12. 1 DNA: The Genetic

Material

1) Griffith injects mice with disease causing bacteria the mice die

2) Griffith injects mice with harmless bacteria the mice live

3) Griffith uses heat to kill disease causing bacteria mouse lives

4) Griffith uses heat killed bacteria mixed with harmless bacteria the mouse dies

Fredrick Griffith, 1928

What caused the harmless strain to change?

Transformation: the harmless strain of bacteria was changed; some factor from the heat-killed harmless bacteria transferred into the live cells

1944, Oswald Avery

• Repeated Griffith’s experiment

• Made an extract or juice from the heat-killed bacteria

• Determined it was DNA that was the transforming factor

• His conclusions were not widely accepted by scientists

1952, Hershey and Chase

• Conclusively proved that it was DNA causing the mice to die in the previous two experiments (Avery and Griffith)

• They used radio-labeled bacteriophages and E.coli

Describe the structure of DNA

• DNA: deoxyribonucleic acid• Genetic material which also

controls protein synthesis• DNA is a very long, thin molecule

made up of nucleotides• Nucleotides are made up of three

components:

1. deoxyribose (sugar)

2. phosphate

3. nitrogenous base

3 Parts of Nucleotides

• By 1900 the basic chemistry of nucleic acids was determined to consist of three basic subunits:– Sugar + phosphate + nitrogenous base

• By the 1920s, it was known that there are two basic forms of nucleic acids that differed in the type of sugar– Ribonucleic acid (RNA) – ribose is the

sugar– Deoxyribonucleic acid (DNA) –

deoxyribose is the sugar

DNA structure• Nitrogen bases: 1. Purines: double ring in structure -Adenine and Guanine 2. Pyrimidines: single ring

structure -Thymine and Cytosine*Base pairing occurs because

adenine only bonds with thymine and guanine only bonds with cytosine

*A-T, G-C; Chargaff’s Rules

DNA structure• The sides of DNA are

alternating sugars and phosphates

• Strands condense into two long chains, which twists to form double helix

• The order of the nucleotides in the center of DNA determines the traits a person has

• The structure of DNA was determined by Watson and Crick