DNA: The Road to
The history behind reaching an
understanding and some basic structure
Science is a process!!!
1928 – Frederick Griffith
2 strains of
Mixing the heat
killed virulent with
the nonvirulent =
disease ability to
1944 – Oswald
Redid Griffiths experiment
to determine which
molecule was transferred.
Treated the heat-killed
with an enzyme that
destoys lipids, proteins,
carbohydrates, and other
molecules (including RNA)
Determined the nucleic
acid DNA transmits the
1940’s – Erwin Chargaff
Chragaff’s Rule: Percentages of
Cytosine (C) and Guanine (G) are
nearly the same. This applies to
Adenine (A) and Thymine (T) as well.
Couldn’t explain why though
1952 – Rosalind Franklin
Used x-rays to get the first
recorded pictures on film
X shaped pattern showed the
strands were twisted around
each other like a spring (helix).
The angle of the X suggested
there are two strands
1953 – Watson & Crick
Tried to build a 3-D model of DNA
Then they saw Franklins work . . .
Within weeks they figured out that it was a
Double Helix: Two strands are wound around
DNA is made up of Nucleotides
Nucleotide is the monomer of DNA
There are four
They are: Guanine,
In DNA this is Deoxyribose
(in RNA it is just Ribose)
Phosphate. This and the sugar make up the
“sugar-phosphate backbone” of DNA.
Certain bases pair up with each other
One Purine and one Pyrimidine
Bond together with Hydrogen bonds
Lets look at a 3-d model.
So what is the big deal?
It is the order of
these base pairs that
makes up you!
is different than
It is code that tells
exactly who you are!
Length of DNA
E. Coli (prokaryote found in your
stomach) has 4,639,221 base pairs
About 1.6mm long . . . Which may not seem
long until you think about the size of a
Have to fold it to one one-thousandth of its
What about humans?
Approximately 3,000,000,000 base pairs
If the DNA sequence of the human genome
were compiled in books, the equivalent of 200
volumes the size of a Manhattan telephone
It would take about 9.5 years to read out loud
(without stopping) the 3 billion bases in a person's
genome sequence. This is calculated on a reading
rate of 10 bases per second, equaling 600
bases/minute, 36,000 bases/hour, 864,000
bases/day, 315,360,000 bases/year.
How do we store it then?
1st = Double helix
2nd = wrapped
3rd = Wrapped up
4th = Wrapped in
coils to make
5th = Supercoiled into
See Figure 12-10 on Page a chromosome
297 for better reference
Well . . . sort of . . .
1. Turn to your partner and introduce yourself
(Need to be sitting next to them)
2. Turn to page 291 in the book
3. Get 1 piece of paper, 1 pair of scissors, 1
glue (may run out) per pair
4. Come up with a key for G, C, T, and A
5. Glue down a model beginning with the
“box” shaped DNA (You will be
combining your DNA with your