1. Deoxyribonucleic Acid
The Molecular Basis of Inheritance
Who are these guys?

2. Who were the scientists?
How did their work contribute to the discovery or understanding of DNA?

3. Griffith’s Transformation Experiment - 1928
Bacteria could get traits from other bacteria “transforming” their traits.

4. Avery, McCarty and MacLeod - 1944
Refined Griffith’s experiment
Proved transforming agent was nucleic acid.

5. Hershey and Chase – “Blender Experiment “ 1952
Further proved that DNA, not protein, is the hereditary material
Bacteriophages! Viruses that infect bacteria

6. Why was this an important discovery?
Maurice Wilkins and Rosalind Franklin
Were using a technique called X-ray crystallography to study molecular structure
Rosalind Franklin
Produced a picture of the DNA molecule using this technique
(a) Rosalind Franklin
Franklin’s X-ray diffraction
Photograph of DNA
(b)
Figure 16.6 a, b
Why was this an important discovery?

7. Watson and Crick – 1953 Discovered the structure of DNA
Nobel prize in 1962 (with Wilkins)
Deduced that DNA was a double helix
Through observations of the X-ray crystallographic images of DNA from Rosalind Franklin

8. Erwin Chargaff – 1950’s
All organisms have the same bases just in different amounts.
In any DNA:
Base pairing is highly conserved through evolution

9. What is DNA? Primary source of genetic information
RNA can be used in some cases
Eukaryotic cells – multiple, linear chromosomes in nucleus
Mitochondria/chloroplast have circular chromosome
Prokaryotic cells – 1 circular chromosomes in cytosol
Plasmids = separate extra-piece of circular DNA (only a few genes)

10. DNA Structure Monomers = nucleotides Nucleotide structure: Phosphate
Sugar (deoxyribose)
Nitrogen base
Adenine, guanine, thymine, cytosine

11. DNA Structure
Antiparallel - Nucleotides on one side run 5’ to 3’ and the opposing side runs from 3’ to 5’
Complementary strands – nucleotides of one side are bonded with their complement not identical.
EX: Reading strand from 5’ to 3’ left top down is the same as right bottom up. 5 3 3 5

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

13. Nitrogen Bases and Pairing in DNA –
Purines
adenine (A)
guanine (G)
Pyrimidines
thymine (T)
cytosine (C)
Base Pairing
A : T
2 Hydrogen bonds
C : G
3 Hydrogen bonds

14. Base Pairing – Chargaff’s Rule
Amount of T = Amount of A
Amount of G = Amount of C
Ex: Approximately how much thymine would be found in the wheat DNA?
The amount of G’s and C’s is approx. 45% so, the amount of A’s and T’s should be close to 55%. Thus, = 27%

15. Semi-Conservative Replication
Replication of DNA
Base pairing allows each side of original strand serves as a template for a new strand
New strand is 1/2 parent template & 1/2 new DNA

16. Prokaryotic DNA Replication
Replication moves in two directions.
Always occurs 5’ to 3’ only.

17. Eukaryotic DNA Replication
Multiple origin sites

18. DNA Replication: The Process
DNA is unwound by Helicase enzyme
Breaks the hydrogen bonds between the nitrogen bases of each strand
Creates the replication forks
SSB’s stabilize replication fork
single-stranded binding proteins
replication fork

19. Making New Strands Free, unbound nucleotides are in the nucleus
RNA Primase puts on RNA Primer to start new strand
Designates start region of replication
DNA Polymerase III – enzyme that bonds new complementary nucleotides to old strand.
makes new 5’ to 3’strands
can only add nucleotides to 3’ end of parent strand
Leading Strand:
Follows replication fork (as helicase unwinds)
One continuous strand
Lagging Strand:
Opposite direction of fork
Lays down multiple strands (fragments)

20. Making Strands….

21. Replication in a Bubble..5 3 3 5
DNA polymerase III
leading strand 5 3 5 3 5 5 3
lagging strand 5 3 5 3 5 3 5
lagging strand
leading strand
growing
replication fork
growing
replication fork 5
leading strand
lagging strand 3 5 5 5

22. Lagging Strands = Okazaki fragments!
DNA polymerase I replaces RNA primers with DNA nucleotides
DNA ligase attaches the DNA pieces together

23. direction of replication
Replication fork
Animation
DNA polymerase
lagging strand 3’
RNA
primase
Okazaki fragments 5’ 5’
DNA ligase
SSB 3’ 5’ 3’
helicase
DNA polymerase 5’
leading strand 3’
direction of replication
SSB = single-stranded binding proteins

24. Editing & Proofreading DNA
Specific Enzymes that can:
Cuts and removes abnormal bases
proofreads
repairs mismatched
Mutations are a permanent change in DNA

25. Telomeres Telomerase – doesn’t let telomeres shorten
Repeating, non-coding sequences at the end of chromosomes
Shorten after each cell division
shorter telomeres = aging cells
Telomerase – doesn’t let telomeres shorten
works in stem cells and cancer cells