Download DNA - South Gibson Science

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DNA by the Numbers
 Each cell has about 2 m of
DNA.
 The average human has 75
trillion cells.
 The average human has
enough DNA to go from the
earth to the sun more than
400 times.
 DNA has a diameter of only
0.000000002 m.
The earth is 150 billion m
or 93 million miles from
the sun.
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DNA – deoxyribonucleic acid is the
nucleic acid that stores and
transmits genetic info. from one
generation to the next.
•present in all organisms, but
different (unique) in each
individual, except for
identicle twins.
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DNA
 DNA is often called the
blueprint of life.
 In simple terms, DNA
contains the instructions
for making proteins
within the cell.
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DNA Structure
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The Shape of the Molecule
DNA is a very long
double stranded
polymer of
nucleotides.
The basic shape is
like a twisted
ladder or zipper.
This is called a
double helix.
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One Strand of DNA
P
nucleotide
B
P
B
P
B
 Each nucleotide monomer
contains
1 phosphate group
1 sugar (deoxyribose)
*1 nitrogenous base
 One strand of DNA has many
millions of nucleotides.
P
B
P
B
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DNA Nucleotide
Phosphate
Group
O
O=P-O
O
5
CH2
O
N
C1
C4
Nitrogenous base
(A, G, C, or T)
Sugar
(deoxyribose)
C3
C2
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One Strand of DNA
• The backbone
of the molecule is alternating
phosphates &
deoxyribose sugar
• The “rungs/teeth” are
nitrogenous
bases (4 possible)
C, T, A, or G
phosphate
deoxyribose
bases
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4 possible Nitrogenous Bases
Double ring PURINES
Adenine (A)
Guanine (G)
A or G
Single ring PYRIMIDINES
Thymine (T)
Cytosine (C)
T or C
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Base-Pairings
• Purines only pair with Pyrimidines
• Hydrogen bonds required to bond
Guanine & Cytosine
3 H-bonds
G
C
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•Hydrogen bonds are required
to bond Adenine & Thymine
2 H - bonds
T
A
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DNA Double Helix
“Rungs of ladder”
Nitrogenous
Base (A,T,G or C)
“Legs of ladder”
Phosphate &
Sugar Backbone
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Two Stranded DNA
• DNA has two
strands that fit
together something
like a ladder or
zipper.
• The rungs or teeth
are the nitrogenous
bases but why do
they stick together?
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DNA
• Two strands coiled called a
double helix
• Sides made of a pentose sugar
Deoxyribose bonded to
phosphate (PO4) groups
• Middle made of nitrogen bases
bonded together by weak
hydrogen bonds
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Hydrogen Bonds
• The bases attract each
other because of
hydrogen bonds.
• Hydrogen bonds are
weak but there are
millions of them in a
single molecule of
DNA.
Cytosine always
pairs with Guanine
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Hydrogen Bonds, cont.
 When making
hydrogen bonds,
Cytosine
always pairs
with Guanine

Adenine
always pairs
with Thymine
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5
DNA
O
3
3
P
5
O
O
C
G
1
P
5
3
2
4
4
P
5
P
2
3
1
O
T
A
3
O
3
5
O
5
P
P 18
The nitrogen bases
of each strand pair
with the bases on the
complementary
strand
The order of the bases
makes up the genetic
code.
A
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Question:
What would be the
complementary DNA strand for
the following DNA sequence?
DNA –C G T A T G-
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Answer:
DNA –C G T A T Gcomp DNA –G C A T A C-
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Question:
If there is 30% Adenine,
how much Cytosine is
present?
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Answer:
If 30% Adenine then
30% Thymine
If 60% A-T; then 40% C-G
Therefore,40% C-G would be
20% Guanine = __% Cytosine
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History of DNA
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History of DNA
• Early scientists thought protein was
the cell’s hereditary material because
it was more complex than DNA
• Proteins were composed of 20
different amino acids in long
polypeptide chains
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Transformation
• Griffith 1928-worked with virulent S
and nonvirulent R strain Pneumoccocus
bacteria
• found that R strain could become
virulent (transform) when it took in
DNA from heat-killed S strain
• Avery 1944- suggested that DNA was
probably the genetic material that
was “transformed”
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Griffith Experiment
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History of DNA
• Chromosomes are made of
both DNA and protein
• Hershey & Chase 1952experiments on
bacteriophage viruses
proved that DNA was the
cell’s genetic material
Radioactive
32P
was injected into bacteria!
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Chargaff’s Rule
 Adenine pairs with Thymine
 Guanine pairs with Cytosine
 The bases form weak hydrogen
bonds
T
A
G
C
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Discovery of DNA
Structure
• Erwin Chargaff 1950 showed the
amounts of the four bases on DNA (
A,T,C,G)
• In a body or somatic cell:
A = 30.5%
T = 30.5%
G = 19.5%
C = 19.5%
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DNA Structure
• Rosalind Franklin & Maurice
Wilkins - 1952 took diffraction
x-ray photographs of DNA
crystals (2 sides, twisted)
• Watson & Crick -1953 built the
first model of DNA using
Franklin’s x-rays
(double helix)
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Rosalind Franklin
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Watson and Crick
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Helix
 Most DNA has a right-hand
twist with 10 base pairs in a
complete turn
 Left twisted DNA is called
Z-DNA or southpaw DNA
 Hot spots occur where right
and left twisted DNA meet
producing mutations
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Antiparallel Strands
• One strand of
DNA goes from
5’ to 3’ (sugars)
• The other
strand is
opposite in
direction going
3’ to 5’ (sugars)
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DNA REPLICATION
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DNA Replication
When a cell divides,
DNA preserves individuality
by passing exact copies of
itself to the new cell
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Replication Facts
DNA has to be copied
before a cell divides
DNA is copied during the S
or synthesis phase of
interphase
New cells will need identical
DNA strands
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Synthesis Phase (S phase)
• S phase during interphase of the
cell cycle
• Nucleus of eukaryotes
S
phase
DNA replication takes
place in the S phase.
G1
interphase
G2
Mitosis
-prophase
-metaphase
-anaphase
-telophase
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DNA Replication
 Begins at Origins of Replication
 One strand serves as a mold for
another strand to be copied
 Two strands open forming Replication
Forks (Y-shaped region)
 New strands grow at the forks
5’
Parental DNA Molecule
3’
Replication
Fork
3’
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5’
DNA Replication
 1.Enzyme DNA Helicase
unwinds & separates the 2 DNA
strands by breaking the weak
hydrogen bonds to unzip the chain
 Single-Strand Binding Proteins attach and keep the 2
DNA strands separated and untwisted
2. Free nucleotides match up &
form H bonds to complete
complementary base strand
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3. Base pairs bond, DNA polymerase
links phosphate of one nucleotide to
the sugar of another.
4. Pairing continues until each
“original” DNA strand has a
complete matching strand
(result 2 identical DNA strands)
IF…
TAGCAT
ATCGTA
THEN…
TAGCAT
ATCGTA
and
TAGCAT
ATCGTA
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DNA makes proteins that are needed
for growth, repair and all life functions
Ex:
collagen - cartilage and tendons
hemoglobin – blood carries oxygen
through the body
keratin - hair and fingernails
insulin – metabolizes blood sugars
…muscles, skin, etc…
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Replication of Strands
Replication
Fork
Point of Origin
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Semiconservative Model of
Replication
 Idea presented by Watson & Crick
 The two strands of the parental
molecule separate, and each acts as a
template for a new complementary
strand
 New DNA consists of 1
PARENTAL (original) and 1 NEW
DNA Template
strand of DNA
Parental DNA
New DNA
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