1. Chapter 12-1, Part III
DNA Structure

2. IV. DNA Structure
A. DNA= Deoxyribonucleic acid
*REMEMBER: nucleic acids are one of the 4 major classes of organic molecules.

3. B. The building blocks of DNA are NUCLEOTIDES
1. Nucleotides consist of 3 parts.
a. Deoxyribose (5-Carbon sugar)
b. Phosphoric acid (phosphate)
c. Nitrogen Base (one of 4)

5. Use your pieces: A’s build: B’s build: one adenine nucleotide and
one thymine nucleotide.
B’s build:
one cytosine nucleotide and
one guanine nucleotide.
Compare the nitrogen bases on each nucleotide. How are they different?

6. The Four Nitrogen bases are
i. Adenine (purine=2 rings)
ii. Guanine (purine=2 rings)
iii. Cytosine (pyrimidine=1 ring)
iv. Thymine (pyrimidine=1 ring)

7. Chargaff’s Rules

8. V. Chargaff
A. Studied nucleotide composition of many different samples of DNA.
B. Found:
1. amount of A=T
2. Amount of C=G
C. This pattern has proven true for almost every organism and is known as Chargaff’s Rules.

9. What are the building blocks of DNA?
A. nucleic acids
B. cells
C. nucleotides
D. nucleus

10. How many parts are there to a nucleotide?
B. 3
C. 4
D. 5

11. What two parts are always the same in a nucleotide?
A. adenine and thymine
B. deoxyribose and cytosine
C. phosphate and deoxyribose
D. phosphate and guanine

12. Chargaff found that the amount of
A. A=C
B. C=G
C. T=C
D. G=T

14. VI. Rosalind Franklin-1951
A. Studied DNA molecule with X-ray diffraction.
B. X-ray diffraction: powerful
X-ray beam aimed at a sample and scattering pattern recorded on film.

15. C. Franklin’s pattern suggested DNA was
1. 2 strands
2. Strands twisted around each other
3. Nitrogen Bases are in center
4. Maurice Wilkins- lab assistant

16. VII. Watson & Crick- 1953
A. Were trying to understand structure of DNA.
B. Used clues from Franklin’s images
1. Two strands
2. Strands are twisted around each other
3. Nitrogen bases are in the center
C. Used Chargaff’s rules to determine the strands are complimentary. A-T and C-G
D. Built structural model that showed:
1. How DNA carries info
2. How DNA makes copies.

17. VIII. Watson & Crick- 1953 continued…
A. They discovered that hydrogen bonds could form between:
1. Adenine (A) & Thymine (T)
2. Guanine (G) & Cytosine (C)

18. Of these scientists who do you think contributed the most to figuring out the structure of DNA?
A. Chargaff
B. Franklin
C. Wilkins
D. Watson and Crick

19. B. They described the structure as a DOUBLE HELIX C
B. They described the structure as a DOUBLE HELIX C. Watson, Crick and Wilkins are awarded the Nobel Prize in for the discovery of the structure of DNA.