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Deoxyribonucleic acid, or DNA, carries the hereditary information. DNA and proteins make up the chromosomes of cells. Although the chemical composition of DNA was known in the 1920s, its structure was not determined until the 1950s. James D. Watson and Francis H. C. Crick worked out the structure of DNA in 1953, after long months of research. Watson, Crick and Maurice Wilkins shared the 1962 Nobel Prize for this important discovery. DNA is made up of molecules of the sugar deoxyribose, phosphate groups, and nitrogen bases. The basic unit of DNA, the nucleotide, is made up of one of each. A molecule of DNA may contain as many as 200,000 nucleotides. The nucleotides make up two chains that are linked and twisted around one another in the form of a double helix. OBJECTIVES In this activity you will:
Learn the basic units and structure of DNA.
Use paper models to understand how the units making up DNA fit together.
Use paper models to learn how DNA makes copies of itself.
scissors 1/2-in transparent tape, or glue stick thumbtacks or masking tape sheets of different colored construction paper cardboard PROCEDURES AND OBSERVATIONS Part I. Structure and Composition of DNA
Imagine that you can untwist the DNA ladder. Then study Figure 1, a diagram of the untwisted ladder. Note that the uprights of the ladder consists of alternating units-.phosphate groups and deoxyribose molecules.
Now study Figure 2 to see the structures of deoxyribose and phosphate, and how they chemically bond together. Their symbols are also shown.
The rungs of the DNA ladder consist of pairs of nitrogen bases. There are two kinds of nitrogen bases: purines and pyrimidines. The purines have a two-ringed structure; they are adenine (A) and guanine (G). The pyrimidines have a one-ring structure; they are cytosine (C) and thymine (T).
Figure 3 shows the structures of the four nitrogen bases found in DNA. Note the symbols for the bases.
A nucleotide consists of one nitrogen base, one phosphate group, and one deoxyribose molecule.
Study Figure 4 to see how the phosphate group, deoxyribose molecule, and nitrogen base are related in a nucleotide. Each nitrogen base is attached to the deoxyribose- side of a phosphate-deoxyribose combination. Note that because there are four different nitrogen bases there are four kinds of nucleotides.
Part II. Making Models of DNA
Cut out the phosphate, deoxyribose, and nitrogen base symbols below. Paste them onto a piece of cardboard and cut them out.
Then rise the cardboard symbols to trace symbols on construction paper. Trace and cut out 20 each of the phosphate and deoxyribose symbols and 5 of each nitrogen base symbol. Use a different color paper for each symbol. Label each nitrogen base with its abbreviation.
Make a nucleotide model by laying a phosphate, a deoxyribose, and a nitrogen base symbol on the pattern in Figure 5. Fasten the symbols together with short pieces of transparent tape. Prepare 20 nucleotides. Be sure to attach the symbols at the correct angles to one another. Otherwise your DNA model will not fit together properly.
In DNA, a particular purine always bonds with a particular pyrimidine. Adenine bonds to thymine and guanine bonds to cytosine. The purines and pyrimidines are bonded together by hydrogen bonds.
Study Figure 6 to see how the nitrogen bases are bonded together in a DNA segment. Then construct a 10-rung model segment of DNA using the nucleotides you have assembled. Match up two nitrogen bases, either A-T or G-C, in each ladder rung. Use short pieces of tape for the bonds. The rungs of the ladder must be of equal length. The nucleotides of each strand can be in any sequence, as long as the two nitrogen bases paired together in the rung are correct. Attach the deoxyribose molecules and the phosphate groups of each strand with tape.
Label Figure 7 to show the nucleotide sequences of the DNA model that you constructed. Draw in the shapes of the nitrogen base symbols and label them A, T, G, or C.
Part III. Learning About DNA Replication
DNA can replicate itself. In this way, the hereditary information encoded in its structure is parsed on to new cells formed by mitosis. During replication, the DNA double helix untwists, and the bonds between the nitrogen bases of each rung break. Nucleotides are normal constituents of cells, and as the DNA double helix splits apart, free nucleotides link up to matching nucleotides of each DNA strand according to the rules of base pairing. The two new double-stranded chains then twist into two separate double helixes. In this way two identical DNA molecules are formed.
Lay your DNA model flat on the table. Starting at one end of the model, cut the pieces of tape that connect the nitrogen bases on five of the rungs. Be careful not to cut the symbols. The effect is something like unzipping a zipper. Lay the unzipped model aside.
Then prepare 20 more nucleotides as you did in Part II. Be sure to use the pattern to assemble the nucleotides at the proper angles.
Matching C with G and A with T, attach new nucleotides to both strands of your DNA model, using short pieces of tape.
Cut apart more rungs as you work along your model. Continue to add new nucleotides to each strand until all the rungs have been cut and new nucleotides attached.
Compare the sequences of the two new segments of DNA that you constructed.
Are the two segments alike?
How do their sequences compare with the sequences shown in figure 7?
Toward the end of the class, carefully fasten one of your model segments of DNA to one of your’ neighbor’s model segments. Work together with the rest of your classmates, fastening segments together until one long, ladderlike segment has been formed. With the help of your teacher, attach one end of the segment to the upper left corner of the classroom bulletin board. Use thumbtacks or heavy masking tape
to do it. Carefully twist the DNA model, starting near the attached end, as tightly as its structure permits. Twist it evenly along its entire length. Then fasten the end to the other side of the bulletin board, draping it as necessary to maintain its form.
CONCLUSIONS AND APPLICATIONS
What determines the sequence of the nitrogen bases in a new DNA strand?
Write out the sequence of the new DNA segment that would form next to the segment GGACTGTTA.
If an incorrect nucleotide is incorporated into a fanning strand of DNA, will this mistake lie transmitted to the next generation of DNA molecules that forms from this strand?
When a DNA molecule replicates, are the two newly finned strands identical to each other? Why or why not?