AP Biology 2006-2007

DNAThe Genetic Material

AP Biology

Scientific History: § The march to understanding that DNA is

the genetic materialu T.H. Morgan (1908)u Frederick Griffith (1928)u Avery, McCarty & MacLeod (1944)u Erwin Chargaff (1947)u Hershey & Chase (1952)u Watson & Crick (Franklin) (1953)u Meselson & Stahl (1958)

AP Biology

Chromosomes related to phenotype: § T.H. Morgan

u working with Drosophila§ fruit flies

u associated phenotype with specific chromosome§ white-eyed male had specific

X chromosome

1908 | 1933

AP Biology

Genes are on chromosomes: § Morgan’s conclusions

u genes are on chromosomesu 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

What’s so impressiveabout proteins?!

AP Biology

The “Transforming Principle”: 1928§ Frederick Griffith

u Streptococcus pneumonia bacteria§ was working to find cure for pneumonia

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

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

AP Biology

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.

AP Biology

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

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

u injected protein into bacteria§ no effect

u injected DNA into bacteria§ transformed harmless bacteria into

virulent bacteria

1944

What’s theconclusion?

mice die

AP Biology

Oswald Avery Maclyn McCarty Colin MacLeod

Avery, McCarty & MacLeod:§ Conclusion

u first experimental evidence that DNA was the genetic material

1944 | ??!!

AP Biology

Confirmation of DNA:§ Hershey & Chase

u classic “blender” experimentu worked with bacteriophage

§ viruses that infect bacteriau grew phage viruses in 2 media,

radioactively labeled with either § 35S in their proteins§ 32P in their DNA

u infected bacteria with labeled phages

1952 | 1969Hershey

Why useSulfurvs.

Phosphorus?

AP Biology

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?

Hershey & Chase:

Which molecule carries viral genetic info?

AP Biology

AP Biology

Blender experiment:§ Radioactive phage & bacteria in blender

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

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

u Confirmed DNA is “transforming factor”

Oh goodie, I like smoothies

AP Biology

Hershey & Chase:

Alfred HersheyMartha Chase

1952 | 1969Hershey

AP Biology

Chargaff:§ DNA composition: “Chargaff’s rules”

u varies from species to speciesu all 4 bases not in equal quantityu 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

AP Biology

Structure of DNA:§ Watson & Crick

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

wRosalind FranklinwMaurice WilkinswLinus Pauling

1953 | 1962

Franklin Wilkins Pauling

AP Biology

Watson and Crick: 1953 article in Nature

CrickWatson

AP Biology

Rosalind Franklin (1920-1958):

AP Biology

But how is DNA copied?§ Replication of DNA

u base pairing suggests that it will allow each side to serve as a template for a new strand

“It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” — Watson & Crick

AP Biology

Models of DNA Replication:§ Alternative models

u become experimental predictions

conservative semiconservative

Can you designa nifty experiment

to verify?

dispersive

1

2

P

AP Biology

Semiconservative replication:§ Meselson & Stahl

u label “parent” nucleotides in DNA strands with heavy nitrogen = 15N

u label new nucleotides with lighter isotope = 14N

“The Most Beautiful Experiment in Biology”

1958

parent replication

15N parent strands

15N/15N

AP Biology

Bacteria culturedin medium with 15N

(heavy isotope)

Experiment:

Results:

Bacteria transferredto medium with 14N

(lighter isotope)

DNA samplecentrifugedafter first

replication

DNA samplecentrifuged

after secondreplication

Less dense

More dense

1 2

3 4

Most impressive!

AP Biology

Conclusion:Predictions: First replication Second replication

Conservativemodel

Semiconservativemodel

Dispersivemodel

Why didn’t I think of this?

AP Biology

Predictions:1st round of replication

conservative

15N/15N

14N/14N

semi-conservative

15N/14N

dispersive

15N/14N

conservative

15N/15N

14N/14N

semi-conservative

15N/14N

dispersive

15N/14N

2nd round of replication

14N/14N

15N parent strands

15N/15N

✘

✘ ✘✔1

2

P

AP Biology

Franklin Stahl

Matthew Meselson

Matthew Meselson Franklin Stahl

Meselson & Stahl:

AP Biology

Scientific History: § March to understanding that DNA is the genetic material

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

u Frederick Griffith (1928)§ a transforming factor can change phenotype

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

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

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

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

u Meselson & Stahl (1958)§ semi-conservative replication

AP Biology

proteinRNA

The “Central Dogma”:

DNAtranscription translation

replication

§ Flow of genetic information in a cell

AP Biology

Science …. FunParty Time!

Any Questions??