Translation

• 1 - Occurs on ribosomes in the cytoplasm; the mRNA molecule becomes attached to the ribosomes at a binding site (groove), two tRNA molecules are held in position on the ribosome at one time
• 2 - Anticodons on the tRNA molecules bind to the complementary codons on the mRNA; held in place by temporary h-bonds, the first codon on mRNA, the start/initiation codon, is always AUG (methionine)
• 3 - The ribosomes move along the mRNA strand reading the mRNA codons and translating them into the sequence of amino acids; amino acids can be added to the polypeptide chain at a rate of 15/sec
• 4 - Energy from breaking bonds between the tRNA and its associated amino acid form peptide bonds between the amino acids producing a polypeptide chain; the formation of the bond also involves the enzyme peptidyl transferase
• 5 - Translation stops when the ribosome reaches a stop codon, for which there is no complementary tRNA, and the polypeptide chain breaks away and begins to fold
• 6 - Completed polypeptides pass into the cisternae of the RER and are budded off into RER vesicles; these vesicles pass to the Golgi where they are modified to form functional proteins, they are packaged into Golgi vesicles
• 7 - Many ribosomes (referred to as a polysome) may become attached to the same mRNA moving along it in sequence to produce multiple copies of the same polypeptide chain
• Transcription Diagram

  A) mRNA

  B) receptor protein

  C) ER membrane

  D) enzyme

  E) signal sequence

  F) polypeptide

  G) endoplasmic reticulum

  H) width

  I) ER

  J) Ribosome

  K) endoplasmic reticulum