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Griffith’s Experiment
► Initial
Part of Experiment
Independent Variable: ______________________
Dependent Variable: ________________________
Controlled Variable (constant): ________________
► Summary of Entire Experiment:
________________________________________
► Griffith
concluded that some part of the
heat-killed cells ( a gene ) was transferred
to the live cells. Harmless cells became
harmful.
► This process was called Transformation
(gene transfer).
Canadian biologist Oswald Avery took Griffith’s
experiment one step further.
► Avery made an extract from the heat-killed
bacteria:
1. Treated extract with enzymes that destroy
proteins, lipids, carbohydrates, and RNA 
transformation still occurred
2. Treated extract with enzymes that break down
DNA  no transformation
► Why did this occur?
►
► Conclusion:
► Avery
and other scientists discovered that
DNA is the nucleic acid that stores and
transmits the genetic information from one
generation of an organism to the next.
DNA & DNA Replication
The Hershey-Chase Experiment
► Bacteriophage:
One kind of virus that
infects and kills bacteria.
► American scientists, Alfred Hershey and
Martha Chase performed this experiment in
1952.
► Bacteriophages consist of a DNA or RNA
core and a protein coat.
Bacteriophage Virus
► Problem:
Are genes made of DNA or
protein?
► A virus injects genes into the bacteria 
They used radioactive markers to see if it
was DNA or protein
► Avery already concluded that genes are
found on DNA. Why are Hershey and Chase
doing this experiment?
►
►
What would
be your
conclusion
after
observing
this?
Why?
► Hershey
and Chase concluded that the
genetic material of the bacteriophage was
DNA, not protein.
Structure of DNA
►
►
1.
2.
3.
DNA is a long molecule made up of
nucleotides.
3 parts of a nucleotide:
A 5-carbon sugar called deoxyribose
A phosphate group
A nitrogenous (nitrogen-containing) base
(There are four kinds of nitrogenous bases
in DNA)
► Erwin
Chargaff, an American biochemist,
made observations about DNA’s nucleotides
that developed into Chargaff’s Rule:
► [A] = [T]
► [G] = [C]
Watson and Crick
► In
1953, Francis Crick, a British physicist,
and James Watson, an American biologist,
made 3D models to try and understand the
structure of DNA.
► They discovered that DNA was a double
helix (two strands wound around each
other)
► Problem:
What forces hold the two strands
together?
► Watson and Crick discovered that hydrogen
bonds could form between certain
nitrogenous bases and provide just enough
force to hold the two strands together.
► Base Pairing: hydrogen bonds form between
adenine and thymine and also between
guanine and cytosine. (Chargaff’s rule)
DNA Length
► Most
prokaryotes, such as an E. Coli
bacterium, have a single circular
chromosome. This chromosome must hold
about 4,639,221 base pairs of DNA.
Chromosome Structure in Eukaryotes
► DNA
in Eukaryotic cells must be packed
even more tightly. The nucleus of a human
cell contains more than 1 meter of DNA.
► Chromatin: tightly packed DNA and protein
► Histones: the proteins in which the DNA is
tightly coiled around
► Nucleosome: a beadlike structure formed by
the DNA and histones.
DNA Replication
► Problem:
How can DNA be copied, or
replicated?
► After Watson and Crick discovered the DNA
structure, they also realized that each
strand of the DNA double helix has all the
information needed to reconstruct the other
half by base pairing.
► The strands are complementary, each
strand can be used to make the other.
► Replication:
the copying process that
duplicates DNA before a cell divides.
► During DNA replication, the DNA molecule
separates into two strands
► Then produces two new complementary
strands following the rules of base pairing.
► Each strand of the double helix of DNA
serves as a template, or model, for the new
strand. (semi-conservative)
► Replication
fork: the sites on DNA where
separation and replication occur.
► DNA Helicase: the enzyme that “unzips” a
molecule of DNA. Hydrogen bonds between the
base pairs are broken and the two strands unwind.
► DNA Polymerase: enzyme that brings more
nucleotides and also proof-reads each new DNA
strand, helping to ensure each is a perfect copy.
► Replication proceeds in both directions until each
chromosome is completely copied.
Replication Practice
► DNA
strand:
TACGTT
Complementary strand: ?
► DNA strand:
CAGGCC
Complementary strand: ?
► DNA strand:
GTAGGC
Complementary strand: ?