13-2 The Structure of DNA

Chargaff analyzed the amounts of the four nucleotides found in DNA (Adenine, Thymine, Guanine, Cytosine) and noticed a pattern.

1.  The amount of  A, T, G, C varies from species to species
2.  In each species, the amount of Adenine equals Thymine,
3. The amount of Guanine equals Cytosine

This is the  Base Pair Rule

Bases come in two types:

pyrimidines (cytosine and thymine)


purines (guanine and adenine)

Rosalind Franklin and Wilkins spent time taking X-ray diffraction pictures of the DNA molecule in an attempt to determine the shape of the DNA molecule.

franklinDNA xray

Watson and Crick  are credited with finally piecing together all the information previously gathered on the molecule of DNA. They established the structure as a double helix - like a ladder that is twisted. The two sides of the ladder are held together by hydrogen bonds.   


The sugar (deoxyribose) and phosphates make up the "backbone" of the DNA molecule.
The phosphate is attached to the 5' carbon (the 5 is a number given to sugar molecules). The DNA strand has a free phosphate on the 5' end, and a free sugar on the 3' end - these numbers will become important later.

Adenine always pairs with Thymine | Guanine always pairs with Cytosine

Side1:: A A T T G G C C A G A T A C
Side2:: T T A A C C G G T C T A T G

DNA is composed of subunits called nucleotides, strung together in a long chain --
Each nucleotide consists of: a phosphate, a sugar (deoxyribose), and a base

Image Credit: Wikimedia

Here the 5' end and the 3' end are seen again, each side of the ladder has an opposite orientation. One side of the ladder as a free sugar (the 3'end) the other side has a free phosphate (the 5'end). This arrangement is called: ANTI-PARALLEL


Other Chapters: 13-1 (Genetic Material), 13-2 (DNA Structure), 13-3 (Replication), 14-1 (Function of Genes), 15 (Mutations), 16 (Biotechnology),