DNA contains the genetic code for all organisms in the form of a twisted ladder structure with sugar-phosphate backbones and nitrogen base rungs that always pair together as A=T and C=G. In 1953, Rosalind Franklin discovered the spiral structure of DNA and James Watson and Francis Crick used her work to build the first DNA molecular model.

2. The Code
 All cells use a code that is stored in its hereditary
material…the code is DNA.
 DNA is a chemical called deoxyribonucleic acid.

3.  DNA contains information for an organism’s
growth and function.
 Every cell that has ever been formed in your body
or in any other organism contains DNA.

4. Who discovered it?
 Mid 1800s: Scientists have known that the nuclei
of cells contain large molecules called nucleic
acids.
 1950: Chemists had learned what DNA was made
of, but they didn’t understand the parts and how
they were arranged.

5. DNA Structure
 1953: Rosalind Franklin discovered that DNA is
two chains of molecules in a spiral form. How did
she discover this?
 Ultimately she discovered that the structure of
DNA is similar to a twisted ladder.

6. 1953: Using the work of Franklin,
James Watson and Francis Crick made
a model of DNA molecule.

7. DNA Model
 Each side of the “ladder” is made up of sugar-
phosphate molecules.
 Each molecule consists of the sugar called
deoxyribose and a phosphate group.

8.  The rungs of the “ladder” are made up of other
molecules called nitrogen bases. Four kinds of
nitrogen bases are found in DNA:
 Adenine (A)
 Guanine (G)
 Cytosine (C)
 Thymine (T)

9. How does it work?
 The amount of cytosine always equals the amount
of guanine. C=G
 The amount of adenine always equals the amount
of thymine. A=T
 This led to the hypothesis that they always occur in
pairs. Each base bonds only with its correct
partner.