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Macromolecules Part IV:
Nucleic Acids
Informational molecules
Introduction: Nucleic Acids
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The amino acid sequence of a polypeptide
is programmed by a gene
A gene consists of regions of DNA, a
polymer of nucleic acids (100’s to 1000’s)
DNA (and their genes) is passed by the
mechanisms of inheritance
Only molecule that can produce identical
copies of themselves
Nucleic acids store and transmit
hereditary information
There are two types of nucleic acids:
 ribonucleic acid (RNA)
 deoxyribonucleic acid (DNA)
 DNA provides direction for its own
replication
 DNA also directs RNA synthesis and,
through RNA, controls protein synthesis
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Roles of DNA
 Protein
expression (Phenotypes)
 Inheritance (Genotypes)
3.Evolution (Mutations)
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Nucleotides
Nucleic acids are
polymers of
monomers called
nucleotides
 Each nucleotide
consists of three
parts:
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a
nitrogen base
 a pentose sugar
 a phosphate group
Sugar-Phosphate
“Backbone”
Nucleic Acids
Polynucleotides are synthesized by
connecting the sugars of one nucleotide to
the phosphate of the next with a
phosphodiester link
 The phosphodiester links create a
repeating backbone of sugar-phosphate
units with the nitrogen bases as
appendages
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DNA
Phosphodiester
link
RNA
Nucleotides
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The nitrogen bases, rings of carbon and
nitrogen, come in two types:
 Purines
 pyrimidines
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Pyrimidines have a single six-membered ring
The three different pyrimidines that differ in the
atoms attached to the ring:
 cytosine
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(C), thymine (T), and uracil (U)
Purines have a six-membered ring joined to a
five-membered ring and there are two types:
 adenine
(A) and guanine (G)
Purine Bases
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Adenine and guanine are purines
Purines are the larger of the two types of bases
found in DNA
Pyrimidine Bases
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Cytosine, uracil and thymine are pyrimidines
Mnemonic: Pyramids CUT
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Nucleotide Bases Pairings
Because of their shapes, only some bases
are compatible with each other
 Purines always pair with pyramidines
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Adenine (A) always pairs with thymine (T) (in
RNA, Adenine pairs with Uracil (U))
 Guanine (G) with cytosine (C)
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Counting Carbons
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Nucleosides VS. Nucleotides
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A nucleoside is one of the four
DNA bases covalently attached
to a sugar
Nucleosides differ from
nucleotides in that they lack
phosphate groups
A nucleotide is a nucleoside
with one or more phosphate
groups covalently attached to
the sugar group
Step 1:
Base
Step 2:
Nucleoside
Step 3: Nucleotide
RNA
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RNA is made
by copying
DNA and is
used to make
proteins
It is a
SINGLE
strand of
nucleotides
DNA
1. Take two nucleic
acid strands
2. Flip one of the
strands
3. Line the strands
together so that
they bond
4. Twist them
5. Now you have DNA!
DNA: Double Helix Structure
• DNA has a double helix structure with two strands that are
antiparallel so that nucleotides can form hydrogen bonds
One Strand
One Strand
H-Bonds
But why is it important
that the strands are Hbonded and not
covalently bonded?
Why H-bonds?
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In order to replicate, the strands
must separate to make new
DNA strands (cannot be broken
if the strands were covalently
bound)
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The sugar-phosphate backbones of the two
polynucleotides are on the outside of the helix
Pairs of nitrogenous
bases, one from each
strand, connect the
polynucleotide chains
with hydrogen bonds
Most DNA molecules
have thousands to
millions of base pairs
Nucleotide Bases Pairings
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Recap: What are the base-pair rules?
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Adenine (A) always pairs with thymine (T) (in
RNA, Adenine pairs with Uracil (U))
 Guanine (G) with cytosine (C)
With these base-pairing rules, if we know
the sequence of bases on one strand, we
know the sequence on the opposite strand
 The two strands are complementary
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DNA - Base pairing
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Adenine and Thymine
always bind together

Guanine and
Cytosine always bind
together
Result: Double Helix Structure
What’s the difference between
RNA and DNA?
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An RNA molecule is single polynucleotide chain
while DNA molecules have two polynucleotide
strands that spiral around an imaginary axis to form
a double helix
The pentose joined to the nitrogen base is ribose in
nucleotides of RNA and deoxyribose in DNA
Nitrogen bases - thymine is replaced by uracil
ATP (adenosine triphosphate)
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ATP is a nucleotide that is used to provide
energy in cells
NAD+ and FAD
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These are nucleotide-containing molecules
that function as electron carriers in a variety
of cellular processes.
NAD+ - nicotinamide adenine dinucleotide
FAD- flavin adenine dinucleotide
Home work
Study the table on page 30
Do Question on page 31 Questions1-12
Macromolecules Quiz Next Thursday 15th
for day 1 classes and Friday 16th for day 2
classes