Dr. Reichler’s Bio 212  MW10-11am    Print Name:_______KEY_____________
Exam #1  February 5, 2003

    Answer each question as succinctly as possible in the space provided.  If needed, continue on the back.  If you use a drawing as part of your answer, be sure to also include a written explanation.  Read each question carefully and don’t hesitate to ask if a question seems unclear.  These questions have specific answers, although for some, more than one answer is possible.  To receive full credit you must clearly and fully answer the question being asked.  Each question is worth 6 pts, unless otherwise noted, for a total of 100 points possible for this exam.

1. a.  Why does Strong Inference encourage the use of multiple hypotheses?
Strong Inference is based on using disproof.  Human nature makes the disproof of a single hypothesis unlikely.  So if multiple hypotheses are used, we can work to disprove some of them.

b. If you were doing an experiment, how would you know whether you were following rule three of Strong Inference or not?
Any of:  Results are repeatable, the proper( negative and positve) controls are used.

2. a.  Helicase does not require energy to perform its function, but DNA polymerase does require energy.  Explain.
Helicase is breaking hydrogen bonds, which are relatively easy to break.  DNA poly is making covalent bonds, and that requires energy.

b. If you look at one end of a DNA double helix, are the ends of the DNA strands the same or different?  Explain.
Either: there are a 5’ and 3’ end.  Or  there are two different bases as per complementary base pairing.

3. What is one reason DNA would be packaged tightly, and one reason it would be packaged loosely?
Any of:  Tight- to fit in cell, during cell division
Loose- for access to the DNA as in DNA replication, transcription/making of proteins/gene expression

4. In the Meselson-Stahl experiment, they labeled DNA with two different forms of nitrogen (15N and 14N).  What was the question they were trying to answer?
How does DNA replicate?  OR  Does DNA replicate by the conservative, semi-conservative, or disruptive hypothesis?

5. a.  Below are two DNA sequences.  Write in the complementary sequence and label the 5’ and 3’ ends.    3’-ATCCGATCAG-5’            3’-GCATAACTTA-5’
            5’-TAGGCTAGTC-3’                5’-CGTATT GAAT-3’


b. Which DNA sequence in 5a would be most easily separated into single strands?  Why?
The sequence on the right would be more easily separated because it has 3 G/C pairs versus 5 G/C pairs for the sequence on the left.  G/C pairs have 3 hydrogen bonds versus 2 hydrogen bonds for an A/T pair, so they are held together more strongly.

6. a. Why do cells need to replicate their DNA?
For cell division.  OR  To have a copy for the two cells created during cell division.

b. What two things occur at the origin of replication?
Any two of:  Helicase unwinds the DNA.  An RNA primer binds so that the leading strand can begin replicating.  The RNA primer is replaced with DNA.  Ligase bonds the fragments together.

c. What is the function of RNA during DNA replication?
It serves as a primer so that DNA polymerase will recognize a double strand with single strand hanging off and begin to replicate the DNA.

d. What two problems would exist if DNA ligase did not work?
The DNA strands, either leading or Okazaki fragments on lagging strand, would not be covalently connected together.  AND  DNA repair would not be able to reconnect the excised and replaced portion of the DNA.

7. a.  What evidence disproves that all DNA polymerases move in the same direction as the helicases?
Any of:  Presence of Okazaki fragments.  That DNA polymerase can only add bases to 3’ end and DNA is anti-parallel.

b. What are two differences between DNA replication on the leading strand and the lagging strand?
Any two of:  Leading- DNA poly moves in same direction as helicase, DNA replicated in continuous strand, Only single primer/ ligase/ DNA poly needed
    Lagging- DNA poly moves in opposite direction as helicase, DNA replicated in many strands (Okazaki fragments), multiple primer/ ligase/ DNA poly needed

8. How does the cell detect a mismatched base-pair, and what are two mechanisms for correcting the mismatch?
The mismatch causes a distortion in the 3-D structure of the double helix.  It can be repaired by DNA polymerase recognizing the distortion and replacing the incorrect base, or by DNA repair enzymes cutting out the incorrect part and then DNA poly replacing the excised bases.

9. Using rules one and two of Strong Inference (the parts prior to actually doing the experiments), answer the following question:  What affect do replication errors have on humans?  (10 pts)
Must make multiple hypos and create an experiment to eliminate one or more of the hypos.  Example:  Hypos- Replication errors are harmful.  Replication errors are not harmful.  Replication errors are helpful.  Expt- Induce errors in some humans; minimize errors in others.  Look to see if it is helpful/harmful/etc.