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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
- 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
- Compare the
sequences of the two new segments of DNA that you constructed.
- Are the two
- 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.
- 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?