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DNA

Structure &

Function

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• Isolated two slightly different strains of

pneumonia bacteria from mice

• Only one of the strains caused

pneumonia = smooth (S) colonies

• Harmless strain = colonies with

rough (R) edges

• Used mice and discovered deadly

bacteria could “transform” harmless

bacteria

Frederick Griffith and

Transformation (1928)

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Transformation• Process: one strain of bacteria is changed

into another strain of bacteria

• Griffith hypothesized since ability to cause

disease was inherited by transformed

bacteria's offspring, the transforming factor

might be a gene (genetic material)

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Oswald Avery’s experiment (1944)

1. Made extract (juice) from heat killed bacteria

2. Used enzymes to destroy

biomolecules & RNA

1. Transformation still occurred

3. Repeated experiment using

DNA destroying enzyme

1. Transformation did NOT occur = DNA must be the

genetic material

Discovered nucleic acid DNA stores & transmits

genetic info from one generation to the next

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Alfred Hershey & Martha Chase

Experiment (1952)• Used bacteriophages (“bacteria eater”)

• Goal: find out which part of the phage (DNA

or protein) produced new phages

• Grew bacteriophages

with radioactive markers

– phosphorus-32 (32P)

only in DNA

– sulfur-35 (35S)

only in Protein

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DNA• Long molecule of units called

nucleotides.

• Nucleotides made of three basic components:

• 5-carbon sugar (deoxyribose)

• a phosphate group

• a nitrogenous base

• 4 kinds of nitrogen bases found in DNA

• adenine (A), guanine (G) = double ringed (purines)

• cytosine (C), thymine (T) = single ringed (pyrimidines)

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Erwin Chargaff

• American biochemist, 1940s

• Discovered percentages of guanine [G]

and cytosine [C] bases are almost equal in

any sample of DNA

– Later found that [A] = [T]

• Scientists had NO idea what this was…

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Chargaff's Rules

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Rosalind Franklin &

Maurice Wilkins (1952)

• Used X-ray diffraction to get info about structure of DNA molecule

• X-shaped pattern showed strands in DNA are twisted around each other,like coils of a spring

– shape known as a helix

– X suggests there are two strands in structure

– Other clues suggest nitrogenous bases are near center of the molecule

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James Watson & Francis Crick

• Trying to understand structure of DNA

– Built 3-D models

• 1953: Shown picture of Franklin’s x-ray &

immediately knew the structure:

• double helix - 2 strands wound around each other

(twisted ladder/spiral staircase)

• Discovered Hydrogen bonds form between

certain nitrogenous bases – base pairing

• Provide just enough force to hold the two strands

together

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Genetic Code

• Genome: organism’s complete set of DNA

(genetic code)

• All living things have a genetic code

– Bacteria – several hundred to several thousand

genes

– Human – ~ 30,000 genes

– Yeast – 6,000 genes

*about 1/3 of yeast genes related to human genes

• Nitrogenous bases hold key to this code

– Order of bases determines what protein is made15

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