AP Biology: Notes and Practice
DNA Structure
What is the structure of nucleic acids like DNA?
The Structure of Nucleic Acids
1. The figure above displays a monomer called a nucleotide.
a. Explain what it means for a nucleotide to be called a “monomer.”
b. What are the three parts of a nucleotide?
c. Which part of the nucleotide can vary? Explain your reasoning.
AP Biology: Notes and Practice
2. The image on the right side of the figure displays a model of a nucleic acid, a type of polymer.
a. Circle a monomer on the nucleic acid model.
b. Explain why a nucleic acid can be referred to as a “polymer.”
3. Just like there are different types of carbohydrates, proteins, and lipids there are different types of nucleic
acids. One type of nucleic acid is called DNA (deoxyribonucleic acid). The structure of DNA is shown to
the right.
a. How is DNA different from the nucleic acid model shown on page
1?
b. How many nucleic acid polymers bond to build DNA?
c. How do the nucleic acid polymers that bond to build DNA attach
to one another?
d. How many nucleotides does this model of DNA contain? Explain
your reasoning.
e. DNA is described as an antiparallel molecule. Antiparallel means “moving in opposite directions.”
Explain why DNA is an antiparallel molecule.
AP Biology: Notes and Practice
th
Throughout the first half of the 20 century, biologists believed that DNA contained equal amounts of adenine,
thymine, guanine, and cytosine. Recall that these are the nitrogenous bases that are part of nucleotide
monomers that build DNA. A scientist named Erwin Chargaff completed a series of scientific studies to test
whether this scientific hypothesis was supported.
Experiment
Chargaff collected DNA samples from several different organisms. The DNA samples were exposed to acid to
break the nucleotides apart. Each nucleotide absorbs different colors of light based on its structure, so scientists
shined different colors of light through the samples. The amount of light absorbed was directly proportional to
the amount of the nucleotide that absorbs that light color. The overall percentages of adenine, thymine, guanine
and cytosine that Chargaff found in different DNA samples are shown in Table 1 below.
Table 1
Organism %Adenine %Guanine %Cytosine %Thymine
Maize
26.8
22.8
23.2
27.2
Octopus
33.2
17.6
17.6
31.6
Chicken
28.0
22.0
21.6
28.4
Rat
28.6
21.4
20.5
28.4
Human
29.3
20.7
20.0
30.0
Grasshopper 29.3
20.5
20.7
29.3
Sea Urchin 32.8
17.7
17.3
32.1
Wheat
22.7
22.8
27.1
27.3
4. What was scientists’ hypothesis regarding the amounts of the different nitrogenous bases in DNA before
Chargaff’s experiment?
5. Adenine absorbs red light while guanine absorbs blue light. According to the data in table 1, how would the
absorbance of red light compare to the absorbance of blue light in a sample of human DNA? Explain your
reasoning.
6. If the hypothesis you wrote in question 4 was correct, what should the percentages be for adenine, guanine,
cytosine, and thymine in each organism? Explain your reasoning.
7. Based on your answer to question 6 and the data in table 1, was the hypothesis scientists held about DNA
structure correct? Explain your answer, citing data from table 1.
AP Biology: Notes and Practice
8. Chargaff noticed that while nucleotides were not all present in equal amounts, nucleotide pairs were always
present in equal amounts with respect to one another.
a. What nucleotide was always present in about the same amount as adenine? Support your answer
with data from Table 1.
b. What other two nucleotides were present in about the same amounts as each another? Support your
answer with data from Table 1.
9. Based on your answers to question 8, what inference can be made regarding the arrangement of nitrogen
bases in DNA? Explain your reasoning. (Hint! Return to the display of nitrogenous bases on page 1 and
the diagram of DNA on page 2 if you need help)
10. Based on your answers to the previous questions, describe how the hypothesis that Chargaff was testing
should be refined.
11. The nucleotide order for one side of a DNA molecule is written below. Based on your answers to questions
9 and 10, predict the nucleotide order for the other side of the DNA molecule. Explain your reasoning.
A
T
G
A
C
T
G
G
C
A
12. The DNA of salmon, a type of fish, contains approximately 29% thymine. Determine the percent
composition of adenine, guanine, and cytosine in salmon DNA. Explain your reasoning.
AP Biology: Notes and Practice
13. Go back to the DNA diagram on page 2. Notices that the nitrogen bases bond together. The bonds are
indicated by a single line.
a. How many bonds are created between the A-T pairs?
b. How many bonds are created between the G-C pairs?
c. Molecules of adenine (left) and thymine (right) are displayed below. They are pictured in the same
orientation as we would see them in a molecule of DNA bound together in the center of the
molecule. Observing the types of atoms from the two molecules that will be close to each other,
what type of bond do you predict forms between these two molecules? Explain your reasoning.
d. Bonds between A-T molecules and G-C bonds can be broken, but one pair is more stable and
therefore harder to break than the other pair. Which pair would you predict to be more stable and
why?
e. Two different segments of DNA are displayed below. Predict which segment of DNA is more likely
to denature due to increasing the environmental temperature. Explain your reasoning.
Segment 1:
AATGGCTAGCGC
TTACCGATCGCG
Segment 2:
AGCCTAGCCGGC
TCGGATCGGCCG