Three Types of RNA catalytic site tRNA docking sites Attached amino acid tRNA transfer Small subunit rRNA ribosomal mRNA Large subunit mRNA messenger Met anticodon 1 2 C A G A U G G A G U U A U G G A G U
3. The first tRNA carrying methionine (and anticodon UAC) binds to the mRNA start codon completing the initiation complex
4. The large ribosomal subunit joins the complex
Translation: Initiation (1) A tRNA with an attached methionine amino acid binds to a small ribosomal subunit, forming an initiation complex.
Translation: Initiation (2) The initiation complex binds to end of mRNA and travels down until it encounters an AUG codon in the mRNA. The anticodon of the tRNA in the initiation complex forms base pairs with the AUG codon.
Translation: Initiation (3) The large ribosomal subunit binds to the small subunit, with the mRNA between the two subunits. The methionine tRNA is in the first tRNA site on the large subunit.
Translation: Elongation 1 The second tRNA enters the second tRNA site on the large ribosomal subunit. Which tRNA binds depends on the ability of its anticodon (CAA in this example) to base pair with the codon (GUU in this example) in the mRNA. tRNAs with a CAA anticodon carry an attached valine amino acid, which was added to it by enzymes in the cytoplasm.
Translation: Elongation 2 The "empty" tRNA is released and the ribosome moves down the mRNA, one codon to the right. The tRNA that is attached to the two amino acids is now in the first tRNA binding site and the second tRNA binding site is empty.
Translation: Elongation 3 The catalytic site on the large subunit catalyzes the formation of a peptide bond linking the amino acids methionine to valine. The two amino acids are now attached to the tRNA in the second binding position.
Translation: Elongation 4 Another tRNA enters the second tRNA binding site carrying its attached amino acid. The tRNA has an anticodon that pairs with the codon. (Here, the CAU mRNA codon pairs with a GUA tRNA anticodon.) The tRNA molecule carries the amino acid histidine (his).
Translation: Elongation 5 Binding of tRNAs, & formation of peptide bonds continues. Ribosome reaches STOP codon (UAG). Protein "release factors" signal the ribosome to release the protein. The mRNA is also released and large & small subunits separate.
Translation: Termination The catalytic site forms a new peptide bond, in this example, between the valine and the histidine. A three-amino acid chain is now attached to the tRNA in the second tRNA binding site. The empty tRNA in the first site is released and the ribosome moves one codon to the right.
Complementary Base Pairing Methionine Glycine Valine etc. template DNA strand (a) complementary DNA strand (b) mRNA (c) tRNA (d) protein amino acids anticodons codons gene etc. etc. etc. etc. G G G A G C G A U U U C A A C A U C C U G G G A G T T C T G A G T C C C C A A A T C
Overview of Information Flow Amino Acids Active Protein Inactive Protein (Cytoplasm) DNA (Nucleus) rRNA tRNA 1 Transcription + Proteins Ribosomes tRNA tRNA-AA mRNA mRNA 2 Translation 3 Modification Product Substrate 4 Degradation