Discovery of DNAGriffith, Avery, Hershey & Chase, Pauling, and Watson and Crick

Refresher Questions

What is DNA?And don’t say Code of Life…

What is it made of?

Big Questions: How did we come across the idea of DNA anyway?

How did we find out that it was DNA that holds the genetic material and not proteins?

What Scientists Knew

WHAT SCIENTISTS KNEW:

DNA Components:Through testing on extracted DNA samples, scientists understood the basic ingredients of DNA but NOT the structure.Made of 4 different kinds of nucleotides. 

Protein ComponentsLarge complex molecule made of 20 different amino acids (much variability)

DNA Nucleotides

Scientists at this point understood the chemical components of the DNA molecule (But not the structure!)

3 Basic Parts5-carbon sugar (deoxyribose)Phosphate group Nitrogen Base (4 kinds)

Adenine

Guanine

Cytosine

Thymine 

Griffith: It has to be something

1928 A British scientist, Frederick Griffith, was working with 2 forms of the same bacteria S. pneumoniae. One form causes pneumonia in mammals while the other form doesn’t.

Smooth S. pneumoniae causes pneumonia

Rough S. pneumoniae does not cause pneumonia

The Experiment

Results

Why is this important to the discovery of DNA?

Think about the last part of Griffith’s experiment- How did that last injection kill the mouse?

Griffith figured out it had to be “something” that altered the R strain of the bacteria but he didn’t know what that “something” was

Avery and Others: Go One Step Further

In 1944 Avery, MacLeod, and McCarthy used a process of elimination style experiment.

Avery was very interested in finding out what the “transforming agent” was that Griffith had discovered

The Experiment

Results

If it isn’t RNA, Sugar Coat, or Proteins it has to be DNA.

DNA is responsible for transforming the bacteria

A lot of scientists were skeptical about this discovery due to how simple a molecule of DNA is.

Why would people think DNA is so simple?DNA is only made up of 3 basic parts… proteins are made up of huge long complex chains of amino acids.

Alfred Hershey and

Martha Chase1952- Hershey and Chase devised an experiment to settle the battle of DNA vs. Protein even though we already had some knowledge from Avery

We know now that DNA is the genetic material, but before this experiment we didn’t know if it was DNA or protein

Used bacteriophages in experiments to determine what the genetic material would be

Bacteriophage….What?

A virus that infects bacteriaSo what?

What infects the bacteria?DNA or Protein?

How are we supposed to tell?

The Experiment

Results

After the solution was centrifuged down we were left with a bacteria pellet.

What do you think showed up in the pellet?The radioactive phosphorous or the radioactive sulfur

Why?!

Does this support Avery’s findings?

The Structure of DNAHow do you do a jigsaw puzzle?

What did Scientists Know?

DNA is the mode of inheritance

Made of nucleotides4 different types

Race is on for the structure!!!

Recipe for DNA

A sugar phosphate backbone

A nitrogenous baseNotice the way it faces

Voila DNA

The Impossible Puzzle

The quest to find the DNA structure is on

Chargaff’s Rule%A = %T %C = %G

And this means what? How does this help realize the structure?

Rosalind Franklin

1951 she was hired by King’s College in London to work on improving their X-ray crystallography

X-ray crystallography can photograph shapes and structures of molecules

Her pictures helped to unravel the mystery of the DNA structure

Watson and Crick

Discovered the shape of the DNA structureWith help from Rosalind Franklin and Chargaff

Watson and Crick deduced a lot of information from one of the pictures and through math, speculation, and chemistry they were able to come up with the “twisted ladder” idea.

What does this mean for science?Did Watson and Crick deserve all the credit?

Fun Question: Why wasn’t Franklin included in the Nobel Prize winning work?

A T C GNitrogenous Bases

PurinesAdenine and Guanine

PyrimidinesCytosine and Thymine

What is different between the two?

What do we know so far?

4 Nitrogenous Bases2 purine

2 pyrimidine

A phosphate sugar backbone

We know it’s a double helix structureWhat holds everything together?

Think Chemistry!

Base Pairing and Bonding!

Who talked about %A=%T and that %C=%G?

What kind of bonds hold the nucleotides together?

DNA Replication the Beginning

In mitosis we talk about DNA replicating to make duplicated chromosomes

Now we get to find out how that DNA is copied and translated

Why is it important for us to understand DNA replication?

What about DNA’s structure is going to make it hard to replicate?

Think about which way the bases are facing

Basic Idea

Think about DNA’s Structure