1. Chapter 12
DNA and RNA
12-1: DNA

2. Structure of DNA
nucleotides: monomers of nucleotides made up of a 5-carbon sugar (deoxyribose), a phosphate group, and a nitrogen-containing base
4 different bases
two purines (double-ringed): adenine and guanine
two pyrimidines (single-ringed): cytosine and thymine

3. Purines Pyrimidines Phosphate group Deoxyribose Adenine Guanine
Cytosine
Thymine
Phosphate group
Deoxyribose

4. What do you notice? Source of DNA A T G C
Pneumonia virus
Yeast
Herring
Human
What do you notice?

5. Chargaff’s Rules X-ray Evidence
The amount of thymine always equaled adenine ( [A] = [T] ) and the amount of guanine always equaled cytosine ( [C] = [G] ).
X-ray Evidence
Rosalind Franklin (1950, Britain): used x-ray diffraction to make an x-ray pattern of DNA which suggested there were two strands.

6. The Double Helix Watson / Crick (1953):
Made a 3-dimensional model to show the structure of DNA (with Franklin’s X-ray photos).
Hydrogen bonds form between the two strands.
Base pairing can ONLY occur between adenine (A) and thymine (T), and between guanine (G) and cytosine (C).
A-T
G-C

7. Double Helix The phosphates and sugars are on the outside
The bases are on the inside

8. Nitrogen
bases
sugar
phosphate

9. Sugar-phosphate backbone
Nucleotide
Hydrogen bonds
Sugar-phosphate backbone
Key
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)