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
Nucleotides consist of 3 parts.
Phosphate
Nitrogen Base
Pentose Sugar

4. Deoxyribose
(Pentose: 5-Carbon sugar)
b. Phosphoric acid (phosphate)
i. Always attached to the 5’Carbon
c. Nitrogen Base (one of 4)

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. DNA Structure C. DNA strands are ANTI-PARALLEL
Phosphates run in opposite directions
1. One strand runs from 3’  5’
2. the other runs from 5’ 3’
3. The phosphate group is the 5’ end

8. Chargaff

9. V. Chargaff A. Studied nucleotide composition of many species.
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.

10. D.Chargaff’s findings determined the NITROGEN BASE PAIR RULES :
1.Guanine<-->Cytosine
2.Adenine<-->Thymine G C T A C G

11. Label the 5’ and 3’ ends in your notes
Label the Carbon #’s for each of the 4 end Deoxyribose pentose structures.
Label the 5’ and 3’ ends in your notes 5’ 3’ 3’ 5’

12. 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.

13. 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

14. 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.

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

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