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What is DNA?
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The Double Helix:
Every
living thing has DNA -- deoxyribonucleic acid -- which comes in
the form of a long, threadlike molecule. A strand of human DNA
found inside a chromosome is about three feet long -- but so
thin that it is measured in angstroms, or hundred-millionths of
a centimeter.
Imagine
that each of these strands is made up of two helixes -- tight
spirals that look almost like tubes -- of molecules. Each helix
is made up of alternating phosphate and deoxyribose (sugar)
units. Now, picture two of these helixes twisted tightly
together to form one strand. This is the famous double helix,
the characteristic shape of DNA, discovered in 1958 by James
Watson and Francis Crick.
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The helixes have
links as a ladder's sides are linked with rungs. There are two different
phosphates (adenine and guanine) and two sugars (thymine and cytosine),
called "bases," that make up all of these rungs, in every plant and
animal and microorganism in the world. Adenine (A) and thymine (T) are
complementary, as are cytosine (C) and guanine (G). This means that
where an A occurs on one helix, immediately opposite it on the other
helix must lie a T. Where C occurs, a G occurs opposite.
Looking at the illustration above, you can see nine "rungs" -- reading
from the top, TA, then CG, GC, TA, TA, AT, TA, AT, CG.
Below that last
CG, the "rungs" are starting to come apart. Finally, the bonds are
completely broken, and each ribbon starts trying to attract a new set of
complementary molecules. This is how DNA replicates --
by "unzipping" and then assembling a new ribbon of sugars and
phosphates, so it can become a double helix again, with a new set of
partners. The replication process is made
possible through enzymes, other molecules, and interrelated metabolic
processes. It is necessary
because when cells divide and multiply, each cell needs its own DNA.
Even though we have perhaps 40 yards of DNA in every one
of our cells, only about 4 yards of
that functions in ways that determine our genetic
(inherited) traits. The rest is sometimes
called "junk DNA" -- how it got there and what it
does is one of the many unknowns in science.
The functioning parts of DNA are called genes. . Most
genes appear to have 5-10,000 base pairs (AT or CG
pairs), although some have several hundred thousand . We
have not yet "found" all the genes.
What Does DNA Determine?
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How does a long row of AT's and CG's translate into
leaves and eyeballs and fur and hummingbird wings? To
shorten a stunningly complex process into a single
statement, we could say that genes determine traits by
"coding for" a sequence of amino acids that, in turn,
make up different proteins.
This means that through a chain of chemical reactions,
hundreds of amino acids stack up to build proteins. Most
proteins are folded into three-dimensional shapes that
depend both on their amino acid composition and on their
cell environment -- that is, concentrations of different
salts, the presence of metal molecules, and other
molecules such as sugars and fats, in the cells. How
proteins function depends both on their amino acid
sequence -- the part controlled by DNA -- and their
shape, which is influenced by other factors.
These other factors, in turn, may be influenced by the
organism's own behavior and its external environment.
This is why there is uncertainty over how much DNA
actually determines. Even in the case of diseases that
are clearly genetic, for instance, the disease may take
quite a different course from one person to the next,
because biology is so complex and people's lives are so
different.
SOURCE: Exploding the Gene Myth by Ruth
Hubbard and Elijah Wald, Beacon, 1993
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