BIOL 1010 Lecture Notes - Lecture 4: Dna Ligase, Hydrophile, Tripe
20 Aug 2018
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lecture 4
DNA: the genetic material
Overview
- The experimental evidence
- Chemical structure of DNA/RNA
- Replication of DNA
Discovery of DNA as the hereditary material
1. Miescher (1869) isolates “nuclein”.
2. Fred Griffith (1928) demonstrates transformation of bacteria.
3. A decade later, Oswald Avery purifies the transforming principle, DNA.
4. Hershey-Chase experiment shows that DNA = genes.
5. Watson & Crick (1953) report structure of DNA.
6. Meselson & Stahl (1958) determine DNA replication is semi-conservative.
7. In 1960’s, Arthur Kornberg and others deduce the molecular components of DNA
replication
2. Fred Fred Griffith (1928) demonstrates transformation of bacteria
Experiment
He was working on trying to make a vaccine against pneumonia.
He was studying Diplococcus pneumoniae, the bacterium that causes pneumonia.
He wished to kill the deadly bacterium by heating to make a safe vaccine1
He wanted to see if the heated bacterium (killed) could be used as the vaccine.
He had 2 strains of the vaccine
1- a harmless R strain of d. pneumoniae.
Noticed that if he placed it on a petri dish, the colonies that formed were rough shaped.
2- A virulent (disease-causing) S-strain of bacterium
Noticed that if he placed them on a petri dish, the colonies that formed were smooth
shaped.
= was able to identify which colonies were virulent and harmless because of their texture
could take sample from the virulent form and the non-virulent form
He used both colonies in his work
lecture 4
Step 1
Injects both colonies into different mice, testing if the colonies are really what he thought
Result:
The mice injected with the harmless R-strain bacterium survives
The mice injected with the killer S-strain die
Step 2
Injects the heat killed S cells into the mice (what he wanted his virus to be) and they survive
BUT
When he injected he live R cells and the heat killed S cells the mice dies??
Both survived in the other steps, so this is unexpected, what happened/changed?
Interpretation of Griffith’s experiment
Came up with theories to explain the last unexpected step
1. Maybe the live heat-killed-S cells in the experiment 4 were not really dead?
Rejected because the mice in experiment 3 survives (same colony)
2. Maybe the R-cells in experiment #4 had mutated into the killer form?
Rejected because the mice survived from the same colony in experiment 1
Conclusion from Griffith’s experiment
The ability to cause infection was transferred from the dead S-cells to the harmless R-cells. The
R cells had been TRANSFORMED. The genetic code for virulence was transferred over to the
harmless bacteria.
Vaccine
Biological preparation that provides immunity to a disease. Contains an agent that resembles the disease-causing bacterium but is often made
from weakened or killed form of the bacterium. Therefore, when these enter the body, antibodies are formed against the bacterium so that if it
enters the body alive and stronger, it will recognize it and kill it right away
GRIFFITH INJECTS LIVE AND
HEAT-KILLED Diplococcus
pneumoniae INTO MICE
lecture 4
VIRUSES ARE COMPOSED OF ONLY NUCLEIC
ACID (DNA OR RNA) AND PROTEIN
Transformation
Permanent change in the hereditary system of the R-strained of the bacterium caused by the
killed S-strain.
What was the transformation principle? What was the material that was transferred over?
- DNA or protein? (don’t know the answer yet)
3. A decade later, Oswald Avery purifies the transforming principle, DNA.
Accomplished a decade afterwards.
Identified the transforming principle as DNA.
Showed that DNA can transform the proprieties of cells (not protein).
Showed this by isolating the transforming principle and checking its content
But he didn’t prove that genes = DNA.
People were not convinced that bacterial genes are similar in complex individuals.
4. Hershey-Chase experiment shows that DNA = genes.
Once the technique of DNA purification was established, it allowed Hersey and Chase to show
that DNA was equal to the genetic material with their experiment:
Evidence that DNA was the genetic material from studies of viruses that infect bacteria.
Background about viruses
At the time, biologist already had information about viruses.
- Knew that they were only made up of nucleic acid (DNA or RNA) and protein.
Proof: They purified some viral organisms and cut them up in pieces and the viruses were able to
reassemble with only nucleic and protein.
1. Idea that viruses were simple
Shown with the tobacco mosaic virus
They had bits and pieces of protein and nucleic acid and the virus was able to
resemble
Other examples
b) adenovirus: made of DNA and proteins
c) influenza virus: made of RNA and proteins
d) **T-even bacteriophase: DNA and protein
kind of virus that infect bacteria
VIRUSES ARE COMPOSED OF ONLY NUCLEIC
ACID (DNA OR RNA) AND PROTEIN
Document Summary
He was working on trying to make a vaccine against pneumonia. He was studying diplococcus pneumoniae, the bacterium that causes pneumonia. He wished to kill the deadly bacterium by heating to make a safe vaccine1. He wanted to see if the heated bacterium (killed) could be used as the vaccine. 1- a harmless r strain of d. pneumoniae. Noticed that if he placed it on a petri dish, the colonies that formed were rough shaped. Noticed that if he placed them on a petri dish, the colonies that formed were smooth shaped. = was able to identify which colonies were virulent and harmless because of their texture could take sample from the virulent form and the non-virulent form. He used both colonies in his work lecture 4. Injects both colonies into different mice, testing if the colonies are really what he thought. The mice injected with the harmless r-strain bacterium survives. The mice injected with the killer s-strain die.
