4. DNA Models

Summary
DNA structure: click on the image to see it rotateDNA structure: click on the image to see it rotateIn 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). The model created by the students makes a lovely classroom decoration and reference for discussing DNA replication, transcription and translation.

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.

Assembling the DNA puzzleAssembling the DNA puzzleVocabulary
DNA
Deoxyribose
Phosphate
Nucleic acid
Adenine
Thymine
Cytosine
Guanine
Base pairs
Nucleotide

AttachmentSize
4dna_models.doc59.5 KB
dna_pieces.pdf1.09 MB

4. DNA Models - Logistics

Time
30 minutes to cut DNA model pieces (It is possible to assign each student a sheet of puzzle pieces to cut out as homework the night before.)
20 minutes to assemble puzzle
10 minutes to tape puzzle together
10-20 minutes to discuss DNA structure and the discovery of the DNA structure

Grouping
Individual students gradually linking their puzzles together to create a long strand.

Materials

  • Scissors
  • Copies of puzzle pieces, each on a different colored paper. For a group of 30 students you will need:
    • 4 copies each of A, C, T, and G
    • 8-9 copies of P
    • 11-12 copies each of S-deoxyribose
  • Several rolls of Scotch tape.
  • Optional: several rolls of 2” packing tape (use if you want to “laminate” the models for display).
  • Bins or trays on which to keep the puzzle pieces. One for every 4-6 students.

Setting
classroom

4. DNA Models - Background

Teacher 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?

Rosalind Franklin: Rosalind FranklinRosalind Franklin: Rosalind FranklinJames 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 DNA model: Physical model built by James Watson and Francis Crick to deduce the structure of DNA. Currently on display in the National Science Museum of London.Watson and Crick DNA model: Physical model built by James Watson and Francis Crick to deduce the structure of DNA. Currently on display in the National Science Museum of London.
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.

Student Prerequisites
None

4. DNA Models - Getting Ready

Getting Ready

  1. Make photocopies of puzzle pieces, each on a different colored paper.
  2. Have students cut out 1 or 2 sheets per person, possibly as homework the night before the activity.

4. DNA Models - Lesson Plan

Lesson Plan

  1. Tell students that today they will be assembling a DNA puzzle. Show students each of the pieces and tell them what the letters on each represent. If you want, tell students the beginning of the DNA discovery story, especially how Watson and Crick 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.
  2. Pass out the puzzle pieces and instruct students to build a DNA molecule. Very little instruction is needed since it is very difficult to put together wrong. The only errors I’ve seen are students who try to flip the pieces so that the letters are hidden and students who try to fit the triangle indent of “T” with the triangle point of “S”.
  3. Circulate around the room and help students as needed. When most students have assembled a molecule 4-5 base pairs long, have students start connecting their molecule together with their neighbors’. At this time, they should also begin taping their molecules together with Scotch tape.
  4. When all the students have merged their pieces into a single long strand, have several students hold up the assembled molecule at the front of the class. Draw conclusions and observations from the group. For instance:
    • What do you notice about the structure overall? What does it look like?
    • How do the two sides of the ladder compare?
    • What are the rungs of the ladder made of?
    • Which nucleic acid pairs with which?
    • How were each person’s individual DNA the same as others’ DNA? How were they different? In what ways you think the real DNA in each person’s cells is the same? In what ways do you think it is different?
  5. Relate the activity back to the real story of the discovery of the DNA structure.
  6. Complete the discussion with a formal definition of the vocabulary words such as base pair and nucleotide.
  7. Optional: “Laminate” the models by covering both sides of the model with packing tape. The model may then be twisted into a helix and hung from the ceiling or walls of the room. You may want to hold off on laminating your model until after discussing DNA replication, transcription, and translation (see Going Further ideas).

Laminated DNA paper puzzleLaminated DNA paper puzzle

4. DNA Models - Assessment

Assessment

  1. Have students draw, label and color pictures of DNA.
  2. Have students build other 3D models of DNA using different materials (beads, candy connected with toothpicks, Styrofoam peanuts, etc.). See the DNA Jewelry Project for one idea or leave the project open ended and have students select their own materials and design.

Going Further

  1. Use the DNA models to illustrate the process of DNA replication, transcription and translation. See Protein Factory lesson.
  2. Show video clips of DNA structure or create a webquest about the search for the DNA structure. The best animations I have seen are available at DNA Interactive. For a great series of web pages telling the story of the discovery, check out the “Finding the Structure” section of DNA Interactive under the main tab “Code”.
  3. For advanced classes such as AP Bio, the 2 hour movie “The Race for the Double Helix”, starring Jeff Goldblum, is a well done dramatization of the many scientists competing against one another in search of the structure of DNA. See this review by Mark Leeper with accompanying discussion questions.

4. DNA Models - Sources and Standards

Sources
This activity was adapted from a DNA model designed by Lori Lambertson of the Exploratorium Teacher Institute.

For additional background materials, see:

  • Wikipedia article on DNA.
  • Read a copy of Watson and Cricks original 1953 article.
  • For the best online DNA resource I’ve seen, go to DNA Interactive. 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:
a. Students know the general structures and functions of DNA, RNA, and protein.
b. 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.