DNA:
Deoxyribonucleic Acid
Biology
Structure of DNA
• DNA nucleotide has
3 parts:
• Sugar molecule
• Deoxyribose
• Phosphate group
• Nitrogen-containing
base
Nitrogen-containing bases
•
•
•
•
Adenine = A
Guanine = G
Cytosine = C
Thymine = T
Nucleotide Structure
• Purines
• Bases that
have two
rings of
carbon and
nitrogen
atoms
• Adenine and
guanine
Nucleotide Structure
• Pyrimidines
• Bases that
have only one
ring of carbon
and nitrogen
atoms
• Cytosine and
thymine
The Origin of the Double Helix
• Rosalind Franklin
• X-ray photographs of
DNA crystals
• Research partner
Maurice Wilkins
The Origin of the Double Helix
• Watson & Crick
• Used Franklin’s
stolen research
and
photographs to
determine the
structure of
DNA
• 1953
The Double Helix
• DNA is composed of
two nucleotide
chains that wrap
around each other
to form a double
spiral, like a spiral
staircase
• Double helix
The Double Helix
• The “backbone” of
the double helix
• Alternating sugar
and phosphate
molecules
• Nitrogen-containing
bases attach to the
sugar molecules in
the “backbone” and
occupy the space in
the center
Complementary Base Pairing
• DNA nucleotides pair
in specific
combinations
• Base-pairing rules
• Cytosine & Guanine
• C-G
• Adenine & Thymine
• A-T
Complementary Base Pairing
• The complementary
base pairs are a
combination
• Purine with a
pyrimidine
• Two rings paired with
a single ring
Complementary Base Pairing
• Base pairs are
connected to one
another by hydrogen
bonds
Replication of DNA
• Replication
• The process of
copying DNA in the
cell
Replication of DNA-Step 1
• The separation
of the two
nucleotide
strands
• The point at
which the two
chains
separate
• Replication
fork
Replication of DNA-Step 1
• The
nucleotide
chains are
separated by
enzymes
• Helicases
• Run down
the strand
and break
the bonds
holding the
complementary bases
together
Replication of DNA-Step 2
• DNA
polymerase
binds to the
separated
chains of DNA
• Runs along
the separated
strands
Replication of DNA-Step 2
• New chains of DNA
are assembled using
nucleotides in the
surrounding medium
that are
complementary to
the existing DNA
chains
Replication of DNA-Step 3
• Once the DNA is
copied the cell is
ready to undergo
cell division
Accuracy and Repair
• Replication is
very accurate
• One error in
every 10,000
pairs of
nucleotides
• Mutations do
occur
• A mistake that
results in a
change in the
nucleotide
sequence
Accuracy and Repair
• The number of
errors and
mutations are
reduced by
proofreading
enzymes
• They detect errors
and replace the
wrong nucleotide
with the correct
nucleotide
• Doesn’t always
work
• Skin cancer