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DNA: Structure and Replication
Deoxyribonucleic acid, or more simply DNA, is a complex molecule found in all living organisms. It is the chemical of
which genes are composed. An understanding of the organization of this molecule has answered many questions.
Scientists now know how chromosomes can duplicate during cell division and transfer their genetic information to new
chromosomes. Scientists also understand how chromosomes can direct the formation of specific proteins outside the
nucleus while still in the nucleus.
In this investigation, it is expected that you:
(a) Use paper models to construct a portion on a DNA molecule
(b) Use paper models to show replication of a DNA molecule
Part A. Structure of a DNA Molecule
The molecules which make up DNA are deoxyribose, phosphoric acid, and nitrogen bases. There are four bases in DNA.
Each base (guanine, thymine, cytosine, and adenine) is chemically joined to a molecule of deoxyribose. A phosphoric
acid molecule is also joined to the deoxyribose. These three molecules form a group called a nucleotide.
A DNA molecule is “ladderlike” in shape. Deoxyribose and phosphoric acid molecules join to form the sides or uprights of
the ladder. Base molecules join together to form the rungs of the ladder.
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Connect six nucleotides together to form a row in the following sequence from top to bottom:
o Cytosine nucleotide
o Thymine nucleotide
o Guanine nucleotide
o Adenine nucleotide
o Guanine nucleotide
o Cytosine nucleotide
Let this represent the left half of a ladder molecule. This should consist of one side or upright plus six half rungs.
1. Name the two molecules which alternate to form the upright or side portion of a DNA molecule.
2. Name the specific molecule to which each base is attached.
3. Name the molecules or parts of the nucleotide which form the half rungs of the ladder.

Complete the right side of the DNA ladder by matching the bases of the other nucleotides to form complete
rungs. It may be necessary to tip molecules upside down in order to join base combinations. Your completed
model should look like a ladder with matched bases as the rungs. Besides being shaped like a ladder, a DNA
molecule is twisted. It looks like a spiral staircase. However, your paper model cannot show this.
4. Is the order of half-rung bases exactly the same from top to bottom on each side on your model?
5. Only two combinations of base pairings are possible for the rungs. Name these molecule combinations or pairs.
6. If four guanine bases appear in a DNA model, how many cytosine bases should there be?
7. Your DNA model has four guanine bases. Does the number of cytosine bases in your model agree with your
prediction?
8. The following are the bases on the left side of a DNA molecule. List the bases that would make the upright side
of a DNA molecule.
Thymine-Adenine-Guanine-Guanine-Cytosine-Thymine-Cytosine-Adenine-Part B. Chromosome and DNA Replication
Your DNA model represents only a short length of the DNA portion of a chromosome. An entire chromosome
has thousands of rungs rather than only six. Although your model is only a small part of a chromosome, its replication is
the same as that of an entire chromosome during mitosis and meiosis.
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
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Open your DNA model along the point of attachment between base pairs (rungs) and separate the two ladder
halves. (A chromosome untwisted and “unzips” in a similar way to prior to replication.)
Using the left half of your model as a pattern, add new nucleotides to form a new right side.
Build a second DNA model by adding new nucleotides to the right half of the original model.
9. Do the two new molecules contain the same number of rungs?
10. Is the order from top to bottom of the base pairs (rungs) different or the same for each new DNA molecule?
11. What is the name for the original strand of DNA that separated and was copied?
12. How many molecules of adenine are in each DNA molecule? How many molecules of thymine are in each DNA
molecule?
13. Do the numbers of adenine and thymine agree?
14. How many molecules of cytosine are in each DNA molecule? How many molecules of guanine are in each DNA
molecule?
15. Do the numbers of cytosine and guanine agree?
16. Are the two DNA molecules exact copies of each other?
17. What is the name of the process in which DNA makes an exact copy of itself?
18. Explain how the pairing of bases serves as a pattern or code for producing exact chromosome copies during cell
division.