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DNA (Deoxyribonucleic Acid)
Transformation of Bacteria
What carries hereditary information?
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By the 1940s, scientists knew that
chromosomes carried genes.
They also knew that chromosomes were
made of DNA and protein.
They did NOT know which of these
molecules actually carried the genes.
Since protein has 20 types of amino acids
that make it up, and DNA only has 4 types
of building blocks, it was a logical
conclusion.
Most Scientists thought protein carried
genes
Chromosomes are made of DNA and protein
Transformation of Bacteria
Avery’s Experiment
1. Avery repeated Griffith’s
experiments with an
additional step to see what
type of molecule caused
transformation.
3. When Avery added
enzymes that destroy
DNA, no transformation
occurred.
So…he knew that
DNA carried
hereditary
information!
2. Avery used enzymes to destroy the sugars and
transformation still occurred—Sugar did not cause
transformation.
Avery used enzymes to destroy lipids, RNA, and
protein one by one. Every time transformation still
occurred—none of these had anything to do with
the transformation.
Hershey-Chase Experiment
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The experiment involved viruses
to see if DNA or protein was
injected into the bacteria in
order to make new viruses.
One group of viruses was
infected with radioactive
protein and another group with
radioactive DNA.
Then the viruses attack the
bacteria.
Radioactive DNA shows up in
the bacteria, but no
radioactive protein.
Chargaff’s Rules
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The amount of adenine
(A) equals the amount of
thymine (T).
The amount of cytosine
(C) equals the amount of
guanine (G).
Rosalind Franklin
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Took X-ray
pictures of DNA.
The photos
revealed the
basic helix,
spiral shape of
DNA.
Maurice Wilkins
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Worked with
Rosalind Franklin.
Took her x-ray
photos and
information to
Watson and Crick
Watson and Crick
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Used Franklin’s pictures
to build a series of
large models.
Stated that DNA is a double-stranded molecule in
the shape of a double helix, or twisted ladder.
Won the Nobel Prize for their work in 1962.
Basic DNA Structure
P
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S
A
P
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S
C
P
S
T
A nucleotide is
the monomer of
DNA
A nucleotide is
made of
– a sugar called
deoxyribose
– a phosphate
– and a base
(ATCG)
Deoxyribose
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Simple sugar
molecule like
glucose that has 5
carbons
The five carbons are
numbered clockwise
starting from the first
one after the oxygen
Phosphate
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The negatively charged phosphate
bonds to the 5’ Carbon of the
deoxyribose.
Bases
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The base bonds to the 1’ Carbon.
Base
Bases
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There are two main types of bases
purines and pyrimidines.
– Purines have two rings in their structure.
• Adenine and guanine are purines.
– Pyrimidines only have one ring.
• Thymine and Cytosine are pyrimidines.
Pyrimidines
Purines
Basic DNA Structure
P
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S
A
S
C
S
T
P
P
To form one strand
of DNA, the
phosphate of one
nucleotide covalently
bonds to the 3’
Carbon of the
deoxyribose from
another nucleotide.
P
S
A
T
S
P
P
S
C
G
S
P
P
S
T
A
S
P
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The two strands of DNA are held
together by hydrogen bonds
Base Pairs
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The nucleotides that bond together by
their bases are called base pairs.
– Adenine only bonds to Thymine
– Guanine only bonds to Cytosine
Does each of your cells
have the same DNA?
YES
DNA Replication
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Before a cell divides,
DNA must make a copy
of itself so that each
new cell has a
complete set of DNA.
What is an enzyme?
A
protein that helps
speed up chemical
reactions in a cell.
Step 1-Unzip DNA
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An enzyme called helicase untwists the
ladder and breaks the hydrogen bonds
between the bases and “unzips” DNA
down the middle.
Helicase Enzyme
Step 2-Prime the DNA
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An enzyme called DNA primase put a
few nucleotides of RNA on the DNA.
This is only to create a starting place
and these will later be removed.
Step 3-Elongation
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The two strands of the
Parent DNA become
templates for the new
strands.
New nucleotides are
added by an enzyme
called DNA polymerase.
Step 3-Elongation
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DNA polymerase
only adds
nucleotides in the 5’
to 3’ direction on
both strands
beginning at the
RNA primer.
Step 4 – Fine tuning
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RNA primer is removed and any gaps
are sealed by an enzyme called ligase.
DNA polymerase proof reads the new
copy and fixes any mistakes.
Helicase unwinds and unzips DNA
P
S
A
T
S
P
P
S
C
G
S
P
P
S
T
A
S
P
DNA Polymerase Adds New Nucleotides
P
P
S
A
T
S
S
P
P
S
C
G
S
T
A
P
C
G
S
P
P
S
S
S
P
S
P
T
P
S
P
A
T
A
S
P
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Are the two copies of DNA the
same?
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Why would it be important for the two
copies of DNA to be the same?
What is a Gene?
A gene is a
code found in
DNA
 Genes code
for proteins
that give
people their
traits.
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How does DNA code for so
many traits with only 4 bases?
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Can you spell 20 words
with the letters A, T, C
and G?
Each combination of
bases codes for a
different amino acid.
Putting the 20 amino
acids in different orders
makes different
proteins.
Making Proteins
Coding for our Traits
What organelle makes proteins?
Ribosomes
Which molecule makes proteins?
RNA
RNA
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Single-stranded nucleic acid
Made of nucleotides
Has ribose instead of deoxyribose
Has uracil instead of thymine
Protein Synthesis
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Two Steps to making a protein
Transcription
– Writing DNA code
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Translation
– Decoding DNA’s message into a protein
Transcription
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Transcription happens in the nucleus
where the DNA is.
Transcription
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DNA’s code is copied by messenger RNA
(mRNA).
mRNA has complementary bases to the DNA
(Remember RNA has a U instead of T).
Three bases of mRNA make a codon.
Translation
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After mRNA is made in Transcription,
the mRNA goes to the ribosome where
the protein is made.
Translation
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Translation begins at the start codon (AUG)
of mRNA.
Then each codon codes for an amino acid in
a protein that is brought in by a tRNA
(transfer).
tRNA has an anticodon with complementary
bases to the mRNA codon.
Translation is terminated by stop codon.
Which Amino Acid does each codon
code for?
GGU
Glycine
 AAA
Lysine
 CUG
Leucine
 UGG
Tryptophan
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Mutations
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Mutation-alteration in
DNA
Mutagens-physical and
chemical agents that
mutate DNA
Deletion-mutation caused by deleting
DNA that should be there
Insertion-mutation caused by inserting
DNA that should not be there
Substitution-mutation caused by
substituting DNA
Gene Regulation
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Genes are not expressed all the time.
Some genes are usually on, but can
be turned off by repressors when they
are not needed.
Some genes are usually off, but they
can be turned on by enhancers when
they are needed.
The End!