Unit Genetics and Evolution Title

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Unit


Genetics and Evolution

Title


Paper DNA Models

Summary


In this activity, students “discover” the structure of DNA by playing with puzzle pieces representing the component pieces of the DNA molecule: the sugar deoxyribose, phosphate groups, and the 4 nucleic acids (adenine, thymine, cytosine and guanine). The process the students go through in putting the puzzle together resembles the way James Watson and Francis Crick deduced the molecular structure of DNA by manipulating molecular models of the component pieces (and a heavy reliance on the prior experimental work of Rosalind Franklin, Maurice Wilkins, and Erwin Chargaff).

Objectives


Can model and describe the general structure of DNA.

Can apply base pairing rules to assemble a DNA molecule.

Can infer that the sequence of the nucleic acids in DNA is the key to how DNA provides instructions to the cell.

Can relate this DNA puzzle activity to Watson and Crick’s original discovery of the structure of DNA.



Vocabulary


DNA

Deoxyribose

Phosphate

Nucleic acid

Adenine

Thymine


Cytosine

Guanine


Base pairs

Nucleotide



Materials


  • Scissors

    • Copies of puzzle pieces, each on a different colored paper.

  • Several rolls of Scotch tape.

  • several rolls of 2” packing tape (use to “laminate” the models for display).


Background

Although DNA was isolated in the 1800s, it was not until the 1900s that scientists believed DNA might store genetic information. By 1929, the 3 major components – the sugar deoxyribose, a phosphate group, and a nucleic acid – had been identified. Furthermore, it was known that the phosphate groups linked the molecule together in a long polymer, however it was assumed that the chains were short and that the bases repeated in the same fixed order.

Towards the late 1940s, more and more came to be known. Erwin Chargaff noticed that in any species he studied, the quantity of adenine was always the same as the quantity of thymine while the amount of guanine was the same as the amount of cytosine. This came to be known as “Chargaff’s ratios”. But what did these rations mean? At around the same time, X-ray diffraction data indicated that DNA was coiled in a helical structure. But how many chains were part of the helix? Did the nucleic acids point in toward the center our face out?
James Watson and Francis Crick deduced the structure of DNA in 1953. There were several events that helped them put together the puzzle. First and foremost, the meticulous X-ray diffraction work of Rosalind Franklin and Maurice Wilkins clearly illustrated that the DNA molecule consisted of 2 strands, a double helix, with the nucleic acids on the inside of the molecule. Moreover, the distance between the strands and the pitch of the helix could be precisely measured. With this information, Watson and Crick were able to build a model of the sugar-phosphate backbone of DNA.
The final step of the solution required the use of cardboard models of the 4 nucleic acids. Watson and Crick cut out precise shapes for each nucleic acid. On the hunch that Chargaff’s rule implied a pairing between adenine-thymine and cytosine-guanine, they played with their puzzle pieces to see how they might fit together. They realized that in just the right orientation, adenine-thymine and cytosine-guanine pairs were almost identical in shape, thus providing equally spaced rungs between the 2 backbones of the ladder.

Watson and Crick published their work in 1953 alongside an article by Franklin and Wilkins showing the X-ray diffraction data. In 1962, Watson, Crick and Wilkins were awarded the Nobel Prize for discovering the structure of DNA. By that time, Franklin had died of ovarian cancer. Since Nobel prizes are not awarded posthumously, Franklin could not share in the honor.

Thus the structure of DNA can be said to be composed of two sugar-phosphate backbones, oriented in opposite directions to one another (notice how the sugars on one side are upside-down compared to the sugars on the other strand). The sugars are then attached to a nucleic acid. The nucleic acids are paired such that adenine is always matched to thymine with 2 hydrogen bonds while guanine is always matched to cytosine with 3 hydrogen bonds. A matching pair of nucleic acids is called a base pair. The assembly of one phosphate, sugar and nucleic acid is called a nucleotide.


Plan


  1. Working by yourself or with a partner, you will be assembling a DNA puzzle. You will act as Watson and Crick who created puzzle pieces to represent the different parts and tried to fit the pieces together in a way that made sense with the data that was known at the time. We have a lot more knowledge today thanks to them.

  2. Your model needs to have 32 base pairs and must be a combination of all four bases!

  3. Your model must be in color, so use construction paper or color your nucleotides. Be sure to include a key for me on the base of the model.

  4. “Laminate” the models by covering both sides of the model with packing tape. The model should then be twisted into a helix and hung from a ceiling and a base. Your model must be sturdy!

  5. Draw conclusions and observations from your model. For instance:

    1. What do you notice about the structure overall? What does it look like?

    2. How do the two sides of the ladder compare?

    3. What are the rungs of the ladder made of?

    4. Which nucleic acid pairs with which?
    5. How were each person’s individual DNA the same as others’ DNA? How were they different? In what ways do you think the real DNA in each person’s cells is the same? In what ways do you think it is different?


  6. Relate the activity back to the real story of the discovery of the DNA structure.

  7. Complete the discussion with a formal definition of the vocabulary words such as base pair and nucleotide.

9. All answers to the above questions must be typed. Both you and your partner should respond to the questions on your own. I will have one model and two sets of answers – make sure any question is written in your own words in order to get credit!
For additional background materials, see:

  • Wikipedia article on DNA (http://en.wikipedia.org/wiki/Dna).

  • For a copy of Watson and Cricks original 1953 article, see (www.nature.com/nature/dna50/watsoncrick.pdf).

  • For the best online DNA resource I’ve seen, go to DNA Interactive (http://www.dnai.org/). You will find interviews with scientists, gorgeous computer animations, lesson plans and fabulous web activities.

  • The classic book, The Double Helix: a personal account of the discovery of the structure of DNA by James Watson.

  • An alternative view of the role played by Rosalind Franklin, Rosalind Franklin and DNA by Anne Sayre.



Standards

Grade 7


Cell Biology

1. All living organisms are composed of cells, from just one to many trillions, whose details usually are visible only through a microscope. As a basis for understanding this concept:

c. Students know the nucleus is the repository for genetic information in plant and animal cells.

Genetics

2. A typical cell of any organism contains genetic instructions that specify its traits. Those traits may be modified by environmental influences. As a basis for understanding this concept:

e. Students know DNA (deoxyribonucleic acid) is the genetic material of living organisms and is located in the chromosomes of each cell.


Grade 8


Chemistry of Living Systems (Life Sciences)

6. Principles of chemistry underlie the functioning of biological systems. As a basis for understanding this concept:

c. Students know that living organisms have many different kinds of molecules, including small ones, such as water and salt, and very large ones, such as carbohydrates, fats, proteins, and DNA.

Grades 9-12


Genetics

5. The genetic composition of cells can be altered by incorporation of exogenous DNA into the cells. As a basis for understanding this concept:



  1. Students know the general structures and functions of DNA, RNA, and protein.

  2. Students know how to apply base-pairing rules to explain precise copying of DNA during semiconservative replication and transcription of information from DNA into mRNA.

A MyScienceBox Lesson Plan by Irene Salter (http://www.mysciencebox.org). This work is licensed under the Creative Commons Attribution-NonCommercial License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc/2.5/ or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.


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