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DNA
Structure
&
Replication
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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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Why do we study DNA?
We study DNA for
many reasons,
e.g.,
• its central
importance to all
life on Earth,
• medical benefits
such as cures for
diseases,
• better food crops.
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Chromosomes and
DNA
• Our genes are on
our
chromosomes.
• Chromosomes
are made up of a
chemical called
DNA.
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The Shape of the Molecule
• DNA is a very long
polymer.
• The basic shape is
like a twisted ladder
or zipper.
• This is called a
double helix.
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The Double Helix
Molecule
• Discovered by
James Watson &
Francis Crick
• The DNA double
helix has two
strands twisted
together.
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One Strand of DNA
• The backbone
of the molecule
is alternating
phosphates and
deoxyribose
sugar
• The teeth are
nitrogenous
bases.
phosphate
deoxyribose
bases
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Nucleotides
O
O -P O
O
O
O -P O
O
One deoxyribose together
with its phosphate and base
make a nucleotide.
O
O -P O
O
Nitrogenous
base
O
Phosphate
C
C
C
O Deoxyribose
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One Strand of DNA
nucleotide
• One strand of
DNA is a polymer
of nucleotides.
• One strand of
DNA has many
millions of
nucleotides.
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Four nitrogenous
bases
DNA has four different bases:
C
• Thymine T
• Adenine A
• Guanine G
• Cytosine
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Two Stranded DNA
• Remember,
DNA has two
strands that fit
together
something like
a zipper.
• The teeth are
the nitrogenous
bases but why
do they stick
together?
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C
N
N
C
N
N
C
C
C
O
• The bases attract each
other because of
hydrogen bonds.
• Hydrogen bonds are
weak but there are
millions and millions of
them in a single
molecule of DNA.
• The bonds between
cytosine and guanine
are shown here with
dotted lines
N
Hydrogen Bonds
N
C
N
C
C
O
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C
N
Hydrogen Bonds,
cont.
• When making
hydrogen bonds,
cytosine always
pairs up with
guanine
• Adenine always
pairs up with
thymine
• Adenine is
bonded to
thymine here
N
O
O
C
C
C C
N
C
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Chargaff’s Rule:
• Adenine and Thymine
always join together
A
T
• Cytosine and Guanine
always join together
C
G
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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
Replication
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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
DNA replication takes
place in the S phase.
phase
G1
interphase
G2
Mitosis
-prophase
-metaphase
-anaphase
-telophase
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DNA Replication
• Begins at Origins of Replication
• Two strands open forming Replication
Forks (Y-shaped region)
• New strands grow at the forks
5’ Parental DNA Molecule
3’
3’
Replication
Fork
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5’
DNA Replication
• As the 2 DNA strands open at
the origin, Replication Bubbles
form
• Prokaryotes (bacteria) have a
single bubble
• Eukaryotic chromosomes have
MANY bubbles
Bubbles
Bubbles
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DNA Replication
• Enzyme Helicase unwinds
and separates the 2 DNA
strands by breaking the
weak hydrogen bonds
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Question:
• What would be the
complementary DNA
strand for the following
DNA sequence?
DNA 5’-CGTATG-3’
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Answer:
DNA 5’-CGTATG-3’
DNA 3’-GCATAC-5’
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PROTEIN
SYNTHESIS
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DNA
and
Genes
DNA
• DNA contains genes,
sequences of nucleotide
bases
• These Genes code for
polypeptides (proteins)
• Proteins are used to build
cells and do much of the
work inside cells
Genes & Proteins
 Proteins are made of
amino acids linked
together by peptide
bonds
 20 different amino acids
exist
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Amino Acid Structure
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Polypeptides
• Amino acid
chains are
called
polypeptides
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DNA Begins the Process
• DNA is found inside the
nucleus
• Proteins, however, are made
in the cytoplasm of cells by
organelles called ribosomes
• Ribosomes may be free in the
cytosol or attached to the
surface of rough ER
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Starting with DNA
• DNA ‘s code must be copied
and taken to the cytosol
• In the cytoplasm, this code
must be read so amino acids
can be assembled to make
polypeptides (proteins)
• This process is called
PROTEIN SYNTHESIS
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RNA
Roles of RNA and DNA
• DNA is the MASTER
PLAN
• RNA is the
BLUEPRINT of the
Master Plan
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RNA Differs from DNA
• RNA has a sugar ribose
DNA has a sugar deoxyribose
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Other Differences
•
•
RNA contains the
base uracil (U)
DNA has thymine
(T)
RNA molecule is
single-stranded
DNA is doublestranded
DNA
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Structure of RNA
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.
Three Types of RNA
• Messenger RNA (mRNA) copies
DNA’s code & carries the
genetic information to the
ribosomes
• Ribosomal RNA (rRNA), along
with protein, makes up the
ribosomes
• Transfer RNA (tRNA) transfers
amino acids to the ribosomes
where proteins are synthesized
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Messenger RNA
• Long Straight chain
of Nucleotides
• Made in the Nucleus
• Copies DNA & leaves
through nuclear
pores
• Contains the
Nitrogen Bases A, G,
C, U ( no T )
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Messenger RNA (mRNA)
• Carries the information for a
specific protein
• Made up of 500 to 1000
nucleotides long
• Sequence of 3 bases called
codon
• AUG – methionine or start
codon
• UAA, UAG, or UGA – stop
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codons
Ribosomal RNA (rRNA)
• rRNA is a single
strand 100 to 3000
nucleotides long
• Globular in shape
• Made inside the
nucleus of a cell
• Associates with
proteins to form
ribosomes
• Site of protein
Synthesis
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The Genetic Code
• A codon designates an amino
acid
• An amino acid may have more
than one codon
• There are 20 amino acids,
but 64 possible codons
• Some codons tell the
ribosome to stop translating
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The Genetic Code
•Use the
code by
reading from
the center to
the outside
•Example:
AUG codes
for
Methionine
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Name the Amino Acids
•
•
•
•
•
GGG?
UCA?
CAU?
GCA?
AAA?
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Remember the
Complementary Bases
On DNA:
A-T
C-G
On RNA:
A-U
C-G
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Transfer RNA (tRNA)
• Clover-leaf shape
• Single stranded molecule with
attachment site at one end
for an amino acid
• Opposite end has three
nucleotide bases called the
anticodon
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Transfer RNA
amino acid
attachment site
U A C
anticodon
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Codons and Anticodons
• The 3 bases of an
anticodon are
complementary to
the 3 bases of a
codon
• Example: Codon ACU
Anticodon UGA
UGA
ACU
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Transcription
and
Translation
Pathway to Making a
Protein
DNA
mRNA
tRNA (ribosomes)
Protein
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Protein Synthesis
 The production or
synthesis of polypeptide
chains (proteins)
 Two phases:
Transcription & Translation
 mRNA must be processed
before it leaves the nucleus
of eukaryotic cells
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DNA  RNA  Protein
Nuclear
membrane
DNA
Transcription
Eukaryotic
Cell
Pre-mRNA
RNA Processing
mRNA
Ribosome
Translation
Protein
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Transcription
• The process of copying
the sequence of one
strand of DNA, the
template strand
• mRNA copies the template
strand
• Requires the enzyme RNA
Polymerase
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Template Strand
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Question:
 What would be the
complementary RNA strand
for the following DNA
sequence?
DNA 5’-GCGTATG-3’
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Answer:
• DNA 5’-GCGTATG-3’
• RNA 3’-CGCAUAC-5’
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Transcription
• During transcription, RNA
polymerase binds to DNA and
separates the DNA strands
• RNA Polymerase then uses
one strand of DNA as a
template to assemble
nucleotides into RNA
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Transcription
• Promoters are regions on
DNA that show where RNA
Polymerase must bind to
begin the Transcription of
RNA
• Called the TATA box
• Specific base sequences act
as signals to stop
• Called the termination signal
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RNA Polymerase
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mRNA Processing
• After the DNA is
transcribed into RNA,
editing must be done to
the nucleotide chain to
make the RNA functional
• Introns, non-functional
segments of DNA are
snipped out of the chain
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mRNA Editing
• Exons, segments of DNA that
code for proteins, are then
rejoined by the enzyme ligase
• A guanine triphosphate cap is
added to the 5’ end of the
newly copied mRNA
• A poly A tail is added to the 3’
end of the RNA
• The newly processed mRNA can
then leave the nucleus
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Result of Transcription
CAP
New Transcript
Tail
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mRNA Transcript
•mRNA leaves the nucleus
through its pores and goes to
the ribosomes
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Translation
• Translation is the process
of decoding the mRNA
into a polypeptide chain
• Ribosomes read mRNA
three bases or 1 codon at
a time and construct the
proteins
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Transcription
Translation
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Ribosomes
• Made of a large and small
subunit
• Composed of rRNA (40%)
and proteins (60%)
• Have two sites for tRNA
attachment --- P and A
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Step 1- Initiation
• mRNA transcript
start codon AUG
attaches to the
small ribosomal
subunit
• Small subunit
attaches to large
ribosomal subunit
mRNA transcript
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Ribosomes
Large
subunit
P
Site
A
Site
mRNA
Small
subunit
A U G
C U A C U U C G
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Step 2 - Elongation
• As ribosome moves, two tRNA with
their amino acids move into site A and
P of the ribosome
• Peptide bonds join the amino acids
Initiation
aa1
aa2
2-tRNA
1-tRNA
anticodon
hydrogen
bonds
U A C
A U G
codon
G A U
C U A C U U C G A
mRNA
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Elongation
peptide bond
aa1
aa3
aa2
3-tRNA
1-tRNA
anticodon
hydrogen
bonds
U A C
A U G
codon
2-tRNA
G A A
G A U
C U A C U U C G A
mRNA
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aa1
peptide bond
aa3
aa2
1-tRNA
3-tRNA
U A C
(leaves)
2-tRNA
A U G
G A A
G A U
C U A C U U C G A
mRNA
Ribosomes move over one codon
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aa1
peptide bonds
aa4
aa2
aa3
4-tRNA
2-tRNA
A U G
3-tRNA
G C U
G A U G A A
C U A C U U C G A A C U
mRNA
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aa1
peptide bonds
aa4
aa2
aa3
2-tRNA
4-tRNA
G A U
(leaves)
3-tRNA
A U G
G C U
G A A
C U A C U U C G A A C U
mRNA
Ribosomes move over one codon
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aa1
peptide bonds
aa5
aa2
aa3
aa4
5-tRNA
U G A
3-tRNA
4-tRNA
G A A G C U
G C U A C U U C G A A C U
mRNA
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peptide bonds
aa1
aa5
aa2
aa3
aa4
5-tRNA
U G A
3-tRNA
G A A
4-tRNA
G C U
G C U A C U U C G A A C U
mRNA
Ribosomes move over one codon
80
aa4
aa5
Termination
aa199
aa3 primary
structure
aa2 of a protein
aa200
aa1
200-tRNA
A C U
terminator
or stop
codon
C A U G U U U A G
mRNA
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End Product –The Protein!
• The end products of protein
synthesis is a primary structure
of a protein
• A sequence of amino acid
bonded together by peptide
bonds
aa2
aa1
aa3
aa4
aa5
aa199
aa200
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Messenger RNA
(mRNA)
start
codon
mRNA
A U G G G C U C C A U C G G C G C A U A A
codon 1
protein
codon 2
methionine
glycine
codon 3
serine
codon 4
isoleucine
codon 5
glycine
codon 6
alanine
codon 7
stop
codon
Primary structure of a protein
aa1
aa2
aa3
peptide bonds
aa4
aa5
aa6
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Protein Synthesis
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