QUESTION IV-1999
4. Scientists seeking to determine which molecule is responsible for the transmission
of characteristics from one generation to the next knew that the molecule must (I) copy itself precisely,
(2) be stable but able to be changed, and (3) be complex enough to determine the organism's phenotype.
• Explain how DNA meets each of the three criteria stated above.
• Selectone of the criteria stated above and describe experimental evidence used to determine that DNA is the hereditary material.
• Describe four of the characteristics of the universal ancestor.
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AP® BIOLOGY
1999 SCORING GUIDELINES
Question IV
Explain how DNA meets each of the three criteria stated above
Select one of the criteria stated above and describe experimental evidence used to determine that DNA is the hereditary material
Note: Part A can earn a maximum of 8 points. Part B can earn a maximum of 2 points.
D. Explain how DNA meets each of the three criteria stated above [8 max]
1. Molecular Properties for Precise COPYING [3 max]
• Template concept (semi-conservative replication)
• Molecular structure[ e.g. complementary base pairing;
A:T,C:G; purine-pyrimidine pairing; antiparallel]
• DNA polymerase function in copying
• Separation concept
• Elaboration of replication [e.g. specific roles of other replication enzymes, proper sequence of steps]
2. Molecular Properties that make it STABLE but ABLE to change [3 max]
Stable
• Energetically favorable arrangement; stable because of
shape of molecule [e.g. double helix; bases in the interior of the helix]
• Energetically favorable arrangement; stable because of bonding [e.g. multiple H bonds; phosphodiester bonds]
• Silent errors [e.g. 'junk" DNA; introns; redundancy of the genetic code]
• Able to be repaired [e.g. proofreading]
Able
• Description of a mutation [e.g. substitution; deletion; insertion; inversion; translocation]
• Crossing over [e.g. during meiosis]
• Base changes [e.g. depurination; deamination; tautomerism]
• Gene rearrangements [e.g. antibody genes in stem cells; transposons]
• Sensitive to mutagens [e.g. UV; X-ray]
• Restriction enzyme recognition sequences
Stable or Able (with justification)
• Methylation
• Telomeres protect ends
3. Molecular Properties that make it COMPLEX enough to determine PHENOTYPE [3 max]
• Colinearity of gene and protein [i.e., base sequence determines aa sequence]
• Infinite base sequence combinations lead to protein variety
• Variable numbers of base pairs per gene lead to different sizes of polypeptides
• Proteins are responsible for phenotype
• Description of transcription and translation
• Chromosome structure as it relates to function [e.g. supercoiling; chromosome or gene inactivation; interaction with histories; etc.]
E. Select one of the criteria stated above and describe experimental evidence used to determine that DNA is the hereditary material [2 max]
Identification of experiment with valid link to any one of the criteria [1 max]:
1) precise copying, 2) stable but able to change, 3) complex enough to determine phenotype, 4) DNA is the hereditary material]
May include but need not be limited to experiments that show:
• DNA can transform bacteria
• viral DNA can re-program cells
• equivalence of A:T and C:G
• double helix structure, [e.g. x-ray crystallography]
• replication is semiconservative
• hereditary enzyme deficiency disorders have genetic links
• DNA codes for protein
• changes in DNA quantities during the cell cycle (mitosis/meiosis)
• chromosome markers linked to disorders
• measurement of mutation rates
• changes in DNA [biotechnology]
Description of EVIDENCE [1 max]
• Evidence provided in the context of an experiment
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