Biology: Initiation Complex Formation

Formation of the Initiation Complex

• The initiation complex is formed when the large 50S subunit combines with the 30S subunit.

Steps of Initiation

• Initiation factors (IFs) bind to the 30S subunit and to the mRNA.
• The tRNA carrying the first amino acid Methionine (Met) binds to the initiation codon AUG on the mRNA.
• The 50S subunit joins the complex, releasing IFs and initiating translation.

Initiation Complex Structure

• The initiation codon AUG is surrounded by the initiation factors IF-1, IF-2, and IF-3.
• The tRNA carrying the first amino acid (Met) is attached to the P site on the 30S subunit.
• The large 50S subunit joins the complex to complete the initiation of translation.

Initiation Factors

• Initiation factors (IFs) help bind other molecules needed for translation and are released when the 50S subunit binds to the 30S subunit.

Ribosome Binding Sites

• The E site is the exit tRNA binding site.
• The P site is the peptidyl binding site.
• The A site is the aminoacyl binding site.

Ingredients for Initiation

• 30S subunit of the ribosome
• Initiator tRNA (tRNAMet)
• mRNA
• Initiation Factors IF-1, IF-2, and IF-3
• One molecule of GTP
• 50S subunit of the ribosome

Shine-Dalgarno Sequence

• The Shine-Dalgarno sequence is a ribosomal binding site in the mRNA, generally located 8 bp upstream of the AUG start codon.
• It is complementary to a sequence in ribosomal RNA.

Formation of the Initiation Complex

• The mRNA is guided to the 30S ribosomal subunit by the Shine-Dalgarno sequence.
• Formylmethionine tRNA (fMet) binds to the peptidyl site (P site), mediated by IF-2-GTP.

Ribosomes

• Site of RNA synthesis and ribosome assembly.
• Bacterial ribosome: 70S, composed of 50S and 30S subunits.
• Eukaryotic ribosome: 80S, composed of 60S and 40S subunits.

Translation Steps

• Initiation
• Elongation
• Termination
• Termination involves STOP codons: UAA, UGA, and UAG, which have no tRNA and involve release factors (RFs).

Transfer RNA (tRNA)

• 73 to 93 nucleotides in length.

Aminoacyl-tRNA Synthetases

• Catalyze the activation of tRNA.
• Each enzyme links a specific amino acid to the matching tRNA.
• There are 20 different aminoacyl-tRNA synthetases, one for each amino acid.
• The "second genetic code" refers to the molecular recognition of a specific tRNA molecule by a specific synthetase.

Learn about the steps involved in the formation of the initiation complex, including the binding of initiation factors and tRNA to the 30S subunit and mRNA.