1. The Race to Discover DNA

2. Central Dogma of Molecular Biology!
Scientists call this the:
DNA
DNA
Central Dogma of Molecular Biology!
RNA
RNA
Protein
Protein

3. How do we know that all of our genetic information comes from DNA?
What type of experiment would you design to determine that DNA is the source of all genetic information?

4. The smooth colonies must carry the disease!
Griffith’s Experiment with Pneumonia and the accidental discovery of Transformation
Frederick Griffiths was a bacteriologist studying pneumonia
He discovered two types of bacteria:
Smooth colonies
Rough colonies
CONCLUSION:
The smooth colonies must carry the disease!

5. Griffith’s Experiment with Pneumonia and the accidental discovery of Transformation
When heat was applied to the deadly smooth type…
And injected into a mouse…
The mouse lived!

6. Griffith’s Experiment with Pneumonia and the accidental discovery of Transformation
Griffith injected the heat-killed type and the non-deadly rough type of bacteria.
The bacteria “transformed” itself from the heated non-deadly type to the deadly type.

7. Griffith’s Experiment did not prove that DNA was responsible for transformation
How would you design an experiment to prove that DNA was responsible for transformation?

8. Avery, McCarty, and MacLeod Repeated Griffith’s Experiment
Oswald Avery
Maclyn McCarty
Colin MacLeod

9. To the Heat-Killed Smooth Type, added enzymes that destroyed…
Avery, McCarty, and MacLeod Added the non-deadly Rough Type of Bacteria to the Heat-Killed Smooth Type
To the Heat-Killed Smooth Type, added enzymes that destroyed…
Carbohydrates
Lipids
Proteins
RNA
DNA

10. DNA was the transforming factor!
S-Type Carbohydrates Destroyed
S-Type Lipids Destroyed
S-Type Proteins Destroyed
S-Type RNA Destroyed
S-Type DNA Destroyed
Conclusion:
DNA was the transforming factor!

11. The Hershey-Chase Experiment
Protein coat
Alfred Hershey & Martha Chase worked with a bacteriophage:
A virus that invades bacteria. It consists of a DNA core and a protein coat
DNA

12. DNA carries the genetic code!
The Hershey-Chase results reinforced the Avery, McCarty, and MacLeod conclusion:
DNA carries the genetic code!
However, there were still important details to uncover…

13. How did DNA: 1. Store information? 2. Duplicate itself easily?
These questions would be answered by discovering DNA’s structure

14. The Race to Discover DNA’s Structure

15. The Race to Discover DNA’s Structure
Linus Pauling
1940s
Discovered the alpha-helical structure of proteins.

16. The Race to Discover DNA’s Structure
Why do you think the bases match up this way?
1950
Chargaff’s Rule: Equal amounts of Adenine and Thymine, and equal amounts of Guanine and Cytosine
Purine + Purine = Too wide
Pyrimidine + Pyrimidine = Too Narrow
Erwin Chargaff
Purine + Pyrimidine = Perfect Fit from X-ray data

17. The Race to Discover DNA’s Structure
X-Ray diffraction image of DNA taken by Franklin in 1951
Maurice Wilkins
Rosalind Franklin

18. The Race to Discover DNA’s Structure
1953
Compiled data from previous scientists to build a double-helical model of DNA
James Watson
Francis Crick

19. DNA Structure Deoxyribonucleic acid
Double helix (twisted ladder or strands) of nucleotides (Sugar (deoxyribose), phosphate and a nitrogen base)
Each strand has a sugar and phosphate backbone covalently bonded to a nitrogen base

20. One single strand of DNA…

21. DNA Structure
Double helix is made of covalently bonded strands that are hydrogen bonded to complementary covalently bonded strands
One strand bonds to the second strand via hydrogen bonds (weak enough to break in order to separate the 2 strands)
Each strand measures 3.4 nm/twist or 10 base pairs

22. DNA Double Helix

23. DNA Structure
Strands of DNA are different – they are oriented in opposite directions to each other – they are ANTIPARALLEL
Each end has a number (5’ or 3’ – you say 5 prime or 3 prime)

24. Four Nitrogen Bases
Adenine (A), Guanine (G), Cytosine (C), Thymine (T)
Purines (double ring structures) – Adenine and Guanine
Pyrimidines (single ring structures) – Cytosine and Thymine
Chargaff rules: A –T and T – A
G – C and C - G

25. Nitrogenous Bases Purines Pyrimidines Adenine Guanine Cytosine Thymine
Phosphate
Sugar (deoxyribose)

26. Chromosome Structure
DNA packs tightly around histones to form chromatin.
DNA and histones form bead-like structures called nucleosomes.
Nucleosomes pack together to form supercoils.
Supercoils condense to form chromosomes.

27. Chromosome Structure Nucleosome Chromosome DNA Coils Supercoils
Histones

28. DNA Replication The double helix did explain how DNA copies itself
We will study this process, DNA replication, in more detail

29. How does DNA replicate? Hypotheses: Conservative Semi-Conservative
Have students stand in one of three corners in the lab: Conservative; Semi-Conservative and Dispersive. Have them design an experiment that would prove their point.
Conservative
Semi-Conservative
Dispersive

30. DNA Replication DNA copies itself in the “S” phase of interphase.
1 parent DNA molecule produces 2 daughter DNA molecules, each daughter being made up of “parent” DNA and a strand of “new” DNA (semiconservative process)

31. Steps of DNA Replication
1. DNA unzips – Helicase enzyme breaks hydrogen bonds, unzipping the double helix at the origin of replication (about 100 on a human chromosome).
A replication bubble is formed when DNA unzips
DNA polymerization is bi-directional because of the antiparallel orientation of the DNA strand.

32. DNA Replication Section 12-2 Original strand New strand Growth
Replication fork
DNA polymerase
New strand
Original strand
Nitrogenous bases

33. DNA Replication

34. Steps of DNA Replication
2. Bases pair up – DNA Polymerase bonds free nucleotides to complementary bases
DNA POL reads DNA in 3’ to 5’ direction, thus a new strand elongates only in the 5’ to 3’ direction
Nucleotides are added at a rate of about 50 per second in mammals and 500 per second in bacteria.

35. Steps of DNA Replication
Leading strand has continuous elongation starting at RNA primer since it is read in 3’ to 5’ direction (towards replication fork) by DNA polymerase
Lagging strand has discontinuous elongation
DNA strand is read 3’ to 5’ away from the replication fork in a series of segments called Okazaki fragments
Once fragments are finished, they are joined to previous fragment with enzyme Ligase.

36. DNA Replication

37. Steps of DNA Replication
3. Proofreading and repair
DNA polymerase “proof-reads” newly created DNA strand and identifies incorrect base pairs.
Nuclease (exonuclease) enzyme cuts out the identified incorrect nucleotides.
DNA polymerase places correct nucleotides into DNA strand.
Ligase fuses these corrected nucleotides into the DNA strand