Step 2 of protein synthesis: Translation “The players” 1.Transfer RNA (tRNA)  Folded into three-lobed shape (clover-like)  At one lobe, resides an anticodon that has three nucleotides complementary a codon on the mRNA  Across from the anticodon is the amino acid attachment site.  When an amino acid is bound to tRNA it’s called amino-acyl tRNA (aatRNA)

2. Ribosomes  Bring together mRNA and tRNA  Made of protein + ribosomal RNA (rRNA)  2 subunits: one large, one small  3 binding sites for tRNA: E site: releases tRNA molecules P site: holds onto one aa-tRNA and growing chain of amino acids A site: holds tRNA bringing the next amino acid

Initiation: 1.mRNA reaches the cytoplasm 2.rRNA binds 5’ end of mRNA at a special sequence 3.Initiator tRNA with anticodon “UAC” binds ribosomal-mRNA complex. **this anticodon matches start codon, AUG and carries a.a. methionine (met)***

Elongation: 1.mRNA codon in A site forms a base pair with incoming aa-tRNA 2.Peptide bond is formed joining new amino acid in A site to growing chain in P site. 3.Ribosome moves a distance of 3 nucleotides along the rRNA molecule (5’  3’ direction) this is called Translocation. Translocation brings the tRNA holding the chain into the P site; a new A site is exposed and the old tRNA from the P site moves to the E site. THIS process is REPEATED many times

Termination:  elongation continues until a stop codon is reached on the mRNA strand in the A site  Previous tRNA, carrying the polypeptide chain remains in the P site until a protein called a release factor binds to the A site. The release factor cuts off the completed polypeptide from the tRNA.  The polypeptide is now free from the ribosome and will take its shape by folding