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In simplified terms,
a single gene in a given organism is the set of instructions for
making a molecular product. The product may be one of the many
macromolecules necessary for the development and life of that
organism or one of the components necessary for the maintenance,
expression, and propagation of the instruction set itself. The
gene uses a chemical code in which the instructions are written,
and those instructions are heritable-they can be passed from one
generation to the next, which thereby explains Mendel's observations.
In physical terms, a gene is a discrete stretch of nucleotides
within a DNA or RNA 
molecule.
Each nucleotide
contains a chemical "base"-guanine, adenine, thymine, or cytosine
(represented as G, A, T, and C, respectively) for the DNA genes
of human beings and other organisms. It is the specific sequence
of these bases that defines the information contained in the gene
and that is ultimately translated into a final product, most often
a protein. The protein may have a structural role, or it may serve
as a catalyst to promote the formation of other macromolecules,
including carbohydrates and lipids. Some functional products of
genes are themselves nucleic acids, demonstrating the power and
versatility of these molecules.
The genome
is
the entire coded genetic blueprint of an organism, the full set
of genetic instructions for making all of the molecules that constitute
it. In the case of humans, the genome is composed of more than
three billion pairs of bases, which have been copied and passed
on letter by letter with gradual modification and expansion for
more than a billion years since life began. The vast majority
of the human genome exists as enormously long DNA molecules that
reside in the form of 23 pairs of elaborately packaged chromosomes
in the nucleus of each cell. The goal of the current genome effort
has been the sequencing of the bases in this nuclear portion of
the genome and a physical mapping of their location on the chromosomes.
Another tiny, but nonetheless essential, chromosome exists outside
the nucleus, in cellular organelles called mitochondria. The sequence
of the human mitochondrial chromosome has already been described.
Leslie
Gornstein
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