1. And the RACE BEGINS!
Once DNA was identified as the genetic molecule the race was on to determine its structure. The combined work of different researchers led to its discovery in February 1953.

2. Important Experiments of the 1950’s
Erwin Chargaff
Analyzed the DNA of many living organisms. In every sample he found equal amounts of certain nitrogen bases.

3. Chargaff’s DNA Nucleotide Sample Testing Results

4. ADENINE (A) = THYMINE (T) CYTOSINE (C) = GUANINE (G)
In every sample of DNA from every organism, the amount of
ADENINE (A) = THYMINE (T)
CYTOSINE (C) = GUANINE (G)

5. Chargaff’s Research Implications
Chargaff finding explained how all organisms have the same universal DNA but still be so different.
All organisms contain the same nitrogen bases in their DNA but the amount of each base is different for each organism.

6. 2. Rosalind Franklin and Maurice Wilkins
Used a technique called x-ray crystallography to capture the first images of DNA which suggested that it had a spiral or
helical shape and an even width.

7. Rosalind’s boss, Maurice Wilkins, showed her Photo 51 to other researchers without her knowledge.

8. .
3. James Watson and Francis Crick Based their model on other’s research.
Used Franklin’s photograph of DNA and Chargaff’s rules of base pairing to develop a model of DNA.

9. In 1953 they published a paper in the scientific journal Nature.

10. Watson and Crick’s findings
DNA’s structure is:
double stranded
two strands complementary to each other
shaped like a double helix
structure would likely allow it to replicate /copy itself.

11. Awarded the the Nobel Prize in Medicine 1962
Francis Harry Compton Crick
James Dewey Watson
Maurice Hugh Frederick Wilkins

12. Rosalind Franklin died of cancer at the age of 38.
Nobel Prize is not awarded posthumously.

13. KEY CONCEPT DNA structure is the same in all organisms.

14. Nucleic Acid Structure
DNA and RNA are polymers made up of billions of repeating monomer units called nucleotides.
Phosphate
Sugar
Nitrogen-containing
base

15. Monomer - Nucleotide
Polymer – DNA

16. Each Nucleotide is composed of 3 molecules bonded together
Nucleotide Structure
Each Nucleotide is composed of 3 molecules bonded together
1. Phosphate (P)
2. 5 Carbon Sugar (S)
3. Nitrogen-containing base (5 types)
Phosphate
Sugar
Nitrogen-containing
base

17. Nucleotides are named for the nitrogen base they contain.
Types of Nucleotides
There are 5 types of nucleotides but each nucleic acid only contains 4 types.
Nucleotides are named for the nitrogen base they contain.
Nitrogen bases are grouped based on their ring structure.

18. Types of Nucleotides depends on base
1. Pyrimidines – Single ring structure
*Cytosine -- C
*Thymine – T (Only in DNA)
*Uracil – U (Only in RNA)
2. Purines – Double ring structure
*Guanine - G
*Adenine - A .

19. Types of Nucleotides depend on their nitrogen base

21. Explained Chargaff’s Rule
Base Pairing in DNA–
One purine (double ring) hydrogen bonds with one pyrimidine (single ring)
Explained Chargaff’s Rule C G T A

22. Hydrogen bonds between the base pairs hold the two strands of DNA together in the double helix
Base pairing allows the DNA molecule to maintain a uniform width along its entire length C G T A

24. What makes each individual organism unique is the order of their base pairs – the base sequence - in their DNA.

25. LET’S COMPARE DNA TO RNA

26. Comparison of DNA and RNA
DNA RNA
Name Deoxyribonucleic Acid Ribonucleic Acid
Sugar type Deoxyribose Ribose
Nitrogen Bases A,T,G,C A,U,G,C
# of Strands Double stranded Single Stranded

27. DNA
2 stranded
The sugar-phosphate backbone is connected by covalent bonds.
Hydrogen bonds between base pairs hold strands together
hydrogen bond
covalent bond

28. RNA
Single strand instead of double strand
Ribose instead of deoxyribose
Uracil instead of thymine

29. Functions of Nucleic Acids
DNA – Serves as the cell’s “blueprint” for making proteins
RNA – 3 types.
Each type has a specific function in
making proteins.