1. Structure of DNA
Unit 5B.1

2. Genetic information is stored and passed to subsequent generations through DNA molecules and, in some cases, RNA molecules.
Prokaryotes, viruses and eukaryotes contain plasmids, which are small extra chromosomal, double stranded circular DNA molecules

3. Proof that DNA is the carrier of genetic information involved a number of important historical experiments

4. Hershey & Chase Viruses were made of only protein and DNA
Performed an experiment using radioactively labeled viruses that infect bacteria.
Viruses were made of only protein and DNA

5. Procedure
Hershey and Chase labeled the virus DNA with a radioactive isotope and the virus protein with a different isotope.
They then watched the virus infect bacteria cells and followed the radioactive isotopes.

6. Results
When the viruses infected bacteria, only the DNA entered the bacteria cell
Evidence that DNA is the genetic material of a cell
DNA caused bacteria to produce more viruses.

7. Franklin & Wilkins Used X-ray crystallography to take pictures of DNA
helped lead to the discovery of the shape of DNA

8. Watson & Crick Double helix shape
DNA is made of two strands of DNA that twist together like a spiral staircase

9. Avery-MacLeod-McCarty
Proved that DNA was the transforming agent that caused nonpathogenic bacteria to turn pathogenic and kill mice
Found that enzymes that destroy DNA did not transform and that enzymes that destroyed only proteins could transform

10. STRUCTURE OF NUCLEIC ACIDS
Arrow from:
Built from
NUCLEOTIDE
SUBUNITS
NITROGEN BASES
CAN BE:
ADENINE GUANINE CYTOSINE THYMINE URACIL
Sugar can be DEOXYRIBOSE (DNA) RIBOSE (RNA)
Image by: Riedell

11. DNA has no URACIL RNA has no THYMINE PURINES (A & G) have 2 RINGS
PYRIMIDINES (T, C, & U) have 1 RING

12. Directionality of DNA You need to number the carbons! nucleotide
it matters!
nucleotide
PO4
N base 5
CH2
This will be IMPORTANT!! O 4 1
ribose 3 2 OH

13. The DNA backbone Made of phosphates and deoxyribose sugars5
The DNA backbone
PO4
Made of phosphates and deoxyribose sugars
Phosphate on 5’ carbon
attaches to 3’ carbon of next nucleotide
base
CH2 5 O 4 1 C 3 2 O –O P O O
base
CH2 5 O 4 1 3 2 OH 3

14. Double helix structure of DNA
“It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” Watson & Crick

15. Anti-parallel strands
Nucleotides in DNA backbone are bonded from phosphate to sugar between 3 & 5 carbons
DNA molecule has “direction”
complementary strand runs in opposite direction 5 3 3 5

16. Bonding in DNA 5 3 3 5 hydrogen bonds covalent phosphodiester
….strong or weak bonds?
How do the bonds fit the mechanism for copying DNA?

17. Base pairing in DNA Purines Pyrimidines Pairing adenine (A)
guanine (G)
Pyrimidines
thymine (T)
cytosine (C)
Pairing
A : T
2 bonds
C : G
3 bonds

18. CHARGAFF’s RULES Erwin Chargaff analyzed DNA from different organisms and found A = T G = C
Now know its because:
A always bonds with T
G always bonds with C
A Purine always bonds to a Pyrimidine

19. Chromosome Structure in Prokaryotes
Approximately 5 million base pairs 3,000 genes
Chromosome
E. coli bacterium
Bases on the chromosome
DNA molecule in bacteria single DOUBLE STRANDED circular loop
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