Discovering DNA

Watson and Crick

1953

2003

DNA is composed of six kinds of molecules; adenine, guanine, thymine, cytosine, deoxyribose, and phosphoric acid. Each adenine, guanine, thymine, and cytosine combines with a molecule of deoxyribose and phosphoric acid. The four combinations are known as nucleotides.

Many of these nucleotides combine to form the huge DNA molecule. (Watson and Crick were attempting to determine the precise nature of their combination.)

The available data on the X-ray diffraction patterns suggested to Watson and Crick that the DNA molecule consists of two long fibers twisted around one another to form a double helix (like double spiral staircases, one for ascent and one for descent).

X-ray data indicated that the diameter of the double helix is about 20 Ångstrom units.

 Each fibre of the double helix consists of phosphate and deoxyribose units alternating with one another: phosphate-deoxyribose-phosphate-deoxyribose, and so on.

The adenine, guanine, cytosine, and thymine units (collectively known as nitrogenous bases) are attached to the phosphate-deoxyribose chain.

Relative Proportions (%) of Bases in DNA

Organism A T G C

Human 30.9 29.4 19.9 19.8

Chicken 28.8 29.2 20.5 21.5

Grasshopper 29.3 29.3 20.5 20.7

Sea Urchin 32.8 32.1 17.7 17.3

Wheat 27.3 27.1 22.7 22.8

Yeast 31.3 32.9 18.7 17.1

E. coli 24.7 23.6 26.0 25.7

In different cells of the same species, the relative amounts of adenine, guanine, thymine, and cytosine are the same.

In different species, the relative amounts of adenine, guanine, thymine, and cytosine vary greatly.

In all cells that had been studied, the amount of adenine was found to equal the amount of thymine and the amount of

guanine was found to equal the amount of cytosine. This was discovered by Erwin Chargaff of Columbia University