DNA QUESTION 1977:				L. PETERSON/AP BIOLOGY

 	Proteins are composed of amino acid subunits which form stable 
	three-dimensional structures.

	a.  Describe how the genetic instructions coded in DNA are translated into 
	     the primary structure (sequence of amino acid subunits) of a protein 
	     molecule.
	b.  Explain how interactions among the individual amino acid subunits 
	     influence the transformation of the molecule into its three-dimensional
	     structure and how they stabilize it.
 
  
  STANDARDS:
   
     Ten was the maximum number of points for part a. Six points were given for a
  good description of the structure and/or function of DNA, mRNA, tRNA (two points each).
  One point was given for a reasonable explanation of amino acid linkage to the ribosomes 
  and one point for mentioning polysomes. The student received an additional point for a good
  discussion of the overall process of protein synthesis.
  
     Ten was the maximum number of points for part b. For mentioning any one of the
  four (primary, secondary, tertiary, quaternary) structures of protein, the student would receive one
  point. For mentioning two or more structures the student received two points. Up to four points
  could be earned for mentioning the different kinds of bonds which hold the protein molecule in its
  various forms (one point for each kind of bond mentioned). Another four points were given for
  describing or explaining the significance of the structures or bonds (one point for each structure or
  bond adequately handled).
  
     Although the number of points totals 20, only a maximum of 15 is awarded because the
  scale used is 0 to 15. The 20 points represent a fair way to treat the students' varied answers.
  However, because the maximum for each part was 10, a student could not answer only half of the
  question and score 15.
  
  LIST OF STANDARDS:
  
  PART A:  STRUCTURE AND/OR FUNCTION IN PROTEIN SYNTHESIS
          DISCUSSION OF DNA                  = 2 points
          DISCUSSION OF mRNA            = 2 points
          DISCUSSION OF tRNA                 = 2 points
      AMINO ACID LINKAGE/RIBOSOME 
  (one point for discussion of each or possible two points for either one if done well/ but
  not two points for each one)
          OVERVIEW OF PROTEIN SYNTHESIS  = 1 point
          MENTION OF POLYSOMES               = 1 point
                                             [Part A  Total = 10 points]
  PART B:  STRUCTURES (primary, secondary, tertiary, quaternary)
          MENTION ONE                    = 1 point
          MENTION OF TWO OR MORE         = 2 points / max. of 2 points
   
     SIGNIFICANCE OR DESCRIPTION OF 
     EACH STRUCTURE OR BOND              = 1 point / max. of 4 points
  
     MENTION OF A BOND                   = 1 point each / max os 4 points
                                    [Part B Total = 10 points]
  THE TERMS PRIMARY, SECONDARY, TERTIARY STRUCTURE OF PROTEINS ARE OFTEN
  USED AS FOLLOWS:
          PRIMARY     = amino acid sequence;
          SECONDARY = coiling of polypeptide chain into alpha helix or interaction
                          of two polypeptides to produce a beta configuration;
          TERTIARY    = folding of the alpha helix to produce more or less globular
                          proteins;
  
  ACTUALLY, THESE TERMS REFER TO TYPES OF FORCES STABILIZING A PROTEIN
  MOLECULE, AND NOT TO ANY ACTUAL GEOMETRIC SHAPE.
     THUS:
          PRIMARY     = covalent bonding of peptides or peptide bond;
  
          SECONDARY = refers to hydrogen bonding as well as various ionic and
                          so-called salt bonds;
  
          TERTIARY    = refers to covalent bonds; interactions between atoms 
                          placed close to each other (i.e. Disulfide bonds);
  
  [QUATERNARY STRUCTURE = alpha & beta chains interacting as in the hemoglobin molecule]