Margaret M. Loos
I.
Objectives
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1. To develop a little awe for patient, hard work paying off.
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2. To develop understanding of the sharing of scientific knowledge and the necessity of its being analyzed with imagination.
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3. To impress on students that today’s understandings form the basis of tomorrow’s discoveries.
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4. To learn the facts that fit together to give us today’s understanding of the DNA structure.
II.
Putting it together
As early as 1907 Fleming noted that there was duplication of the nucleic acids in the nucleus of the cell. A scientist, E. Chargaff, established that there was a quantitative relationship between
Adenine
and
Thymine
and between
Guanine
and
Cytosine
. (In RNA, Uracil replaces Thymine) X-ray people (crystallographers) including
Maurice Wilkens
and his colleagues at King’s College worked to determine the three dimensional aspects of the DNA structure for over 20 years. Research determined that the adenine
only
bonds with thymine (or uracil in RNA) and quinine
only
bonds with cytosine. Further, there are two weak hydrogen bonds between A and T (- . . . ..) and three weak hydrogen bonds between C and G ( . . . . . . ..) After many years of separate scientific studies, two scientists, Watson and Crick, consulted with Wilkens and together they published that DNA is a double helix structure, parallel and complementary in structure. It winds and unwinds and replicates itself. We receive this simple statement and accept it but it took the dedication, perseverance and imagination of many people to bring about “this discovery”. And if that effort isn’t impressive enough remember that these simple combinations of A = T and G = C probably took billions of years to find each other and arrange the code that brings about the replication of life.
Let’s take a look at a diagrammatic representation of the double strand diagram, or chains going in opposite directions. The bonds between them are the “weak” hydrogen bonds that can only hold together because of the many linkages and that allows for easy breaking under the right conditions. These chains separate and each will become a mold or
templateon
which another strand can form. Watson and Crick and Wilkens were awarded the Nobel prize for their model piecing together this twisted ladder of phosphates, sugars, and nucleotides and information from many individual scientists’ research.
(figure available in print form)
III.
Demonstration
: Overlays from Allyn and Bacon “
Molecular
Biology
on DNA Replication.”
Use each chain in the diagram, fill in the possible initials of each nucleotide and predict the chain each is the template for. When you take the two new strands that are formed, what has happened?
IV.
Assignment
:
It is easier to see some of these features of the double helix in colorful drawings than in black and white so each student is to do one of the following:
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1. Bring in an article from a magazine published within the last two years with a visual conceptualization of the double helix and write a paragraph presenting his understanding of the drawing, or
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2. Prepare his own model, drawn on three dimensional conceptualization. (lego can work) I’m bringing zippers myself.
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3. Prepare a list of questions on the material thus far and predict where we’re going.
(figure available in print form)