DNA

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

Batch 1: radioactive sulfur (35S)

Batch 2: radioactive phosphorus (32P)

Empty protein shell

Phage DNA

Fig. 16-4-3

EXPERIMENT

Phage

DNA

Bacterial cell

Radioactive protein

Radioactive DNA

Batch 1: radioactive sulfur (35S)

Batch 2: radioactive phosphorus (32P)

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.