Transcript DNA – Deoxyribonucleic Acid

DNA –
Deoxyribonucleic Acid
DNA – The Genetic Storehouse
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DNA occurs as a double
stranded string of
nucleotides that are bound
together in the shape of a
double helix.
If it was unwound it would
appear as a ladder, with
the sugar-phosphate
groups as the two supports
and the nitrogen containing
base groups as the ladder
rungs.
Stucture
From Franklin’s X-ray analysis, we know
that there are several repeating
sequences that must be accounted for in
the structural arrangement.
 Levene’s and Chargaff’s work previously
identified the structures and pairing rules
by the nucleotide bases.
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Structure cont.
Adenine and Guanine make up a double
ring structure of nucleotides known as the
Purines.
 Cytosine and Thymine are a single ring
nucleotide making up the Pyrimidines.
 The chemical configuration of the bases
allows for only certain base pairings.
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Complementary Base Pairs
Adenine (double ring with 2 H binding
sites) can only form a stable association
with Thymine (single ring with 2 H sites)
 Likewise, Cytosine (single ring, 3 H
binding sites) forms a stable relationship
with Guanine (double ring, 3 H sites)
 These pairings create what is known as
Complementary Base Pairs.
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Two weak
Hydrogen
Bonds
Three Weak
Hydrogen
Bonds
Complementary not Identical
The two opposite strands of DNA are
complementary. They carry
complementary base pairs in such a way
that the Hydrogen bonding sites line up.
 The strands run antiparallel, that is the
sugar phosphate backbone point in
opposite direction. We call these the 5’
and 3’ ends of the DNA strands.
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You are the scientist!!!!!!
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Read the
Thinking Lab on
the bottom of
Page 575. Do
only the first
question.