Name: __________________________________
Date: ___________________________________
Flynt ___ - Period
____th Grade Science
LESSON OBJECTIVES
Identify and describe the chemical components of DNA.
Visualize and describe the structure of DNA.
Explain the rules of base pairing in a DNA molecule.
Explain how the structure of DNA and the sequence of chemical bases enables information to be securely
stored, copied and transmitted.
Describe the evidence and identify the key individuals that contributed to the double-helix model of DNA.
LESSON SUMMARY
The Components of DNA
 DNA is a nucleic acid made up of nucleotides joined into long strands or chains by covalent bonds. Nucleotides
may be joined in any order.
 A DNA nucleotide is a unit made of a nitrogenous base, a 5-carbon sugar called deoxyribose, and a phosphate
group.
 DNA has four kinds of nitrogenous bases: adenine, guanine, cytosine, and thymine.
Solving the Structure of DNA
 Erwin Chargaff showed that the percentages of adenine and thymine are almost always equal in DNA. The
percentages of guanine and cytosine are also almost equal.
 Rosalind Franklin’s X-ray diffraction studies revealed the double-helix structure of DNA.
 James Watson and Francis Crick built a model that explained the structure of DNA.
The Double-Helix Model
 The double-helix model explains Chargaff’s rule of base pairing and how the two strands of DNA are held
together. The model showed the following:
 The two strands in the double helix run in opposite directions, with the nitrogenous bases in the center.
 Each strand carries a sequence of nucleotides, arranged almost like the letters in a four-letter alphabet for
recording genetic information.
 Hydrogen bonds hold the strands together. The bonds are easily broken allowing DNA strands to separate.
 Hydrogen bonds form only between certain base pairs–adenine with thymine, and cytosine with guanine. This
is called base pairing.
Getting to Know DNA
1. What does DNA stand for?
2. Where in the cell is DNA found?
3. Describe—in complete sentences—the roles/functions of DNA.
4. Which of the following statements are correct? (You may circle more than one.)
A. Chromosomes are made of DNA.
I.
B. Chromosomes make up DNA.
J. Proteins help to replicate DNA.
C. Chromosomes are made of DNA + protein.
K. Genes are made of several hundred
chromosomes.
D. DNA is a highly coiled chromosome.
E. Chromosomes are made of highly coiled
DNA.
F. Proteins are made of DNA
DNA builds proteins.
L. Each chromosome includes several thousand
genes.
M. Each strand of DNA consists of several
thousand genes.
G. DNA is made of proteins.
H. DNA carries instructions for how to make
proteins.
5. Why is DNA called a double helix?
6. What are the full names of the four different types nitrogen bases found in DNA? Which nitrogen bases always
pair with each other?
7. Which bases are purines and which bases are pyrimidines? How are purines and pyrimidines different?
For Questions 8–13, on the lines provided, answer the questions to correctly label the parts of the DNA molecule
that correspond to the numbers in the diagram.
What type of bond joins the nitrogen base pairs
together?
8. _____________________________________
A sugar, a phosphate and nitrogenous base combine to
form one monomer of DNA. This unit is called what?
9. _____________________________________
What is the structure being identified?
10. _____________________________________
What is the structure being identified?
11. _____________________________________
What is the structure being identified?
12. _____________________________________
What is the structure being identified?
13. _____________________________________
14. The drawing below shows half of a DNA molecule. Fill in the appropriate
letters for the other half. Then explain why you drew your sketch the way you
did.
15. The “handrails” or “backbone” of a DNA molecule are made an alternating
chain of what two different molecules?
16. Research and complete this table to explain how the structure of the DNA molecule allows it to perform each
essential function.
Function
How the Structure of DNA Supports this Function
Store
information
Copy
information
Transmit
information
17. Complete the table to describe each scientist’s contribution to solving the structure of DNA.
Scientist
Erwin Chargaff
Rosalind Franklin
James Watson and
Francis Crick
Contribution
18. Research Chargaff’s Rules. Then complete the table by estimating the percentages of each nitrogen base that
should be present based on Chargaff’s rules.
DNA sample
Percent of
adenine
1
31.5
2
3
Percent of
thymine
Percent of
guanine
30
20
Percent of
cytosine
17