DNA

DNA

DNA

RNA

RNA

Protein

Protein

Scientists call this the:

Central �theme of �Molecular �Biology!

Draw and Label: DNA, Gene, Chromosome

DNA packaging

Vocabulary that you must know

• DNA- A molecule that stores genetic information

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• Gene- A segment of DNA that has the instructions for a certain protein

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• Chromosome- A large piece of DNA tightly coiled up. Humans typically have 46 chromosomes stored in their nucleus

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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?

The Genetic Code

• Biologists call the program of the cell the genetic code.
• That genetic code is

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DNA

(Deoxyribonucleic acid)

The Components and Structure of DNA

• The Components and Structure of DNA
• DNA is made up of nucleotides.
• A nucleotide is nucleic acids made up of three basic components:
• a five-carbon sugar called deoxyribose
• a phosphate group
• a nitrogenous base.

The Components and Structure of DNA

• There are four kinds of nitrogenous bases in
• DNA:
• adenine
• guanine
• cytosine
• thymine

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The Components and Structure of DNA

• The backbone of a DNA chain is formed by sugar and phosphate groups of each nucleotide.

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Some Key contributors to the understanding of DNA

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• Fredrick Griffith
• Discovered transformation
• Avery, McCarty, and MacLeod
• Repeated Griffith’s experiment and concluded DNA was the transforming factor
• Alfred Hershey & Martha Chase
• Repeated Avery,McCarty, and MacLeod’s experiment using bacteria and concluded DNA carried the genetic code

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:

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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 transformation

How would you design an experiment to prove that DNA was responsible for transformation?

Avery, McCarty, and MacLeod�Repeated Griffith’s Experiment

Oswald Avery

Maclyn McCarty

Colin MacLeod

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

Carbohydrates

Lipids

Proteins

RNA

DNA

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

S-Type Carbohydrates Destroyed

S-Type Lipids Destroyed

S-Type Proteins Destroyed

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:

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A virus that invades bacteria. It consists of a DNA core and a protein coat

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DNA

Protein coat

Protein coats of bacteriophages labeled with Sulfur-35

DNA of bacteriophages labeled with Phosphorus-32

Bacterium

Bacterium

Phage

Phage

1. 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…

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

The Race to Discover DNA’s Structure

Linus Pauling

1940s

Discovered the alpha-helical structure of proteins.

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?

Purine + Purine = Too wide

Pyrimidine + Pyrimidine = Too Narrow

Purine + Pyrimidine = Perfect Fit from X-ray data

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 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

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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?

Conservative

Semi-Conservative

Dispersive

Hypotheses:

1. Bacteria cultured in medium containing a heavy isotope of Nitrogen (¹⁵N)

Meselson-Stahl Experiment

2. Bacteria transferred to a medium containing elemental Nitrogen (¹⁴N)

Meselson-Stahl Experiment

Meselson-Stahl Experiment

3. DNA sample centrifuged after First replication

Meselson-Stahl Experiment

4. DNA sample centrifuged after Second replication

DNA Replication

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

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

A with T

G with C

DNA Replication

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

DNA Replication

Each parental strand now serves as a template for the new strand.

DNA Replication

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

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

Replication: 2nd step

DNA

Polymerase III

But…

We’re missing �something!

What?

Where’s the�ENERGY�for the bonding!

• Build daughter DNA strand
• add new complementary bases
• DNA polymerase III

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Recognize this?

Energy of Replication

Where does energy for bonding usually come from?

energy

ATP

GTP

TTP

CTP

ADP

AMP

GMP

TMP

CMP

modified nucleotide

energy

We come�with our own�energy!

And we�leave behind a�nucleotide!

You�remember�ATP!�Are there �other ways�to get energy�out of it?

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Are there�other energy�nucleotides?�You bet!

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Energy of Replication

• The nucleotides arrive as nucleosides
• DNA bases with P–P–P
• P-P-P = energy for bonding
• DNA bases arrive with their own energy source for bonding
• bonded by enzyme: DNA polymerase III

ATP

GTP

TTP

CTP

Replication

• Adding bases
• can only add nucleotides to �3′ end of a growing DNA strand
• need a “starter” nucleotide to �bond to
• New strand only grows �5′→3′

DNA

Polymerase III

DNA

Polymerase III

DNA

Polymerase III

DNA

Polymerase III

energy

energy

energy

energy

3′

3′

5′

B.Y.O. ENERGY!

The energy rules�the process

5′

energy

3′

5′

5′

5′

3′

need “primer” bases to add on to

energy

energy

energy

3′

no energy to bond

energy

energy

energy

3′

5′

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