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DNA History
and Structure
Unit 6
The year is 1928.
•Frederick Griffith tries to determine why certain
bacteria give people pneumonia.
•He figures out something is able to be passed from
harmful bacteria to harmless ones, making them
DEADLY.
Discovery:
•Genetic information could be transferred from one
bacteria to another. This is called transformation.
The year is 1944.
•Oswald Avery repeats the
experiments that Griffith did.
•He is trying to figure out which
molecule was transforming the
harmless bacteria into killers.
•He uses enzymes to break
down different molecules.
•When he breaks down DNA,
the harmless bacteria stay
harmless.
What does this tell us about
DNA?
Discovery:
DNA stores and
transmits the
genetic
information from
one generation of
an organism to
the next!!!
!
It is now 1952.
•Alfred Hershey and Martha Chase are busy studying
viruses called bacteriophages. (they infect bacteria!)
•They knew viruses infect cells by injecting their own
genetic material inside, but what is the genetic material?
•Hershey and Chase radioactively marked the viral DNA.
When the viruses infected bacteria, they saw that the
bacteria now contained radioactive material.
What could Hershey and Chase deduce from these
results?
Discovery:
DNA is the
genetic material
of the viruses, not
the protein coat!
That’s two
experiments
that say DNA is
the genetic
material!!!
So what is DNA, anyway?
DNA = DeoxyriboNucleic Acid
Long molecule made up of units called nucleotides
3 Basic Parts of
DNA:
5-carbon sugar
(deoxyribose)
A phosphate group
Nitrogenous base
Who figured out the structure of DNA?
Erwin Chargaff’s
discovered that the
percentage of adenine
present in DNA is the same
as the percentage of
thymine. Also, the
percentages of cytosine
and guanine are equal to
one another.
Chargaff’s Rule: A=T and C=G
It’s 1952
•Rosalind Franklin is
using X-rays to study the
structure of DNA.
DNA
• Franklin’s X-ray pictures show a pattern
that suggests two strands of DNA coiled
around one another in a helix and that the
nitrogenous bases are near the center of
the molecule .
Should we
give some
credit to
Ros?
Nah.
She’s just
a
woman…
•Meanwhile, Francis Crick and
James Watson are building 3D
models of DNA using wire and
cardboard.
• They are stumped until they
see a copy of Rosalind
Franklin’s X-ray picture.
Where is it found?
DNA is tightly coiled up around
histone proteins so that it
can all fit in the nucleus. The
coiled DNA is called
chromatin.
DNA is located in the nucleus.
During mitosis, the DNA is
duplicated and the
chromatin condenses into
chromosomes.
Watson and Crick
• Watson and Crick immediately figure out
that the DNA molecule has to be a
double-helix with two strands of DNA
coiled around one another, nitrogenous
bases facing inward, paired according to
Chargaff’s Rule.
DNA is made up of two
complementary strands.
Remember, the base pairing
rules!
Due to specific base
pairing, if you know the
sequence of one
strand, you can figure
out the sequence of the
other!
This is why replication can
occur!