Lesson 3: Translation
From RNA to protein: translation
The genetic code
Three possible “reading frames”
THE ABC FOR THE DNA
THE AXB CFO RTH EDN A
Insertion (X) or
Deletion (B)
Ribosomes in the cytoplasm of a eukaryotic cell
Composition of eukaryotic ribosomes
RNA-binding sites in the ribosome
Each ribosome has:
• a binding site for mRNA
• three binding sites for tRNA
• A-site: aminoacyl-tRNA
• P-site: peptidyl-tRNA
• E-site: exit
tRNA molecules:
matching amino acids to codons in mRNA
Translation of the genetic code:
two adaptors that act one after another
mRNA translation mechanism
Step1: An aminoacyl-tRNA molecule binds to the A-site on the ribosome
Step2: A new peptide bond is formed
Step3: The small subunit moves a distance of three nucleotides along the mRNA chain ejecting the
spent tRNA molecule
Step4: The next aminoacyl-tRNA molecule binds to the A-site on the ribosome
Step5: . . .
The initiation phase of protein synthesis in eukaryotes
1. Initiation complex (small ribosomal subunit + initiation factors) binds DNA and searches for start codon
2. Large ribosomal subunit adds to the complex 3. Translation starts
4. . . .
Prokaryotic vs eukaryotic mRNA molecules
Structure of a typical prokaryotic mRNA molecule
5´ end capping of eukaryotic mRNA molecules
The final phase of protein synthesis
binding of release factor to a stop codon terminates translation
the completed polypeptide is released
the ribosome dissociates into its two separate subunits
Polyribosomes:
several ribosomes can simultaneously translate
the same mRNA molecule
Proteasomes :
degradation of “unwanted” proteins in eukaryotic cells
The production of a protein by a eukaryotic cell
Many levels of regulation/variation
Protein import by membrane-bounded organelles
ATP !!!
The role of signal sequences in protein sorting
proteins destined for the ER: N-terminal signal sequence that directs them
proteins destined to remain in the cytosol: no signal sequence
