Ribosomes and Protein Synthesis

What Do Ribosomes Do?

  • DNA contains genetic information for protein synthesis.
  • DNA is converted into messenger RNA (mRNA) via transcription in the nucleus.
  • mRNA moves to the cytoplasm for translation into protein.
  • Ribosomes are involved in translation: they bind to mRNA, decode its information, and allow amino acids to assemble with the help of transfer RNA (tRNA).
  • Ribosomes create peptide bonds between amino acids, forming a protein chain.

Ribosome Definition

  • Ribosomes are cellular organelles composed of proteins and ribosomal RNA (rRNA).
  • They consist of a large and a small subunit that join during translation.
  • Prokaryotes (no true nucleus):
    • 50S larger subunit
    • 30S smaller subunit
    • Forms a 70S ribosome
  • Eukaryotes (true nucleus):
    • 60S larger subunit
    • 40S smaller subunit
    • Forms an 80S ribosome
  • The "S" stands for Svedberg unit.

The Ribosome's Function: Three Stages

  • Ribosomes produce proteins via translation, which includes initiation, elongation, and termination.
  • They form amino acid chains at a rate of 200 amino acids per minute.

Initiation

  • In prokaryotes, initiation factor IF3 stimulates mRNA to bind with the smaller ribosomal subunit.
  • Initiation factor IF2 binds with GTP and tRNA carrying N-formylmethionine.
  • This allows the tRNA to bind with the P-site of the larger ribosomal subunit.
  • GTP is hydrolyzed by a ribosomal protein, allowing ribosomal assembly to move forward.
  • Initiation factors are released.

Elongation

  • Elongation begins with GTP binding to elongation factor EF-Tu.
  • EF-Tu binds with amino acid-bearing tRNA and helps it arrive at the A-site of the larger ribosomal subunit.
  • Transfer of aminoacyl-tRNA to the A-site occurs via GTP hydrolysis.
  • EF-Tu is released with the help of elongation factor EF-Ts, which restores the EF-Tu-GTP complex.
  • The polypeptide chain on the tRNA at the P-site forms a peptide bond with the amino acid at the A-site.
  • The ribosome moves towards the next mRNA codon with the help of elongation factor EF-G (translocation).
  • Uncharged tRNA (without amino acid) is released from the P-site.
  • The P-site then gains the peptidyl tRNA.

Termination

  • Termination begins with the recognition of termination codons by release factors: RF1 (UAG and UAA) and RF2 (UGA and UAA).
  • tRNA cannot recognize these codons.
  • Release factors attach to the A-site when the P-site has peptidyl tRNA and a termination codon arrives at the A site.
  • The peptide chain is released from the P-site of the larger subunit, and the subunits separate.
  • Separation occurs through GTP hydrolysis.

Ribosome Structure

  • Ribosomes contain many proteins and ribosomal RNA, forming the ribosomal complex.
  • The amount of RNA and proteins varies in different species.
  • In prokaryotes (50S subunit):
    • 23S rRNA
    • 5S rRNA
    • 31 proteins
  • In prokaryotes (30S subunit):
    • 16S rRNA
    • 21 proteins
  • In eukaryotes (60S subunit):
    • 5.8S rRNA
    • 28S rRNA
    • 5S rRNA
    • 49 proteins
  • In eukaryotes (40S subunit):
    • 18S rRNA
    • 33 proteins

Ribosome Location

  • Ribosomes are present in two forms inside the cell:
    • Free ribosomes: suspended in the cytosol, can move freely (except in other organelles and the nucleus).
    • Membrane-bound ribosomes: attached to the endoplasmic reticulum (rough ER).
  • Proteins synthesized by free ribosomes remain in the cytosol.
  • Proteins synthesized by membrane-bound ribosomes are transferred to the ER and then to the Golgi apparatus for processing, before being sent to specific locations.

Two Subunits of Ribosomes

  • Smaller subunit: decodes mRNA.
  • Larger subunit: forms peptide bonds.
  • Smaller subunit has less rRNA and protein content than the larger one.
  • Smaller subunit has the mRNA binding site.
  • Larger subunit has the E-site, P-site, and A-site:
    • A-site: tRNA-bearing amino acid arrives.
    • P-site: peptide chain formation takes place.
    • E-site: uncharged tRNA is removed.
  • The two subunits remain separated in the cell but bind together during translation to form a complete ribosome unit.

Lesson Summary

  • Ribosomes are cellular organelles made of proteins and rRNA involved in translation.
  • They bind with mRNA, allow tRNA containing amino acids to arrive, and synthesize peptide bonds to form polypeptide chains.
  • Protein synthesis occurs in three stages: initiation, elongation, and termination.
  • Initiation: first amino acid-bearing tRNA arrives at the P-site.
  • Elongation: peptide bond forms, and the ribosome translocates on mRNA.
  • Termination: release factors bind with the termination codon, and the polypeptide chain is released.
  • Ribosomes exist as free ribosomes (in the cytosol) and membrane-bound ribosomes (on the ER).
  • Ribosomes contain a larger and a smaller subunit with different rRNA and protein content in eukaryotes and prokaryotes.
  • The smaller subunit binds mRNA, and the larger subunit contains E, P, and A sites.
  • Subunits join to form a ribosomal unit for protein synthesis.