Nucleic Acids

Deoxyribonucleic Acid (DNA)and

Ribonucleic Acid (RNA)

By Kristie AklBy Kristie Akl

Nucleic Acids

• Nucleic Acids– Informational Polymers: Code for all of the

proteins in an organism– 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 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? (instead of protein)

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

1928 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 harmless rough type of bacteria.

• The bacteria “transformed” itself from the harmless type to the deadly type.

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

Hypothesis:

• When the 2 strains were mixed, some FACTOR was transferred from the heat killed cells into the live cells!

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?

1944 Avery, McCarty, and MacLeodRepeated Griffith’s Experiment

Oswald AveryOswald Avery Maclyn McCartyMaclyn McCarty Colin MacLeodColin MacLeod

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

Bacteria

CarbohydratesCarbohydrates LipidLipidss

ProteinProteinss

RNARNA DNADNA

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

1952 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

moviemovie

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.

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

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

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

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 liquid (all the pellet and in the liquid (all activity was from the activity was from the phosphorus so DNA not phosphorus so DNA not protein was genetic material protein was genetic material that infected the cell!)that infected the cell!)

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

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?

Adenine + Guanine = Too Adenine + Guanine = Too widewide

Thymine + Cytosine = Too NarrowThymine + Cytosine = Too Narrow

Adenine + Thymine = Perfect Fit from X-ray Adenine + Thymine = Perfect Fit from X-ray data or Cytosine + Guaninedata or Cytosine + Guanine

The Race to Discover DNA’s Structure

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

The Race to Discover DNA’s Structure

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

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

An enzyme called “helicase” binds to a specific site An enzyme called “helicase” binds to a specific site on the DNA and separates the two strands.on the DNA and separates the 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 polymerase enzyme moves down the strands, and inserts the correct base to pair with each half of DNA molecule.

DNA Replication

Another enzyme called ligase forms bonds Another enzyme called ligase forms bonds between the sugars and phosphates in the DNA between the sugars and phosphates in the DNA backbonebackbone

“Proofreading enzymes” double check the new strands, then strands “zip up” and two new “daughter” DNA molecules are present.

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.