation, a key topic in molecular biology exams. Understanding its composition differentiates prokaryotic from eukaryotic ribosomes, essential for life sciences students studying cell biology and genetics.

Option Analysis

Each option tests knowledge of ribosomal Svedberg units (S), which measure sedimentation rates rather than mass additively.

• 50S and 30S subunits
  Correct composition: larger 50S subunit (23S + 5S rRNA, ~31 proteins) joins smaller 30S subunit (16S rRNA, ~21 proteins) to form functional 70S ribosome in prokaryotes.

• 50S and 40S subunits
  Incorrect; 40S is the small subunit of eukaryotic 80S ribosomes (cytoplasmic), not prokaryotic 70S.

• 60S and 30S subunits
  Wrong pairing; 60S is the large eukaryotic subunit, while 30S belongs to prokaryotes—mixing creates non-functional hybrids.

• 60S and 40S subunits
  Describes eukaryotic 80S ribosomes (60S large + 40S small), found in mitochondria/chloroplasts or cytoplasmic translation, not standard bacterial 70S.

Ribosome Structure Details

Svedberg units reflect shape and density, so 50S + 30S ≠ 80S but sediments as 70S together. The 30S subunit decodes mRNA via anticodon-codon matching; 50S catalyzes peptide bonds (peptidyl transferase center).​

Prokaryotic ribosomes lack 5.8S rRNA present in eukaryotic large subunits.​

Biological Significance

70S structure enables antibiotic targeting (e.g., tetracycline binds 30S, chloramphenicol binds 50S) without affecting eukaryotic 80S. Mitochondria/chloroplasts retain bacterial-like 70S ribosomes from endosymbiotic origins.

Exam Tip

Memorize: Prokaryotes/Mitochondria = 70S (50S+30S); Eukaryotes = 80S (60S+40S). Common distractors mix subunits across kingdoms.