Modeling DNA..."It's Elementary Watson...AND Crick!"
Lesson 8 of 9
Objective: Students will be able to trace the evolution of the discovery of the structure of the DNA.
In 1962 James Watson (b. 1928), Francis Crick (1916–2004), and Maurice Wilkins (1916–2004) jointly received the Nobel Prize in physiology or medicine for their 1953 determination of the structure of deoxyribonucleic acid (DNA). Determining the structure of DNA was an engineering mystery for the dynamic trio. The final structure had to be able to contain instructions for assembling proteins and had to be easily and accurately duplicated, so that when a cell divides, it can pass on the correct copy of its genetic information to each new daughter cell. Although making models is fun, building models is also a technique scientists really use to help them figure out how things are put together or how they might work. Watson and Crick used cut-and-paste paper models to help them ultimately solve the mystery of the structure of the the DNA molecule.
The goal of this lesson is to construct a paper model of a DNA helix by creating the fundamental unit of DNA, called a nucleotide. This unit consists of one sugar molecule, one phosphate group, and one nitrogenous base which will be joined together to form the ladderlike helix. Watson and Crick describe this structure in detail in their original Nature magazine publication and close read of the article in part or in its entirety goes perfectly at the beginning or end of this lesson!
As we move into the modeling phase of the lesson, I guide students through a strategy called Diagram Dialogue so that I can present the overall structure of the DNA double helix as well as preview the vocabulary and terminology that will be used in the remainder of the lesson.
For Student or Student Groups:
1. Cut out the pattern for the nucleotide(s) assigned to you or your group.
2. Place the pattern on the appropriate-color paper.
3. Label your pieces in the same way that the pattern is labeled.
4. Glue your nitrogen base to your sugar molecule by matching up the dots.
5. Glue your phosphate group onto your model by matching up the stars.
6. Your teacher will join the nucleotides of all groups in the class together to form a double helix.
At this point in the lesson I pause and survey my students by asking many of the questions listed below. Based on the types of responses students provide, we may extend our learning with an additional Modeling DNA Hands-on Lab activity.
Students can respond to the following inquiries individually or with a partner in writing and the inquiries can be made during a whole group discussion:
1. Do you see any consistent relationship between the DNA bases (puzzle pieces) in one strand of your puzzle and the bases with which they are paired in the other strand? If so, state the nature of the relationship(s) you see.
Yes. T is always paired with A, and C is always paired with G.
2. Half of the puzzle pieces that you were given (the A’s and G’s) were much larger than the other pieces (the C’s and T’s). Did this size difference cause your DNA model to be significantly wider in some parts than in others? If not, why not?
No. Because one of the large bases (A or G) is always paired with one of the small ones (T or C), a double-stranded DNA molecule always has a constant width.
3. Is there any consistent difference in the way that the puzzle pieces in the right-hand strands and the left-hand strands of your model are oriented? If so, what is the difference?
Yes. The two strands run in opposite directions.
4. How can you account for the fact that no matter which bases were selected for the left-hand strand of a DNA molecule, everyone had just the right pieces left over to assemble a matching right-hand strand?
We all started with equal numbers of each kind of nitrogenous base. Because G always pairs with C and T always pairs with A, you always can build a two-stranded molecule if you start with as many A’s as T’s and as many C’s as G’s.
5. Are the two DNA puzzles that you now have the same or different? How can you account for this?
They are identical. Because of the base-pairing rules, each individual strand of a two-stranded DNA molecule contains all of the information required to build a new two-stranded molecule that is just like the starting molecule.
6. What do you suppose biologists call this process of making two identical double stranded DNA molecules from one when it occurs in cells?
They call it “DNA replication.”