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DNA
DeoxyriboNucleic Acid
What can DNA do?
• Carries information from one generation to
the next
• Determines the heritable characteristics of
organisms
• Genes must be easily copied
DNA Structure
• DNA is a long molecule
made up of units called
– Nucleotides
• 3 basic components
– 5-carbon sugar
» Deoxyribose
– Phosphate group
– Nitrogenous base
Nitrogenous Bases
• There are four
Nitrogenous Bases
– Purines: Two rings in
their structures
• Adenine
• Guanine
– Pyrimidines: One ring
in their structure
• Cytosine
• Thymine
Backbone of DNA
• Formed by sugar and
phosphate groups of
each nucleotide
• The nitrogenous bases
stick out sideways from
the chain
• Nucleotides can be joined
together in any order
– Any sequence is
possible!!
Chargaff’s Rules
• Adenine always
pairs with Thymine
• Cytosine always
pairs with Guanine
Double Helix
• Rosalind Franklin
– Used X-ray diffraction
– Had the evidence that the double helix existed
but didn’t put it all together
• Watson and Crick
– Tried to put together a model of DNA
– Wasn’t until they saw Franklin’s X-rays that
they discovered the double helix pattern
Double Helix cont.
• Looks like a twisted
ladder
• Hydrogen bonds between
certain nitrogenous bases
provide the force to keep
the structure together
• Hydrogen bonds can only
form between certain
base pairs
– Adenine and thymine
– Guanine and cytosine
– Proves Chargaff’s rules