In 1952, Rosalind Franklin obtained a photograph of DNA known as Photograph 51 that helped prove its double helix structure. In 1953, James Watson and Francis Crick described the double helix structure of DNA in the journal Nature. The DNA molecule contains nucleotides with nitrogenous bases that form two antiparallel and complementary strands that twist together in the shape of a double helix.

1. Lesson #13
DNA

2. The DNA molecule
Double helix model

3. Rosalind Franklin - 1952
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Photograph 51

4. James Watson and Francis Crick
1953
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J. Watson F. Crick

5. Double helix model
 In 1952 Rosalind Franklin proposed the DNA molecule is a
double helix and she made a photograph that helped
prove it.
 In 1953 James Watson and Francis Crick described the
structure of the DNA molecule on NATURE magazine.
 DNA is a molecule which packs and condenses with the
help of proteins (histones) to form a structure called
chromosome.
 The DNA molecule contains the code or genes which make
an organisms structure and function.

7. Components of the DNA molecule
 Formed by thousands of smaller units called
nucleotides
 Each nucleotide is made up of deoxyribose, a
nitrogenous base and phosphate
 There are 4 different nucleotides which differ
according to their nitrogenous base.
 Adenine (A), cytosine (C), thymine (T) and guanine (G)
 When nucleotides join together, they form a chain
where deoxyribose alternates with phosphate groups.

8. Nucleotides: Nitrogenous bases

9. Deoxyribonucleic Acid (DNA)
 DNA molecule made up of two,
complimentary, antiparallel chains (strands) of
nucleotides.
 Antiparallel: The strands run in opposite
directions (5’  3’ and 3’  5’)so that the
nitrogenous bases look towards the interior of
the molecule.
 Complimentary: The nitrogenous bases on
opposite chains are complimentary and join
through hydrogen bonds.
Adenine (A) --- Thymine (T)
Cytosine (C) --- Guanine (G)
 Both chains twist around each other in the
shape of a double helix.