1. History of DNA
H Biology
Winter 2018

2. What does DNA stand for?
Deoxyribonucleic acid

3. What is DNA made of? Which one? DNA is a macromolecule Double stranded
Made up of nucleotides
Covalently bonded together
Double stranded
Helix
“Spiral”
Which one?

4. What is a nucleotide? Molecule made of Deoxyribose sugar
A phosphate group
A nitrogenous base

5. How was DNA identified as genetic material?
1928 – Griffith notices that bacteria can transfer some type of material to other types of bacteria
“Transforming
principle”
Smooth bacteria had a cover of sugar = smooth looking; rough bacteria lacked this sugar coating
Only LIVE S strands of bacteria (not heated, dead forms) killed the mice
Griffith concluded that some of the material must have been transferred from the heat-killed S strand to the LIVE R strand (which made it living again)  he called this mystery material the “transforming principle”

6. How was DNA identified as genetic material?
1944 – Oswald Avery discovered that the transforming principle was DNA (genetic material)
1952 – Hershey and Chase studied a virus that affects bacteria (bacteriophage) and confirmed that DNA was genetic material
Studied protein and dna – didn’t know which was the material that transferred info – discovered it was DNA after 10 years!

7. How was the structure of DNA discovered?
1950 – Erwin Chargaff
Noticed that the same 4 bases are found in the DNA of all organisms
Also noticed that:
# of Adenine = # of Thymine
# of Cytosine = # of Guanine
“Chargaff’s Rule”
Pure As Gold
Cing Tut lives in a Pyramid

8. How was the structure of DNA discovered?
1950s – James Watson and Francis Crick
Worked to figure out DNA’s structure
Thought that DNA might be a helix, but had no evidence
Idea that DNA was a
helix came from Linus
Pauling

9. How was the structure of DNA discovered?
– Rosalind Franklin
Used x-ray diffractions to show DNA was truly a double helix
Worked with Maurice Wilkins

10. How was the structure of DNA discovered?
1953 – Watson and Crick
Wilkins (a colleague of Franklin) shows Watson and Crick the x-ray pictures
This information gave Watson & Crick the evidence needed to conclude DNA has a helical shape
Made a model of DNA which was made up of two chains of nucleotides

12. DNA Structure

13. DNA - Basics Deoxyribonucleic Acid Stores and transmits genetic info
Tells the cells which proteins to make and when to make them

14. DNA - Basics Made up of nucleotides: Double helix structure
Phosphate group
Sugar
Nitrogen bases (4 total)
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
Double helix structure
(twisted ladder)

15. All about that base, ‘bout the base…

16. Cing Tut lived in a Pyramid
Nitrogen Bases
2 groups that bases are put in based on structure
Purines → 2 carbon rings
Adenine (A)
Guanine (G)
Pyrimidines → 1 carbon ring
Cytosine (C)
Thymine (T)
Pure As Gold
Cing Tut lived in a Pyramid

17. Base Pairing Rules Adenine (A) always matches w/ Thymine (T)
Cytosine (C) always matches w/ Guanine (G)
covalent: between the sugar,base and phosphor goups - hydrogen - from base to base of the different strands

18. Base Pairing Rules Why these base pairings? Bases are specific!
Sizes of the bases (rings)
Number of H bonds formed with each other
The sizes of the bases (and how they pair) also determine the structure of the larger DNA molecule
C forms 3 hyrdogen bonds with G
A forms 2 H bonds with T

19. Purine + Purine = Too wide Pyrimidine + Pyrimidine = Too Narrow
Size of Bases
Purine + Purine = Too wide
Pyrimidine + Pyrimidine = Too Narrow
A – T forms TWO hydrogen bonds
C-G forms THREE hydrogen bond

20. Purine + Pyrimidine = Perfect FitNO
Purine
+ Pyrimidine
= Perfect Fit NO
YES!

21. What makes up the “backbone” of DNA?
The sides of the ladder are made up of:
1) Sugar
2) Phosphate
Alternate along backbone

22. What Holds Everything Together!?
Weak Hydrogen bonds connect the nitrogenous bases to each other
Covalent bonds connect the sugars and phosphates to each other!

23. DNA Structure Summary Double Helix (twisted ladder)
Sides of ladder = sugar/ phosphate backbone
Rungs of the ladder = nitrogen bases