Deoxyribonucleic Acid (DNA)
Deoxyribonucleic Acid is the code of life. It is double helical in shape, which means that it looks like a ladder that has been twisted
into a spiral. The structure of DNA was discovered in 1953 by James Watson and Francis Crick. They shared a Nobel Prize in Medicine
with Maurice Wilkins (whose work aided their discovery) in 1962 for their achievement.
The sides of the "ladder" are made up of bonds between alternating deoxyriboses and phosphates. Deoxyribose, from which DNA derives
part of its name, is what is the five carbon sugar depicted below. The four carbon atoms not labeled are at the unlabeled vertices
of the pentagon.
A phosphate is an ion with the formula PO4. It connects to the deoxyribose at the fifth carbon (the labeled carbon in the
diagram above) and the leftmost hydroxyl ion (OH). See the diagram below for the structure of phosphate.
The information in DNA is held in the sequence of nitrogenous bases (like letters of the alphabet). The DNA alphabet has four letters: A,
T, C, and G. These letters stand for Adenine, Thymine, Cytosine, and Guanine respectively. Pairs of
these bases form the rungs of the "ladder." The bases are connected with hydrogen bonds (the dotted lines in the diagram below). Adenine
always pairs with Thymine, and Cytosine always pairs with Guanine.
Adenine paired with Thymine
Guanine paired with Cytosine
In a cell, DNA is wrapped around special proteins called histones. The entire human genome is divided into 46 groups called chromosomes.
There are actually only 23 different kinds of chromosomes, and there are two copies of each, called homologous pairs. This means that
a normal human has two copies of each gene, a section of DNA that codes for a single protein. However, the types of mutations
in each copy can be different. Homologous chromosomes are not identical.