1. Griffith’s Experiments
1928: Frederick Griffith – bacteriologist
Goal: To prepare a vaccine for pneumonia
Vaccine:
injectable substance created with a weakened disease to help produce memory cells to protect against future infections

2. Griffith’s Experiments Cont.
Worked with two strains of S. pneumoniae
Disease-causing strain (S) contains a capsule composed of polysaccharides allows it to be virulent
Virulent: ability to cause disease
Second strain (R) does not contain the capsule – which in turn doesn’t cause disease
Mice injected with S = death
Mice injected with R = no harm

3. Griffith’s Experiments Cont.
What specifically causes the death of the mice?
Capsule?
Griffith treated the S bacteria with heat (no reproduction) and injected the vaccine
No – the mice still lived
What happens if we mix heat-killed S (harmless) and R strain (harmless)?
R bacteria changed to a virulent bacteria – why?
Transformation:
A change in genotype caused by cells taking up foreign genetic material
No known cause of transformation

4. Avery’s Experiments 1944: Oswald Avery – Rockefeller Institute
What substance is responsible for the process of transformation?
Discovered DNA is the material responsible for transformation
DNA contains the genetic instructions for producing the capsule

5. Genetic Material & Replication
Chapter 9 DNA
Genetic Material & Replication

6. DNA’s Role Revealed
The scientific community was skeptical of DNA and its purpose
Alfred Hershey & Martha Chase (1952)
Proved that DNA indeed is the source of transformation using phages
Bacteriophage: virus that infects bacteria
Used bacteriophage T2 and E. coli to perform their experiment

7. DNA’s Role Revealed Cont.
The Experiment:
Step 1: Cultures
Grew one batch of T2 and E. coli in a nutrient medium containing radioactive sulfur (35S)
Another batch in radioactive phosphorous (32P)
Step 2: Infection
Each labeled batch of phages were used to infect separate batches of E. coli
Step 3: Blending
After some time for infection they used a blender to pull off the phages
Ask Why use radioactive sulfur? Tagging
Draw the picture for visual
Step 2: radioactive tags can track the type of material – protein or DNA

8. DNA’s Role Revealed Cont.
Examination:
In 35S phage – most of the phage (upper layer) was still labeled
This means that protein was not injected into the bacteria
In 32P phage – most of the 32P was found in the inner bacterial layers
The DNA was affected and will be passed on to the offspring
Ask Why use radioactive sulfur? Tagging
Draw the picture for visual
Step 2: radioactive tags can track the type of material – protein or DNA

9. Double Helix James Watson & Francis Crick (1950s)
DNA was modeled as a double helix
Meaning it was two strand of DNA twisted with nucleotides linking them together
Nucleotides:
subunits containing a phosphate group, a five carbon sugar & a nitrogen-containing base
Deoxyribose:
The five carbon sugar that is contained within DNA (the D in DNA)

10. Double Helix Cont. Purines Pyrimidines
Purines: two rings of carbon and nitrogen (shorter names)
Pyrimidines: one ring (longer names)

11. Chargaff’s Observations
Erwin Chargaff – Biochemist (1949)
Data from different organism DNA analysis concluded that:
Adenine = the amount of Thymine
Guanine = the amount of Cytosine
However the amount of each pair varied from organism to organism

13. Base Pairing
Rules and Chargaff’s observations allow for only two complementary pairs:
Adenine and Thymine
Guanine and Cytosine
Complementary base pair concluded that the sequence of one strand determines the sequence of its complementary strand
Let the student work one out

14. Roles of Enzymes DNA Replication: Step 1: Unwinding
Process of copying DNA using enzymes
Step 1: Unwinding
DNA Helicase:
an enzyme that unwinds the DNA strand by breaking hydrogen bonds between the pairs
Other enzymes are laid down to prevents the strands from reattaching
Replication Forks:
The areas were the strands are split forming a Y
Draw on the board

15. Roles of Enzymes Step 2: Nucleotide Addition Step 3: Finishing
DNA polymerase:
An enzyme that adds complementary nucleotides to each exposed strand follow the base pairing rules
Two new double helices are formed
Step 3: Finishing
Polymerases are signaled to end with a special nucleotide sequence
The new DNA strand is identical to the template

16. Checking for Errors DNA polymerases have a “proofreading” ability
The ability allows it to only continue if the previous pair is correctly paired
If a mismatch is found – the polymerase can go back and fix it
Proofreading reduces the odds of an error to one error in every one billion nucleotides

17. Rate of Replication Prokaryote – Circle DNA Eukaryote – Helix
Two replication forks to increase rate
Eukaryote – Helix
If there were only one replication fork it would take 33 days to copy
Each section has a replication fork allowing a replication to take 100 seconds

18. Video https://www.youtube.com/watch?v=itsb2SqR-R0