The DNA stored in the nucleus of a single human
cell spans over six feet in length if stretched
from end to end. Made up of four chemical building
blocks called A, C, T and G, for short, DNA contains
the instructions for making all living things.
The building blocks link to form the molecule's
famous "double helix" structure,
which allows genetic information to be copied
and passed down from one generation to the next.
Faithful copying of DNA information accounts
for why we look like our parents and why dogs
don't give birth to cats, for example.
Occasionally exposure to toxins or malfunction
of cellular processes, among other things, does
cause copying mistakes. Such changes over long
time periods provide opportunities for organisms
to adapt to new surroundings--or, cause
them to die out.
Discrete segments of DNA, called genes, encode the
instructions for making proteins. Work horses of
the cell, proteins, serve as structural material,
hormones, enzymes and neurotransmitters as well as play many
other roles. Efforts are underway to decipher the
complete set of genetic information, known as the
genome, in a variety of agriculturally--and--environmentally
important plants and other organisms and
to learn the function of the proteins those genes