The segments of DNA that we call genes usually code for proteins. The region of
a strand of DNA, which is "upstream" of the coding sequence either turns on or
off the expression of a particular gene. When we say a gene is "on" we mean
that it is making mRNA and that protein is translated from the mRNA. In humans
and in other animals, there must be eye specific genes, liver specific genes,
and genes that are expressed in all cells. Similarly in plants there must be
flower specific genes, leaf specific genes, root specific genes and so on.
These genes make the proteins that are specific for each function in each
tissue. These "upstream" sequences that regulate cell and tissue specific
expression are called:
