Here is a CT lesson Paula Hodges uses with her biology students. She comments:

I think that this is a particularly good thinking exercise and one which requires metacognition because most of the kids already have an idea of what DNA looks like and how it replicates. By asking them to connect their model with the evidence, I think they get to better understand why the molecule is constructed the way that it is. In addition, researchers actually did propose three models for replication and the one which was originally thought to be accurate turned out not to be. So it's a good exercise in seeing how science actually works and how scientists think and need to be aware of their thinking.

 


UNRAVELING THE MYSTERIES OF DNA

In 1953, James Watson and Francis Crick proposed a model (based partly on the X-ray diffraction studies of Rosalind Franklin and M.H.F. Wilkins) for the structure of the DNA molecule and an explanation for DNA replication. Strong support for the Watson/Crick replication model came from the experiments of M.S. Meselson and F.W. Stahl.

Part I

 

Using the following evidence, established by these and other scientists, design a DNA molecule.

1. Make a drawing or model to show your hypothesis. Your DNA molecule should be 9 "rungs" long.

2. Explain why you designed the model the way you did by making connections between your design and the evidence you used to support your hypothesis.

EVIDENCE

1. DNA is composed of units called nucleotides. Each nucleotide consists of:

(A) - one sugar molecule (this sugar has 5 carbon atoms)

(B) - one phosphate group molecule (composed of oxygen and phosphorus)

(C) - one nitrogen base molecule (it contains the element nitrogen)

(D) - nucleotides are named by the nitrogen base present.

 
2. There are four different kinds of nitrogen bases:

adenine (A)

guanine (G)

cytosine (C)

thymine (T)

 
3. Molecules of A and molecules of G are the same size as each other.

4. Molecules of C and molecules of T are the same size as each other

5. A and G are LARGER molecules than C and T.

6. The DNA nucleotides are arranged to form a large double-stranded molecule. The arrangement of the nucleotides in the DNA molecule is such that the DNA molecule looks like a ladder with many rungs.

(A) The "rungs" (horizontals) are nitrogen bases (2 nitrogen bases per "rung".
(B) Each nitrogen base is bonded to a sugar.
(C) The "uprights" (verticals) are alternating molecules of the 5-carbon sugar and the phosphate group (the sugar is bonded to the phosphate group).
(D) These "uprights" are informally called the "sugar-phosphate backbone".

 7. In any given sample of DNA

(A) the amount of A equals the amount of T
(B) the amount of C equals the amount of G
(C) however the amount of A+T does not equal the amount of C+G

8. The nitrogen bases "on the rungs" of the DNA "ladder" form weak bonds with each other. (The bonds are not difficult to break or to make.)


UNRAVELING THE MYSTERIES OF DNA

Part II

In order for genetic information to be passed from one generation to the next, the DNA in cells must replicate, in other word, make an exact copy of itself. At first researchers did not know what the replication process looked like. How did DNA replicate? How did it make an exact copy of itself?

Researchers did know the following:

  • 1. The nitrogen bases "on the rungs" of the DNA "ladder" form weak bonds with each other. (The bonds are not difficult to break or to make.) 

    2. Within the cell are many "free" nucleotides of A, T, C, and G. These nucleotides are considered "free" because they are not bonded to other molecules. Each of these "free" nucleotides has 3 phosphate groups instead of one.

    3. Molecules of DNA can be made in the laboratory. Free nucleotides can be made in the laboratory.

    4. Molecules of DNA and the free DNA nucleotides which are made in the laboratory can be made using nitrogen which can be identified. 

    5. Nitrogen has two isotopic forms (N14 and N15). These isotopes can be distinguished from one another in a compound by chemical testing. A scientist can tell if a molecule (nucleotide) was made with nitrogen-14 or with nitrogen-15 or with both N14 and N15.

     

    •


    For this simulation you will have N14 nucleotides to build a DNA molecule and N14 free nucleotides. You will also have N15 nucleotides to build a DNA molecule and N15 free nucleotides. Nucleotides containing N14 are green and nucleotides containing N15 are yellow. The circle on each nucleotide represents the phosphate groups, the long rectangle represents deoxyribose (the sugar), and the box with the letter represents the nitrogen bases of A, T, G, or C.

    Your Assignment

     1. Propose at least two (2) hypotheses to explain how DNA can make an exact copy of itself. REMEMBER: the two "new" DNA molecules must be identical to each other and identical to the "original" DNA molecule. To make the two new DNA molecules you may use as building blocks-- the original DNA (as shown below) and "free" nucleotides available in the cell.

    2. Explain how you would use the N14 and N15 nucleotides to test each of your hypotheses?

     

     

    Use the following DNA molecule as the original which will then be replicated.

     

    +          +

    |-T-A-|

    +          +

    |-C-G-|

    +          +

    |-C-G-|

    +          +

    |-G-C-|

    +          +

    |-A-T-|

    +          +

    |-T-A-|

    +          +

    |-T-A-|

    +          +

    |-C-G-|

    +          +

    |-G-C-|

    + is the symbol for the phosphate
    | is the symbol for the sugar

     

     

     

    A is the symbol for the adenine base
    T is the symbol for the thymine base
    C is the symbol for the cytosine base
    G is the symbol for the guanine base