Cell Biology Facts
Kevin C. Hartzog
here for a printable version
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
- 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
- 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
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.
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
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.
- 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
- 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
- When a DNA molecule
replicates, are the two newly finned strands identical to each other?
Why or why not?