NAME __________________________________________________ PERIOD ______ DATE _____________
MODELING DNA STRUCTURE AND DNA REPLICATION
IntroductionIn 1953 and 1954, American James Watson and Englishman Francis Crick published reports describing
the structure of DNA and how it replicates. A scientist in London named Maurice Wilkins, revealed an X-ray
photograph of DNA taken by his colleague Rosalind Franklin to Watson. By 1962, Watson, Crick and Wilkins
received the Nobel Prize for their contribution to the discovery of the double helix. Since Franklin had died of
cancer years earlier, she was not awarded the Prize. Historians still speculate whether or not Franklin would
have received the Nobel Prize had she been alive.
DNA is the blueprint of life. DNA found in the nucleus determines all of the cell’s activities. Ultimately,
it controls all of the proteins to be produced by the cell. In order to appreciate fully the power of the DNA
molecule, we must first examine its structure and how it replicates.
Identifying the MoleculesMolecule
Shape
Color
Molecule
Phosphate
Guanine
Deoxyribose
Thymine
Adenine
Cytosine
Constructing a DNA Molecule Instructions and QuestionsQ: Which molecules are the nitrogenous bases?
Q: Which molecules are purines?
Q: Which molecules are pyrimidines?
Q: In actual DNA, which molecules are larger?
Q: Which bases pair (or bond) together?
Q: What is a nucleotide? What molecules make up a nucleotide?
1. Construct a nucleotide, using any nitrogenous base, a
phosphate and deoxyribose molecules. Color, paste it
in the space to the right and label the molecules.
Q: On what molecule is the nitrogenous base bonded
to on the nucleotide?
Shape
Color
2. A DNA molecule is made up of two chains of nucleotides. It looks like a ladder.
Q: Which molecules of the nucleotides make up the sides or backbone of the “ladder”?
Q: Which molecules make up the rungs or steps of the “ladder”?
Q: What kind of bonds holds the backbone together?
3. Construct a strand of nucleotides 3 bases long. Be sure to glue the pieces together and place them near the
top-center of your paper. Then construct the complementary strand and paste the ends together. Don’t glue
it down yet. Remember Chargaff’s base pairing rules!
4. The two strands of nucleotides are bonded together by hydrogen bonds between the nitrogenous bases.
Guanine and Cytosine: three hydrogen bonds
Adenine and Thymine: two hydrogen bonds
5. Now place and glue your complimentary strands together and use dark lines in between the bases to
represent the appropriate number of hydrogen bonds. Your model must show the hydrogen bonds and not
be joined directly together.
Q: DNA resembles a twisted ladder. Thus, the structure of the DNA molecule is called a
6. An illustration of a double helix is shown below. Be sure to examine page 233 of your text as a reference.
Modeling DNA Replication Instructions and Questions7. Before a cell divides, it must first replicate its DNA. “Unzipping” or separating the strands of nucleotides
starts the process. This is accomplished through an enzyme that breaks the hydrogen bonds between the
complimentary strands. Once the bonds are broken, the DNA unravels. Show this by constructing two
strands of nucleotides 3 bases long, glue them below your already constructed “ladder”, and arrange them
so that they look like a zipper being opened. (Use pg. 237 as a reference)
8. Now construct and glue down 3 more bases below, the unzipped portions, except ADD a complimentary
strand for each of the “unzippered” strands (sides). You should now have 2 complete DNA models.
Q: How do you know which nucleotides to use for the new complimentary strand?
Q: How do the new DNA molecules compare to the parent DNA strand?