Role of Ribosome in Protein Synthesis

Ribosomes are essential molecular machines in cells that are responsible for protein synthesis, a process known as translation. Here is an overview of their role:

1. Structure of Ribosomes

Ribosomes are made of ribosomal RNA (rRNA) and proteins.

They consist of two subunits:

Small subunit: Binds to the mRNA (messenger RNA).

Large subunit: Contains sites for tRNA (transfer RNA) binding and catalyzes peptide bond formation.

2. Stages of Protein Synthesis Involving Ribosomes

a. Initiation

The small ribosomal subunit binds to the mRNA.

A specific initiator tRNA carrying methionine binds to the start codon (AUG).

The large subunit then joins to form a functional ribosome.

b. Elongation

tRNAs bring amino acids to the ribosome.

The ribosome moves along the mRNA, reading codons.

For each codon, a matching tRNA delivers the correct amino acid.

The ribosome catalyzes peptide bond formation, extending the growing polypeptide chain.

c. Termination

When a stop codon (UAA, UAG, or UGA) is reached, no tRNA matches.

Release factors bind, triggering the release of the polypeptide.

The ribosomal subunits then dissociate and can be reused.

3. Ribosome Binding Sites (in prokaryotes)

A site (Aminoacyl site): Binds incoming tRNA.

P site (Peptidyl site): Holds the tRNA with the growing peptide chain.

E site (Exit site): Where the used tRNA exits the ribosome.

4. Ribosomes in Different Cells

Prokaryotic ribosomes: 70S (30S + 50S subunits)

Eukaryotic ribosomes: 80S (40S + 60S subunits)

Ribosomes can be free-floating in the cytoplasm or attached to the rough endoplasmic reticulum (RER) in eukaryotic cells.

Summary

Ribosomes are the sites of translation, converting the genetic code carried by mRNA into functional proteins by assembling amino acids in the correct order. They play a central role in gene expression and cellular function.