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1
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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Watson & Crick’s Model
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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.
5
The Shape of the Molecule
• DNA is a very long
polymer.
• The basic shape is
like a twisted
ladder.
• This is called a
double helix.
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The Double Helix Molecule
• The DNA
double helix
has two
strands
twisted
together.
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One Strand of DNA
• The backbone
(sides) of the
DNA molecule
are alternating
phosphates and
deoxyribose
sugar
• The rungs 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
Phosphate
Nitrogenous
base
O
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 Kinds of Bases in DNA
• Pyrimidines are
single ring bases.
• Purines are
double ring
bases.
N
N C
O C
C
N C
N
N C
C
C
N
N C
N C
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Thymine and Cytosine are
pyrimidines
• Thymine and cytosine each have one
ring of carbon and nitrogen atoms.
• They are the smaller.
N
O
C
C
O
C C
N
C
thymine
N
O
C
C
N
C
N
C
cytosine
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Adenine and Guanine are
purines
• Adenine and guanine each have two
rings of carbon and nitrogen atoms.
• They are the larger bases
N
C
Adenine
N
C
C
N
O
N
C
N
N
C
N
C
C
C
N
Guanine
C
N
N
C
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Two Stranded DNA
• Remember, DNA
has two strands
that fit
together
something like a
ladder.
• The rungs are
the nitrogenous
bases but why
do they stick
together?
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C
N
N
C
N
C
C
C
C
N
N
N
C
C
C
O
• The bases attract to
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
O
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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
C
C
O
C C
N
C
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Chargraff’s Rule:
• Adenine and Thymine
always join together
A
T
• Cytosine and Guanine
always join together
C
G
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DNA Double Helix
5
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
BASE-PAIRINGS
H-bonds
G
C
T
A
The Code of Life…
• The “code” of the chromosome is the
SPECIFIC ORDER that bases occur.
A T C G T A T G C G G…
Replication Quiz
A---?
G---?
2. When does replication occur? C---?
T---?
3. Describe how replication works. A---?
G---?
4. Use the complementary rule to A---?
G---?
create the complementary
C---?
strand:
A---?
G---?
T---?
1. Why is replication necessary?
Replication Quiz
1. Why is replication necessary?
So both new cells will have the correct
DNA
2. When does replication occur?
During interphase (S phase).
3. Describe how replication works.
Enzymes unzip DNA and complementary
nucleotides join each original strand.
4. Use the complementary rule to
create the complementary strand:
A---T
G---C
C---G
T---A
A---T
G---C
A---T
G---C
C---G
A---T
G---C
T---A
DNA Trivia
• 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.
The earth is 150 billion m
or 93 million miles from
the sun.
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✔ Check for Understanding
Figure out the complementary DNA strand for
the following:
• A A T T G G C G A T C C G G C G T G G C C
• A A T T G G C G A T C C G G C G T G G C C
• T T A A C C G C T A G G C C G C A C C G G
SUPER JOB!
Before Replication can occur:
STEP-1
• Double helix must be unwound.
• This occurs by the enzymes known as
helicases.
• Helicases break the hydrogen bonds
that link the complementary bases
together.
Step-2
• Next DNA polymerases are enzymes that
move along the exposed DNA strands
adding nucleotides according to the base
paring rules.
»A=T
»C=G
•
As the DNA polymerases move along the
strand 2 NEW double helixes are formed.
• The Enzymes will remain attached until a
stop signal is reached.
• DNA polymerase also acts as a proof
reader. This enzyme can backtrack
and remove an incorrect nucleotide
and replace it with the correct one.
✔ Check for Understanding
• What happens when the DNA
polymerase doesn’t catch a mistake?
• A mutation occurs!
✔ Check for Understanding
What are some mutations we
have already discussed?
Inversion
Deletion
Duplication
Translocation
DNA is a Prisoner:
• If DNA can’t get out of the nucleus
some other molecule has to be able to
do so.
What do you think it is?
RNA
What do you
notice about
RNA?
What is RNA?
• RNA is chemically similar to DNA but
there are a few differences.
RNA
Ribose sugar
(extra oxygen atom)
A,U,C,G
Single strand
DNA
Deoxyribose
sugar
A,T,C,G
Double strand
✔ Check for Understanding
Why do you think that RNA being only a single
strand is so important?
Fantastic!
• So it can carry the genetic
information out of the nucleus.
• RNA takes copies of genes out of the
nucleus, into the cytoplasm so
proteins can be produced.
• RNA is present in the cell in 3 different
forms:
– 1. Messenger RNA (mRNA) comes from
the nucleus
– 2. ribosomal RNA (rRNA) makes up
ribosomes
– 3. transfer RNA (tRNA) is found in the
cytoplasm of the cell
mRNA
Transcription is the process in which the hereditary
information of the DNA is copied into the mRNA
(enzymes are needed)
• messenger RNA passes from the nucleus, where it is
synthesized, into the cytoplasm and becomes attached
to the ribosomes
• messenger RNA is a short-lived intermediate in
protein production
• messenger mRNA can be produced very quickly and
broken down just as quickly
✔ Check for Understanding
• Practice Problem:
•
• DNA: A A C C G C G A T T T A G G C C A T A T
• DNA
Comp:
• mRNA:
Ribosomal and transfer RNA
• Ribosomes are made up of rRNA
and some proteins
• Transfer RNA is found in the cell’s
cytoplasm, tRNA picks up amino
acids and carries them to the
ribosomes
✔ Check for Understanding
• DNA: A A T G C G T C A T
• DNA:
Comp
mRNA:
43
The Genetic Code and Protein Synthesis
DNA (gene)
transcription
RNA (mRNA) translation
Protein (peptide sequence)
• Gene Expression: Is the use of genetic
information to produce proteins.
• Gene Expression works through two
stages:
1. Transcription
2. Translation
How RNA Makes Proteins
• Translation
– Takes genetic information and
gets it deciphered, changes the language
– During translation
• mRNA + rRNA + tRNA work together to
build proteins by joining amino acids
• Translation occurs at the ribosomes
(rRNA)
Translation occurs at the ribosomes
• Ribosomes are made up of 2
sections or subunits
• mRNA attaches to the smaller
subunit and then the larger section
or subunit connects forming a
complete ribosome
2 pieces + mRNA= ribosome
The Genetic Code
• Every 3 nucleotides in
mRNA codes for an amino
acid.
• Each triplet of mRNA is
called a codon.
The Genetic Code Continued
• There are 64 possible codon
combinations but only 20 amino acids.
MORE than ONE codon can specify a
specific amino acids.
• Stop codon specify the end of the
amino acid sequence.
• TRANSFER RNA (tRNA)
• attaches to the ribosome at a special location
• tRNA carries the amino acid to the ribosome
• one end of the tRNA has the amino acid and the
other end is a 3 nucleotide sequence called an
anticodon.
• Anticodon is complementary to the
mRNA
• As the mRNA passes through the
ribosomes one tRNA after another is
matched to the mRNA codons
• Amino acids are added to the protein
chain until a stop message is reached
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Using the amino acid chart:
• 1. A U G
Amino Acid=
• 2. C A A
Amino Acid=
• 3. G U U
Amino Acid=
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✔ Check for Understanding
• mRNA
• Amino Acid
CCG
• mRNA
• Amino Acid
TAT
• mRNA
• Amino Acid
UGC