Prokaryotic Ribosomes | 70S Ribosomes
Prokaryotic Ribosomes: Structure and Function
Overview of Prokaryotic Ribosomes
- Prokaryotic ribosomes are approximately 20 nanometers in diameter, composed of proteins and RNA, specifically ribosomal RNA (rRNA).
- The composition of prokaryotic ribosomes is about 65% rRNA and 35% ribosomal proteins.
- These ribosomes can either be associated with the cell membrane or exist freely in the cytoplasm, influencing their function.
Protein Synthesis Locations
- Ribosomes attached to the cell membrane synthesize proteins that are transported outside the cell.
- In contrast, free ribosomes in the cytoplasm produce cytoplasmic proteins.
Sedimentation Profile and Subunit Composition
- Prokaryotic ribosomes are classified as 70S based on their sedimentation profile during centrifugation.
- The 70S ribosome consists of two subunits: a larger 50S subunit containing about 31 ribosomal proteins and a smaller 30S subunit with around 21 ribosomal proteins.
rRNA Components
- The 50S subunit comprises two types of rRNA: 23S and 5S rRNA.
- The smaller 30S subunit contains only one type of rRNA, which is the 16S rRNA.
Role of Factors in Translation
- Ribosomes do not operate independently; they require various factors for translation:
- Initiation factors for starting translation.
- Elongation factors for extending the polypeptide chain.
- Termination factors for concluding protein synthesis.
Functional Sites within Ribosomes
- Ribosomes have three critical sites involved in protein synthesis:
- A site: Accepting charged tRNAs during translation initiation.
- B site: Responsible for peptidyl transfer activity crucial during elongation.