The Structure of DNA
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Inherited characteristics are determined by genes.
Genes are found on chromosomes.
Chromosomes are made of protein and DNA.
DNA stands for deoxyribonucleic acid.
DNA is the genetic material that determines
inherited characteristics
• DNA is made of nucleotides.
• A nucleotide consists of:
 A sugar (ribose)
 A Phosphate
 A base
• There are 4 bases:
 Adenine
 Guanine
 Thymine
 Cytosine
• Each base has a different
shape (like puzzle pieces)
• Chargaff was a biochemist who
paved the way to the discovery of
the double helix structure of DNA.
• Chargaff used chemistry to discover
that the amounts of Adenine and
Thymine were always present in the
same proportion in DNA and that
Guanine and Cytosine were also
always present in DNA in the same
proportional amounts.
• In other words, there is always a 1:1
ratio of A to T and of C to G.
• These are now known as
“Chargaff’s Rules.”
Rosalind Franklin used X-Ray diffraction to
discover the “double helix” structure of
DNA. Her assistant, Maurice Wilkins,
showed the x-ray image to James Watson –
without her knowledge or permission. She
died in 1958 at age 37 of cancer – possibly due
to radiation exposure from her work with X-ray
diffraction.
Rosalind Franklin
http://www.yourgenome.org/stories/giants-ingenomics-rosalind-franklin
Watson and Crick used
Rosalind Franklin’s x-ray
diffraction work Chargaff’s 1:1
ratio, to build a model of DNA
and prove the chemical
makeup and double helix
structure of DNA. This
discover led to an
understanding of how DNA is
copied and how it works in the
cell.
They won a Nobel Prize for
their work.
Neither Erwin Chargaff nor
Rosalind Franklin were
awarded the prize.
A HELIX is a 3 dimensional
spiral.
A DOUBLE HELIX
then is TWO spirals.
• The two sides of the ladder are
made of alternating sugar parts
and phosphate parts.
• The rungs of the ladder are the
base pairs.
• Adenine always pairs with
Thymine and Guanine always
pairs with Cytosine.
• Knowing the base pairs allows the cell to
REPLICATE or make copies.
• Each base ALWAYS pairs with its
COMPLEMENT. Thus each pair of bases are
complementary.
• For example, AGC will always pair with TCG.
• What is the complement of AAT?
• Copies of DNA are made by
splitting the DNA molecule
down the middle.
• Then, the bases on each side
are used as the pattern for the
new strand.
• Half of EACH DNA molecule is
OLD DNA and the other half is
NEW DNA.
• DNA is copied every time the
cell divides so that the new cell
gets a complete copy of DNA.
1. Who is this and what is this a picture of?
Rosalind Franklin; The picture is a n X-ray diffraction
of DNA.
2. Who are these two scientists and what did they
do?
Watson and Crick proved the double helix shape of
DNA.
3. What are the 3 components of DNA?
Sugar, phosphate, nitrogen base
4. What are the 4 bases?
Adenine, Guanine, Thymine and Cytosine
5. What bases always pair
together?
Adenine and Thymine
Guanine and Cytosine