DNA

Discovering the Molecule of Inheritance

QUESTION:

People had long realized that offspring tend to resemble their parents.

Question for scientists: “What is the hereditary molecule that gets passed from parent to offspring?”

WHAT SCIENTISTS KNEW:

1. DNA Components:

- Through testing on extracted DNA samples, scientists understood the basic ingredients of DNA but NOT the structure.

- made of ______ different types of nucleotides

2. Protein Components

- Large 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 Parts 5-carbon sugar (deoxyribose) Phosphate group Nitrogenous Base (4 kinds)

Adenine Guanine Cytosine Thymine

BIG DEBATE

PROTEIN OR DNA?????

Uh oh…sounds like the fixin’s for a science fight…..I hear snapping…..

Frederick Griffith 1928 British Medical Officer Discovers that genetic information can be transferred from heat-killed

bacteria cells to live ones. Pneumococcus (which infects mice),

S (smooth) lethal R (rough) strain non-lethal

Experiment S strain were killed by heat, and their remains were added to R strain bacteria. formerly harmless R strain now was able to kill its host. CONCLUSIONS?

This phenomenon, called transformation, provides the first evidence that the genetic material is a heat-stable chemical.

Oswald Avery

1944 Used two different enzymes

One digested DNA One digested Proteins

  Using different enzymes how can we modify

Griffith’s experiment?

Alfred Hershey and Martha Chase 1952 - Studies viruses,

Bacteriophage – infects bacteria

Viruses composed of a nucleic acid (DNA or RNA) core protein coat (capsid)

Viruses Inject hereditary information (DNA) into

host cell host cell create many more virus particles.

HERSHEY CHASE EXPERIMENT Used radioisotope labels P32 and S35

Phosphorus(P32) is a component of __________________

Sulfur (S35) is a components of ____________.

HMMM…….What could we do with this?????

Results

Observed that the molecule that enters the bacterial cell, causing the infection was …………….

!!!!!!There is a clear correlation between DNA and genetic information.!!!!!!!

DNA Structure Now what? Scientists realized that the DNA molecule

needed to do 3 things

Carry information generation to generation Code had to be translatable into proteins Easily copied during cell division

When you do a puzzle, what strategies do you use?

We knew the parts Sugar, phosphate group, nitrogenous bases

(A, T, C, G)

SHAPE???????

Chargaff’s Data!

Erwin Chargaff

1949

Discovery: amount of adenine equals the amount of thymine amount of guanine equals the amount of cytosine

CHARGAFF’S LAW!

What did scientists know?

DNA molecule of heredity DNA made of nucleotides 4 different types of nucleotides

Difference involves which nitrogenous base it has. Chargaff’s rule; amount of A=T, amount of

C=G Big Question: Now we know the

pieces….how does all this go together?

Rosalind Franklin (1920-1958) Early 1950’s King’s College, London, England Using X-ray photography and crystalized

DNA, took pictures of the molecule. Hinted that the DNA molecule is two strands

wrapped around each other.

James Watson & Francis Crick Working on developing model

of DNA molecule Figured out the “ladder”

1953, shown one of Franklin’s X-ray picture indicating shape of molecule “twisted ladder”

Published paper in 1953 of DNA being a double helix with two strands being wound around each other

Double Helix

Each DNA strand has a Sugar-Phosphate backbone

Nitrogenous bases face inward towards each other

Hydrogen bonds that develop between bases on adjoining strand keep helix together

Nitrogenous Bases

Purines Adenine and Guanine

Pyrimadines Thymine and Cytosine

This gives us 4 different types of nucleotides!

Base Pairing

Chargaff’s rule, A bonds to T G bonds to C.

Weak hydrogen bonds connect the two bases.

NOW WHAT?

How does a simple molecule made of four pieces code for a whole organism?

DNA Protein