Nucleotides, The Building Blocks of
Nucleic Acids, Have Three Parts
? Pentose (5-carbon) sugar
- 2’ carbon
- no oxygen on DNA’s
deoxyribose
- hydroxyl (-OH) on RNA’s ribose
- 3’ carbon contains a hydroxyl (OH)
phosphate
nitrogenous
base
? Nitrogenous base
- cyclic, nitrogen-containing
- attached to 1’ carbon of pentose
pentose
gray = carbon
blue = hydrogen
red = oxygen
blue = nitrogen
yellow = phosphate
? Phosphate group
- attached to 5’ carbon of pentose
- negatively charged at neutral pH
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DNA Is Built Out Of Four Specific
Nucleotides
A = adenine
? purines
C = cytosine
- nitrogenous
base has a
double ring
- adenine and
guanine
G = guanine
? pyrimidines
- nitrogenous
base has a
single ring
- cytosine and
thymine
T = thymine
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Nucleic Acids are Polymeric Strands
? Polynucleotides are
polymers of nucleotides
connected by
phosphodiester bonds.
5’ phosphate
- Phosphate group at 5’ end
of one sugar reacts with
the hydroxyl group at the 3’
end of another sugar.
? Forms a sugarphosphate backbone
phosphodiester
bonds
3’ hydroxyl
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Dehydration Synthesis Reactions Release
Water Molecules
? Synthesis involves
removing the OH- group
from one subunit and a
H+ from another.
- Called dehydration
synthesis or condensation
reaction.
? H+ combines with OH- to
reform H2O (water).
X
O
H
Y
O
H
H
X
Y
H
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2
Hydrolysis Reactions Require Water to
Break Linkages.
? Hydrolysis introduces
water at the linkage
between subunits.
X
O
H
Y
O
H
H
- The polymer is
"hydrolyzed".
? One subunit gains a
H+, and the other
subunit gains the OH-.
X
Y
H
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The Atoms within Molecules are
Connected by Chemical Bonds.
? Ionic Bonds
- One atom loses an electron, one atom gains an
electron, then the two opposite-charged ions attract.
? Covalent Bonds
- Atoms are held together by sharing electrons both need
to fill up their outer orbital of electrons.
? Hydrogen Bonds
- A hydrogen atom covalently attached to one atom is
weakly drawn toward another atom.
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3
Strands Are Held Together By Hydrogen
Bonds Between Base Pairs
T
? Nitrogenous bases on
opposite strands form
specific base pairs.
G
- Hydrogen bonds are weak and
easily broken and reformed.
? Watson-Crick base pairing
rules
A
- A binds to T through two
hydrogen bonds.
- C binds to G through three
hydrogen bonds.
C
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The Two Strands of a DNA Molecule are
Complementary
5’ AGGCTGAATCGATACGT 3’
|||||||||||||||||
3’ TCCGACTTAGCTATGCA 5’
? Watson-Crick base pairing rules require that
the nucleotide sequences of opposite strands
be complementary.
? The sequence of one strand determines the
sequence of the opposite strand.
- This allows one strand to serve as a template for
accurately building an opposite strand.
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4
DNA is a Double Helix of Two
Antiparallel Polynucleotide Strands
3’
5’
5’
? Two strands wind
around each other.
? Strands are held
together by hydrogen
bonds between
nitrogenous bases.
? Bases are inside.
? Sugar-phosphate
backbones are
3’
outside.
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The Cyclic Rings Of The Bases Are
Stacked Within The Double Helix
? Nitrogenous bases align
themselves parallel to each
other.
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5
DNA is ...
? two polynucleotide strands
? with complementary
sequences of nucleotides
? in a double helix
? in antiparallel orientation
? held together by weak
hydrogen bonds
Rick Hershberger ? www.rickhershberger.com
Watson and Crick, Nature, 2 April 1953
Molecular Structure Of Nucleic Acids: A Structure for
Deoxyribose Nucleic Acid
? It has not escaped our
notice that the
specific pairing we
have postulated
immediately suggests
a possible copying
mechanism for the
genetic material.
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6
Enzymatic Requirements of DNA Polymerase
and its Constraints on DNA Replication
? DNA polymerase only adds a nucleotide that is
complementary, forming a correct Watson-Crick base
pair.
- Requires a template strand.
? DNA polymerase only adds a nucleotide onto the 3’
end of a strand.
- Requires a deoxynucleotide triphosphate (dNTP) as a substrate.
? DNA polymerase only adds a nucleotide onto the 3’
end of a base that is correctly based-paired.
- Requires a primer.
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