DNADNA

People have always wondered …

� How do traits get passed from one

generation to the next?

When did we discover that DNA existed?

� 1869- Johann Freidrich Meischer

� Found in the nucleus

� To simple for heredity

� No one was really sure what DNA did

Early 1900’s

� Other scientists discover the nucleotide

components

� Sugar, nucleotide, nitrogenous bases

Fredrick Griffith - 1928

� Transformation experiment

� What was he trying to do?

� What did he discover?� What did he discover?

� How was his discovery important?

Oswald Avery- 1944

� Avery, MacLeod, McCarty

� What were they trying to do?

� What did they discover?� What did they discover?

� How was his discovery important and how

did people react?

Alfred Hershey and Martha Chase-1944

� What were they trying to do?

� What was their experiment like?

� What did they discover?� What did they discover?

� How was this discovery important?

Now we know lots more

� The nature of science

� Science builds on itself

� Leads to more questions

� Technology allows for more discovery

Once people knew DNA was the genetic material it became a race to discover its structure

� Linus Pauling

� Maurice Wilkins and Rosalind Franklin

� James Watson and Francis Crick� James Watson and Francis Crick

Why has the discovery of DNA structure been so important?

1. Griffith Experiment

� The genetic material in cells, was not clear to

scientists for many years.

� Remember Chromosomes consist of

proteins.proteins.

OR

Scientists were trying to figure which one out

of DNA or protein played the role of the

genetic material. . . ?

This led to several experiments, two of which This led to several experiments, two of which

are very important.

Griffith Transformation Experiment

� He was trying to find a vaccine against

Streptococcus pneumonia (bacteria that

caused pneumonia)

Isolated two different strains of bacteria� Isolated two different strains of bacteria

� Rough surface appearance (R); no

disease/harmless (no coating)

� Smooth surface appearance (S); disease

causing/pathogenic (coating)

He added the

rough strain

And the mouse

lived!

He added the

smooth strain

And the mouse

died

He added the

smooth strain

that had been

killed by heat

And the

mouse lived!

He added smooth

strain killed by heat

with rough strain

And the mouse died.

What happened?

Set up four experiments

� Live R injected=mice lived

� Live S injected=mice died

� Dead S injected=mice lived

� Live R + dead S=mice diedLive R + dead S=mice died

Griffith thought that the killed virulent bacteria had passed on a molecule of inheritance to the non-virulent bacteria.

� Griffith found from this experiment that even though he had killed the S cells, he hadn’t destroyed their hereditary material, which was the one part that caused the which was the one part that caused the disease!

Transformation

� An organism is transformed by gaining genetic material from another organism

� How was the rough bacteria transformed? � How was the rough bacteria transformed?

Oswald Avery 1944

� Continued work of Griffith and found that

extracts of killed bacteria could transform

healthy bacteria cells

Showed the heritable material in extracts � Showed the heritable material in extracts

was DNA (stores and transmits genetic

information)

� People did not believe his results

The Hershey-Chase Experiment

What is a bacteriophage?

� A virus that infects bacteria.

How do viruses work?

� The bacteriophage attaches to the surface of

the bacteria Escherichia coli (E. coli).

� Once attached, the bacteriophage injects DNA into the bacterium.

The DNA instructs � The DNA instructs the bacterium to produce masses of new viruses. So many are produced, that the E. colibursts.

What did Hershey and Chase know about viruses?

1. Viruses are made of DNA and protein

2. Bacteriophages infect bacteria and the cell dies when the new viruses leave

3. The virus injects its genetic material into 3. The virus injects its genetic material into the host cell in order to take it over

4. The purpose of their experiment

� Bacteriophages can be used to

determine what the heritable information

of the cell is because they are made of of the cell is because they are made of

proteins and DNA

5. The experimental procedure� Radioactive sulfur was

added to the proteins.

� Radioactive phosphorous

was added to the DNAwas added to the DNA

Their final results

6.Conclusion: DNA is the heritable material,

not protein

� People finally acknowledge that DNA is the

genetic material!!

Erwin Chargaff 1949

� Studied composition of DNA

� The amount of adenine always equals the

amount of thymine and the amount of

cytosine always equals the amount of cytosine always equals the amount of

guanine.

A=T

C=G

Rosalind Franklin

� X-ray diffraction images of DNA fibers

� Position of atoms can be calculated from

images

� Franklin's data was provided without her knowledge to other researchers named Watson and Crick. These scientists were also working on discovering the structure of DNA. discovering the structure of DNA.

� These scientists used her data and that of other scientists to build their ultimately correct and detailed description of DNA's structure in 1953.

James Watson & Francis Crick 1953

� Put together model for DNA structure; double helix (two strands of DNA bonded together wound in a helix)

Sugar-phosphate backbone (from the outside of the molecule)outside of the molecule)

Nitrogenous base form the middle of the molecule

purines bond with pyrimidines

Won Nobel prize for their discovery!

Answer the Questions

1. Explain what is meant by the term transformation. Why are Griffiths experiments on S. pneumococci considered to be so important?considered to be so important?

2. What's a bacteriophage How does it infect bacteria?

3. Describe the procedure by which Hershey & Chase demonstrated that DNA is the genetic material.