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DNA

• Contained in chromosomes containing DNA and protein

• Nucleic acid is made up of nucleotides– Nitrogenous base– Deoxyribose sugar– Phosphate

Important Scientists in the Discovery of DNA

• Frederick Griffith • Oswald Avery• Alfred Hershey and Martha Chase• Rosalind Franklin• Francis Crick and James Watson

Mice and Transformation:

Griffith’s Experiment

Fredrick Griffith

• 1928• Studied Streptococcus pneumoniae• 2 strains

– One pathogenic– One harmless

Frederick Griffith

• Hypothesized that when the live harmless bacteria was mixed with heat-killed disease causing, some “factor” was transferred from the heat-killed bacteria into the live bacteria– Transforming factor might be a gene

Fredrick Griffith

• Transformation: – Definition: change– Harmless bacteria became harmful when mixed

with heat killed bacteria

Oswald Avery• Repeated Griffith’s work (1944)• Made extract from the heat-killed bacteria

• Tested three possible factors for transformation– DNA– RNA– Protein

• What was his conclusion? What factor allowed transformation to occur?

Alfred Hershey and Martha Chase• 1952• The Hershey-Chase Experiment• Studied viruses

(bacteriophages)– Viruses: non-living particles

smaller than a cell that can invade living cells

Fig. 16-3

Bacterial cell

Phage head

Tail sheath

Tail fiber

DNA

100

nm

Hershey and Chase• Specifically looked at T2• T2 invades Escherichia

coli bacteria

• Radioactive isotope of sulfur marked protein coat

• Radioactive isotope of phosphorus marked DNA

• What did they find?

Fig. 16-4-1

EXPERIMENT

Phage

DNA

Bacterial cell

Radioactive protein

Radioactive DNA

Batch 1: radioactive sulfur (35S)

Batch 2: radioactive phosphorus (32P)

Fig. 16-4-2

EXPERIMENT

Phage

DNA

Bacterial cell

Radioactive protein

Radioactive DNA



Empty protein shell

Phage DNA

Fig. 16-4-3

EXPERIMENT

Phage

DNA

Bacterial cell

Radioactive protein

Radioactive DNA



Empty protein shell

Phage DNA

Centrifuge

Centrifuge

Pellet

Pellet (bacterial cells and contents)

Radioactivity (phage protein) in liquid

Radioactivity (phage DNA) in pellet

X-Ray Evidence• Rosalind Franklin • Used X-Ray

crystallography to find out structure of DNA molecules

• Diffracts light to reveal image

X-Ray Evidence• X near center shows DNA

twists around center• Angle of the X suggests

two strands and the nitrogenous bases (hydrophobic) are near the center of the molecule

• Shows diameter of the double helix

The Double Helix

• James Watson and Francis Crick

– Built three-dimensional models of DNA

– Used Rosalind Franklin’s x-ray pictures of DNA to assist in the model

– The Double Helix

The Double Helix

• Two complementary strands of DNA wrapped around each other

• Will have a uniform diameter• Realized that hydrogen bonds held the two

strands together (A with T; C with G)– Two hydrogen bonds between A and T– Three hydrogen bonds between C and G

Nitrogenous Bases• Make up DNA

molecules• Two Types

– Purines – two rings in the structure

• Adenine (A)• Guanine (G)

– Pyrimidines – one ring in the structure

• Cytosine (C)• Thymine (T)

Chargaff’s Rule• Chargaff studied

percentages of nitrogenous bases (1950)

• Percentage of guanine and cytosine are almost equal

• Percentages of adenine and thymine are almost equal

• Chargaff’s Rule supports idea that Adenine (A) bonds to Thymine (T) and Cytosine (C) bonds to Guanine (G)

Fig. 16-5 Sugar–phosphate backbone

5 end

Nitrogenous

bases

Thymine (T)

Adenine (A)

Cytosine (C)

Guanine (G)

DNA nucleotide

Sugar (deoxyribose)

3 end

Phosphate

p. 310 1-31. A fly has the following percentages of

nucleotides in it’s DNA– 27.3% A– 27.6% T– 22.5% G– 22.5% CHow do these numbers demonstrate Chargaff’s rule?

p. 310 1-3

2. How did Watson and Crick’s model explain the basis for Chargaff’s rule?

3. If transformation had not occurred in Griffith’s experiment, how would the results have differed? Explain.