Nucleic Acids– “Informational Polymers”: Code for all of the

proteins in an organism

– Polymer: Nucleic Acid• Monomers: Nucleotides

–Each Nucleotide is made up of:1) Phosphate Group2) Pentose 5-C Sugar

» Ribose or deoxyribose

3) Nitrogenous bases» Adenine (A), Cytosine(C), Thymine (T), Guanine(G), and

Uracil (U)

Nucleic Acids• Polymers– DNA – RNA (tRNA, mRNA,

rRNA)• The nitrogenous bases (A,

C, G, T) are what makes up the DNA “code”

• RNA codes are transcribed or made from DNA codes

• Proteins are then translated or made from RNA codes

DNADNA

RNARNA

ProteinProtein

Scientists call this the:

Central

Dogma of

Molecular

Biology!

How do we know that all of our genetic information

comes from DNA? (instead of protein)

• 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!

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

• 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 with Pneumonia and the accidental discovery of Transformation

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 Avery Maclyn McCarty Colin MacLeod

They then added Heat-Killed Smooth to the non-deadly Rough Type of

Bacteria

Carbohydrates Lipids

Proteins

RNA DNA

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

destroyed…

S-Type Carbohydrates

Destroyed

S-Type Lipids

Destroyed

S-Type Proteins Destroye

d

S-Type RNA

Destroyed

S-Type DNA

Destroyed

Conclusion:

DNA was the 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

DNA

Protein coat

movie

Protein coats of bacteriophages labeled with Sulfur-35

DNA of bacteriophages labeled with Phosphorus-32

Bacterium

Bacterium

Phage

Phage1. Hershey and Chase

mixed the radioactively-labeled viruses with the bacteria

The viruses infect the bacterial cells.

Protein coats of bacteriophages labeled with Sulfur-35

DNA of bacteriophages labeled with Phosphorus-32

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

Protein coats of bacteriophages labeled with Sulfur-35

DNA of bacteriophages labeled with Phosphorus-32

3. Centrifuged the mixture so that the bacteria would form a pellet at the bottom of the test tube

4. Measured the radioactivity in the 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…

But first, Review -- What are the

monomers of DNA?

• Composed of repeating nucleotides containing:– Deoxyribose 5-Carbon sugar– Phosphate group– Nitrogen base: (4 kinds)

Adenine (A) Thymine (T)Cytosine (C) Guanine (G)

The Race to Discover DNA’s Structure

The Race to Discover DNA’s Structure

In the 1940’s, Linus Pauling discovered the alpha-helical structure of proteins.

Stop here to discuss conclusion to Chargaff’s Inquiry Activity

The Race to Discover DNA’s Structure

1950

Chargaff’s Rule: Equal amounts of Adenine and Thymine, and equal amounts of Guanine and Cytosine

Erwin Chargaff

Why do you think the bases match up

this way?

Adenine + Guanine = Too wide

Thymine + Cytosine = Too Narrow

Adenine + Thymine Perfect Fit Cytosine + Guanine

The Race to Discover DNA’s Structure

Maurice Wilkins

Rosalind Franklin

X-Ray diffraction image of DNA taken by Franklin in

1951

The Race to Discover DNA’s Structure

James Watson Francis Crick

1953

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

The Race to Discover DNA’s Structure was Over

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

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

• Purines bond with Pyrimidines:– Adenine bonds with Thymine– Guanine bonds with Cytosine

– A sugar-phosphate backbone• A sugar-phosphate backbone plus

one nitrogenous base is called a nucleotide.

• DNA is arranged in a double-helix, made up of nucleotide monomers.

Purines Pyrimidines

DNA OverviewCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

P

SS

S

S

P

S

P

S

P

P

S

P

S

P

S

S

S

S

a. Double helix c. One pair of bases

P

S

P

P

P

P

P

OC

CC

purine basepyrimidine basephosphate3' end5' end

3' end5' end

5'1'

3' 2'

5'

4' 1'

1'

3' 2'

4'

2' 3'

5'

4'

deoxyribose

b. Ladder structure

3' end 5' end

C C