DNA Structure

DNA is a large, complex organic molecule that is constructed from smaller organic molecular building blocks--nucleotides. Each nucleotide contains a phosphate group, a sugar molecule (named deoxyribose), and one of four bases.

Chromosomes (lengths of DNA) are large compared to other organic molecules, but still much smaller than a cell. To appreciate this relative size difference, watch this video.

We refer to the bases by the first letter of their chemical names:

A = Adenine

T = Thymine

G = Guanine

C = Cytosine

The specific structure of the DNA molecule was discovered in 1953 by American James Watson and his British research partner Francis Crick. In the following video, watch Jim Watson explain how they used chemistry and their molecule models to work out the shape of DNA--the double helix.

Replication

When mitosis occurs, each of the chromosomes must be duplicated exactly so that the two daughter cells will be identical. All 3 billion nucleotides must be copied, in the correct sequence. This process is called replication and only happens right before mitosis occurs.

First, the double helix is unzipped at its hydrogen bonds. The two original strands of the unwound DNA are used as templates to construct the opposite sides of the two new molecules. Because of the base-pairing rule (A always pairs with T, and G always with C) the two new double helix molecules are identical copies and this process takes very little time.

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