AP Biology 2006-2007

DNAThe Genetic Material

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History of Genetic Science

Create a time chart in your notebook- use the book to fill it in-20 min!

Scientist(s)Scientist(s) ExperimentExperiment DateDate ConclusionsConclusions

1850s1850s

19081908

19281928

19441944

19471947

19521952

Early 1950sEarly 1950s

19531953

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Scientific History The march to understanding that DNA is

the genetic material Mendel (1857) T.H. Morgan (1908) Frederick Griffith (1928) Avery, McCarty & MacLeod (1944) Erwin Chargaff (1947) Hershey & Chase (1952) Franklin (Early 1950s) Watson & Crick (1953)

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Mendel’s laws of heredity Law of segregation

each allele segregates into separate gametes

Law of independent assortment genes on separate chromosomes

assort into gametes independently

EXCEPTION linked genes

1857

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Chromosomes related to phenotype T.H. Morgan

working with Drosophila fruit flies

associated phenotype with specific chromosome white-eyed male had specific

X chromosome

1908 | 1933

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Genes are on chromosomes Morgan’s conclusions

genes are on chromosomes but is it the protein or the

DNA of the chromosomes that are the genes? initially proteins were thought

to be genetic material… Why?

1908 | 1933

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The “Transforming Principle” 1928

Frederick Griffith Streptococcus pneumonia bacteria

was working to find cure for pneumonia

harmless live bacteria (“rough”) mixed with heat-killed pathogenic bacteria (“smooth”) causes fatal disease in mice

a substance passed from dead bacteria to live bacteria to change their phenotype “Transforming Principle”

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The “Transforming Principle”

Transformation = change in phenotypesomething in heat-killed bacteria could still transmit disease-causing properties

live pathogenicstrain of bacteria

live non-pathogenicstrain of bacteria

mice die mice live

heat-killed pathogenic bacteria

mix heat-killed pathogenic & non-pathogenicbacteria

mice live mice die

A. B. C. D.

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DNA is the “Transforming Principle” Avery, McCarty & MacLeod

purified both DNA & proteins separately from Streptococcus pneumonia bacteria which will transform non-pathogenic bacteria?

injected protein into bacteria no effect

injected DNA into bacteria transformed harmless bacteria into

virulent bacteria

1944

What’s theconclusion?

mice die

AP BiologyOswald Avery Maclyn McCarty Colin MacLeod

Avery, McCarty & MacLeod Conclusion

first experimental evidence that DNA was the genetic material

1944 |

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Confirmation of DNA Hershey & Chase

classic “blender” experiment worked with bacteriophage

viruses that infect bacteria grew phage viruses in 2 media,

radioactively labeled with either 35S in their protein coat 32P in their DNA

infected bacteria with labeled phages

1952 | 1969Hershey

Why useSulfur

vs.Phosphorus?

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Protein coat labeledwith 35S

DNA labeled with 32P

bacteriophages infectbacterial cells

T2 bacteriophagesare labeled with

radioactive isotopesS vs. P

bacterial cells are agitatedto remove viral protein coats

35S radioactivityfound in the medium

32P radioactivity foundin the bacterial cells

Which radioactive marker is found inside the cell?

Which molecule carries viral genetic info?

Hershey & Chase

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Blender experiment Radioactive phage & bacteria in blender

35S phage radioactive proteins stayed in supernatant therefore viral protein did NOT enter bacteria

32P phage radioactive DNA stayed in pellet therefore viral DNA did enter bacteria

Confirmed DNA is “transforming factor”

Taaa-Daaa!

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Hershey & Chase

Alfred HersheyMartha Chase

1952 | 1969Hershey

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Let the race begin! 1950s – scientific community racing to

find 3-D structure of DNA Major players

Erwin Chargaff James Watson & Francis Crick Linus Pauling Maurice Wilkins & Rosalind Franklin

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Chargaff DNA composition: “Chargaff’s rules”

varies from species to species all 4 bases not in equal quantity bases present in characteristic ratio

humans:

A = 30.9%

T = 29.4%

G = 19.9%

C = 19.8%

1947

That’s interesting!What do you notice?

RulesA = TC = G

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Rosalind Franklin (1920-1958) Wilkins & Franklin used X-ray

crystallography to study DNA structure

Diffraction pattern used to deduce 3-D shape of molecules

DNA was helical in shape!

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Structure of DNA Watson & Crick

developed double helix model of DNA other leading scientists working on question:

Rosalind FranklinMaurice WilkinsLinus Pauling

1953 | 1962

Franklin Wilkins Pauling

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Watson and Crick1953 article in Nature

CrickWatson

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DNA Structure Revealed 1953 – the puzzle pieces are assembled Watson put sugar-phosphate chain on the

outside & nitrogen bases on the inside Sugar-phosphate backbone like the side

ropes of a rope ladder. Pairs of nitrogen bases, one from each

strand, form rungs The ladder forms a twist every ten bases

Watson (age 25) & Crick (age 37) publish a 1 page paper in Nature Franklin’s name was not mentioned

Hear from Watson

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Learning Check Summarize the evidence that DNA is

the genetic material that allows traits to be passed on from one generation to the next

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Scientific History March to understanding that DNA is the genetic material

Gregor Mendel (1857) Traits are carried from one organism to another

T.H. Morgan (1908) genes are on chromosomes

Frederick Griffith (1928) a transforming factor can change phenotype

Avery, McCarty & MacLeod (1944) transforming factor is DNA

Erwin Chargaff (1947) Chargaff rules: A = T, C = G

Hershey & Chase (1952) confirmation that DNA is genetic material

Watson & Crick (1953) determined double helix structure of DNA

AP Biology 2006-2007

Any Questions??