By Kristie Akl. DNA RNA Protein Scientists call this the: Central Dogma of Molecular Biology!

By Kristie Akl

DNADNA

RNARNA

ProteinProtein

Scientists call this Scientists call this the:the:

Central Central

DogmaDogma of of

Molecular Molecular

Biology!Biology!

How do we know that all of our genetic information

comes from DNA?

• What type of experiment would you design to determine that DNA is the source of all genetic information?

Griffith’s Experiment with Pneumonia and the accidental discovery of Transformation

• Frederick Griffiths was a bacteriologist studying pneumonia

• He discovered two types of bacteria:– Smooth colonies– Rough colonies

CONCLUSION:

The smooth colonies must carry

the disease!


• When heat was applied to the deadly smooth type…

• And injected into a mouse…

• The mouse lived!

• Griffith injected the heat-killed type and the non-deadly rough type of bacteria.

• The bacteria “transformed” itself from the heated non-deadly type to the deadly type.


Griffith’s Experiment did not prove that DNA was responsible for

transformationHow would you design an

experiment to prove that DNA was responsible for

transformation?

Avery, McCarty, and MacLeodRepeated Griffith’s Experiment

Oswald AveryOswald Avery Maclyn McCartyMaclyn McCarty Colin MacLeodColin MacLeod

http://osulibrary.orst.edu/specialcollections/coll/pauling/dna/pictures/portrait-avery.jpg

http://profiles.nlm.nih.gov/CC/A/A/C/A/_/ccaaca.jpg

Avery, McCarty, and MacLeodAdded the non-deadly Rough Type of Bacteria to the Heat-Killed Smooth

Type

CarbohydratesCarbohydrates LipidLipidss

ProteinProteinss

RNARNA DNADNA

To the Heat-Killed Smooth Type, To the Heat-Killed Smooth Type, added enzymes that added enzymes that

destroyed…destroyed…

S-Type S-Type Carbohydrates Carbohydrates

DestroyedDestroyed

S-Type S-Type Lipids Lipids

DestroyeDestroyedd

S-Type S-Type Proteins Proteins DestroyeDestroye

dd

S-Type S-Type RNA RNA

DestroyeDestroyedd

S-Type S-Type DNA DNA

DestroyeDestroyedd

Conclusion:Conclusion:

DNA was the DNA was the transforming factor!transforming factor!

The Hershey-Chase Experiment

Alfred Hershey & Martha Chase

worked with a bacteriophage:

A virus that invades

bacteria. It consists of a

DNA core and a protein coat

DNADNA

Protein coatProtein coat

Protein coats of bacteriophages labeled with Protein coats of bacteriophages labeled with Sulfur-35Sulfur-35

DNA of bacteriophages labeled with Phosphorus-32DNA of bacteriophages labeled with Phosphorus-32

BacteriumBacterium

BacteriumBacterium

PhagePhage

PhagePhage1.1. Hershey and Chase Hershey and Chase

mixed the mixed the radioactively-radioactively-labeled viruses labeled viruses with the bacteriawith the bacteria

The viruses infect The viruses infect the bacterial cells.the bacterial cells.


DNA of bacteriophages labeled with Phosphorus-DNA of bacteriophages labeled with Phosphorus-3232

2.2. Separated the Separated the viruses from the viruses from the bacteria by bacteria by agitating the virus-agitating the virus-bacteria mixture in bacteria mixture in a blendera blender


DNA of bacteriophages labeled with Phosphorus-DNA of bacteriophages labeled with Phosphorus-3232

3.3. Centrifuged the mixture so Centrifuged the mixture so that the bacteria would form a that the bacteria would form a pellet at the bottom of the test pellet at the bottom of the test tubetube

4.4. Measured the radioactivity in Measured the radioactivity in the pellet and in the liquidthe pellet and in the liquid

The Hershey-Chase results reinforced the Avery,

McCarty, and MacLeod conclusion:

DNA carries the genetic code!

However, there were still important details to uncover…

How did DNA:1. Store information?

2. Duplicate itself easily?

These questions would be answered by

discovering DNA’s structure

The Race to Discover DNA’s Structure


Linus Linus PaulingPauling 1940s1940s

Discovered the Discovered the alpha-helical alpha-helical structure of proteins.structure of proteins.


19501950

Chargaff’s Rule: Chargaff’s Rule: Equal amounts of Equal amounts of AAdenine and denine and TThymine, and equal hymine, and equal amounts of amounts of GGuanine uanine and and CCytosineytosine

Erwin Erwin ChargaffChargaff

Why do you think Why do you think the bases match up the bases match up

this way?this way?

Purine + Purine = Too widePurine + Purine = Too wide

Pyrimidine + Pyrimidine = Too Pyrimidine + Pyrimidine = Too NarrowNarrow

Purine + Pyrimidine = Perfect Fit from X-ray Purine + Pyrimidine = Perfect Fit from X-ray datadata


Maurice Maurice WilkinsWilkins

Rosalind Rosalind FranklinFranklin

X-Ray diffraction image of X-Ray diffraction image of DNA taken by Franklin in DNA taken by Franklin in

19511951


James WatsonJames Watson Francis CrickFrancis Crick

19531953

Compiled data Compiled data from previous from previous scientists to scientists to build a double-build a double-helical model of helical model of DNADNA

The Race to Discover DNA’s Structure was Over

• DNA is made up of:– Four nucleotides: Adenine, Thymine,

Guanine and Cytosine– These follow the rules of base-pairing:

• Adenine bonds with Thymine• Guanine bonds with Cytosine

– A sugar-phosphate backbone

• DNA is arranged in an double-helix

DNA Replication

• The double helix did explain how DNA copies itself

• We will study this process, DNA replication, in more detail

How does DNA replicate?

ConservativeConservative Semi-Semi-ConservativeConservative

DispersiveDispersive

Hypotheses:

1.1.Bacteria cultured in medium Bacteria cultured in medium containing a containing a heavyheavy isotope of isotope of Nitrogen (Nitrogen (1515N)N)

Meselson-Stahl ExperimentMeselson-Stahl Experiment

2.2.Bacteria transferred to a medium Bacteria transferred to a medium containing elemental Nitrogen containing elemental Nitrogen ((1414N)N)



3.3.DNA sample centrifuged after First DNA sample centrifuged after First replicationreplication


4.4.DNA sample centrifuged after DNA sample centrifuged after Second replicationSecond replication

DNA Replication

The “parent” molecule has two complementary The “parent” molecule has two complementary strands of DNA.strands of DNA.

Each is base paired by hydrogen bonding with its Each is base paired by hydrogen bonding with its specific partner:specific partner:

A with TA with T

G with CG with C

DNA Replication

The first step in replication is the separation of the The first step in replication is the separation of the two strands.two strands.

DNA Replication

Each parental strand now serves as a template Each parental strand now serves as a template that determines the order of the bases along a new that determines the order of the bases along a new complementary strand.complementary strand.

DNA Replication

The nucleotides are connected to form the sugar-The nucleotides are connected to form the sugar-phosphate backbones of the new strands.phosphate backbones of the new strands.

Each “daughter” DNA molecule consists of one Each “daughter” DNA molecule consists of one parental strand and one new strand.parental strand and one new strand.

The Race to Replicate DNA

• Two teams: A and B• Individually, each team member

will run to the board to add a nucleotide to the “unzipped” strand of DNA.

• The first team to finish base-pairing their DNA correctly will win the game.

By Kristie Akl. DNA RNA Protein Scientists call this the: Central Dogma of Molecular Biology!

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