How is information encoded in DNA used? Central to understanding this
is figuring out gene expression mechanisms. The main idea is that every
gene contains the information to produce a protein, which is a linear
arrangement of amino acids. There are 20 types of amino acids used in
proteins, so our task is to figure out how a sequence of 
in DNA is converted to a sequence in the amino acid alphabet. The really
interesting (and hard) part is learning what triggers and regulates gene
expression.
The story is slightly different for prokaryotic and eukaryotic organisms .
Prokaryotes lack a nucleus in their cells and their genome is floating
around somewhere in the cell, while eukaryotes have a nucleus in which the genome is
contained. The basic story of gene expression can be divided into two stages,
transcription and translation. In the first stage, DNA is
transcribed to produce RNA (ribonucleic acid), in the second stage the RNA is
translated to produce a protein. As far as we are concerned, the difference
between RNA and DNA is that RNA is usually a single stranded molecule, and
the nucleotides it uses are 
where Thymine used in DNA is replaced
by Uracil. This intermediary RNA is know as messenger RNA (mRNA for short).