Translation

• tRNA

  Has anticodon region which is complementary and antiparallel to mRNA. It carries the AA that mRNA codon codes for.

  Enzyme aminoacyl-tRNA synthetase is what attaches the AA to the tRNA. 20 in total, one per AA

  When it has an AA, it is charged
• Ribosomes (sites)

  Large subunit has 3 sites:

  A site= AA site, holds the next tRNA carrying an AA

  P site= polypeptide site, holds the tRNA carrying the growing polypeptide chain

  E site= exit site
• Initiation

  Begins when the small ribosomal subunit binds to mRNA and a charged tRNA binds to the start codon, AUG on mRNA. The tRNA carries methionine (Met), then the large subunit binds

  Very first tRNA that carries Met goes to the P site, every other will go to A site first
• Elongation

  Starts when next tRNA comes into A site. mRNA is moved through the ribosome and its codons are read.

  Each mRNA codon codes for a specific AA. Since all organisms use same genetic code, supports common ancestry
• Elongation (steps)

  First: Codon recognition - the correct anticodon of the next tRNA goes into A site

  Second: Peptide bond formation - peptide bonds are formed that transfer the polypeptide to A site tRNA

  Third: Translocation - tRNA in A site moves to P site, the tRNA in P site goes to E site. A site is open for next tRNA
• Termination

  Occurs when a stop codon in the mRNA reaches the A site of ribosome. Stop codons don't code for AA, but do signal for a release factor by hydrolyzing the bond that holds the polypeptide to P site. Releasing polypeptide, and the whole translational units disassemble.
• Retroviruses


  Exception to standard flow of genetic info. Goes from RNA to DNA, using an enzyme known as reverse transcriptase. Couples viral RNA to DNA, and DNA becomes part of RNA
• Polyribosome


  Multiple ribosomes trailing along mRNA far past the start codon. Allowing a cell to rapidly make copies of a polypeptides