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Transcript DNA_and_Replication

Unlocking the Mystery of
The Ghost of Science Past…
I will take you on a
journey to unravel the
molecular mysteries of
The year is 1928.
We are in the lab of Frederick Griffith as he tries to determine why
certain bacteria give people pneumonia.
Little does he know, he is about to discover something even more
Something was able to be passed from harmful bacteria to
harmless ones, making them DEADLY.
•Genetic information could be transferred from one bacteria to
another. This is called transformation.
The year is 1944.
We are in the lab of Oswald Avery
in New York.
Avery is repeating the experiments
that Griffith did.
But 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?
DNA stores and
transmits the
information from
one generation of
an organism to
the next!!!
It is now 1952.
Alfred Hershey and Martha Chase are busy in their laboratory
studying viruses called bacteriophages. (they infect bacteria!)
The scientists knew that viruses infect cells by injecting their own
genetic material inside, but they weren’t sure what this genetic
material actually was.
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?
DNA is the
genetic material
of the viruses, not
the protein coat!
That’s two
that say DNA is
the genetic
So what is DNA, anyway?
DNA = DeoxyriboNucleic Acid
Long molecule made up of units called nucleotides
3 Basic Parts of DNA:
1. 5-carbon sugar (deoxyribose)
2. A phosphate group
3. Nitrogenous base
Nitrogenous Bases
Adenine pairs with Thymine
(A with T)
Cytosine pairs with
Guanine (C with G)
The bases are held
together by hydrogen
The deoxyribose sugar
and the phosphate group
combine to make up the
Who figured out the
structure of DNA?
A lot of different scientists made
discoveries that led to a model of DNA…
To find out more about
the discoveries about
DNA we’ll visit Erwin
Chargraff’s lab. Erwin
was a biochemist and
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.
Chargraff’s Rule: A=T and C=G
It’s 1952 and Rosalind
Franklin is using Xrays to study the
structure of DNA.
Her X-ray pictures
show a pattern that
suggests two strands
of DNA coiled around
one another in a helix.
The pictures also
suggest that the
nitrogenous bases are
near the center of the
Should we
give some
credit to
Nah. She’s
just a
Meanwhile, Francis Crick
and James Watson are
working in their lab building
3D models of DNA using
wire and cardboard. They
can’t figure anything out
until they are shown a copy
of Rosalind Franklin’s Xray picture.
They 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
Chargraff’s Rule.
Nitrogenous Bases
Adenine pairs with Thymine
(A with T)
Cytosine pairs with
Guanine (C with G)
The bases are held
together by hydrogen
The deoxyribose sugar
and the phosphate group
combine to make up the
Remember that…
DNA is located in the nucleus.
DNA is tightly coiled up around histone
proteins so that it can all fit in the
nucleus. The coiled DNA is called
During mitosis, the DNA is
duplicated and the chromatin
condenses into chromosomes.
DNA is made up of two
complementary strands.
Remember, the base pairing
Due to specific base
pairing, if you know the
sequence of one
strand, you can figure
out the sequence of the
This is why replication can
DNA Replication occurs a replication forks.
The DNA is “unzipped” by an enzyme called
Helicase breaks the hydrogen bonds that
hold the bases together.
Primase gets the strand ready, so that
DNA polymerase can do its job…
DNA polymerase adds nucleotides to the strand,
creating a double-stranded DNA molecule that is a
copy of the original.
It also proofreads its work, to avoid
mistakes in replication.
Replication is
This means that each new
strand of DNA created is
half parental or “old” DNA
and half new DNA.