DNA QUESTION 1990:				       L. PETERSON/AP BIOLOGY
     
	Describe the steps of protein synthesis, beginning with the attachment of a
	messenger RNA molecule to the small subunit of a ribosome and ending with
	the release of the polypeptide from the ribosome. Include in your answer a
	discussion of how the different types of RNA function in this process.

  
  STANDARDS:
  
  INITIATION (until peptide bond formation):           Max. = 4 points
  __  site of attachment on mRNA (near start codon or near 5' end, or groove in ribosome)
  __  usually AUG is start codon
  __  1st tRNA enters P site directly or 1st aa is met or fmet
  __  initiation factors
  __  large subunit then binds
  __  A + P sites (Entry/Donor)
  
  ELONGATION (from 1st peptide bond until last aa added)         Max. = 4 points
  __  anticodon/codon; complementary to mRNA
  __  elongation factors
  __  peptidyl transferase
  __  bond broken between tRNA and aa
  __  formation of peptide bond
  __  tRNA leaves
  __  translocation (movement of mRNA relative to ribosome
  
  TERMINATION                                Max. = 4 points
  __  stop codon is reached
  __  stop codons are UGA, UAA, or UAG
  __  no corresponding tRNA for stop codons
  __  releasing factors
  __  hydrolysis of tRNA from polypeptide
  __  dissociation of ribosomal subunits
  
  OTHER (may be found in above sections):                   Max. = 3 points
  __  tRNA:  e.g. tRNA carries aa to ribosome; charging of tRNA with cognate aa; 
         recycling of tRNA; formation of aminoacyl tRNA;
  __  mRNA:  what a codon is; mRNA carries information from DNA; cap/tail;
         leader/trailer; signal sequence;
   __  rRNA:  sites on ribosome for attachment; protein plus rRNA forms a 
         complete functional ribosome;
  __  GTP as energy source
  __  rate of translation
  __  polysomes