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Genomics 101: A
Primer
Genomics and Its Impact on Science and Society: The Human
Genome Project and Beyond
Cells are the fundamental working units
of every living system. All the instructions needed to direct their
activities are contained within the chemical DNA (deoxyribonucleic
acid).
DNA from all organisms is made up of the
same chemical and physical components. The DNA sequence is the particular side-by-side
arrangement of bases along the DNA strand (e.g., ATTCCGGA). This
order spells out the exact instructions required to create a
particular organism with its own unique traits.
The genome is an organism’s complete
set of DNA. Genomes vary widely in size: the smallest known genome
for a free-living organism (a bacterium) contains about 600,000 DNA
base pairs, while human and mouse genomes have some 3 billion (see
"Early Insights"). Except for mature red
blood cells, all human cells contain a complete genome.
DNA in the human genome is arranged
into 24 distinct chromosomes--physically separate
molecules that range in length from about 50 million to 250 million
base pairs. A few types of major chromosomal abnormalities,
including missing or extra copies or gross breaks and rejoinings
(translocations), can be detected by microscopic examination. Most
changes in DNA, however, are more subtle and require a closer
analysis of the DNA molecule to find perhaps single-base
differences. 
Each chromosome contains many
genes, the basic physical and
functional units of heredity. Genes are specific sequences of bases
that encode instructions on how to make proteins. Genes comprise
only about 2% of the human genome; the remainder consists of
noncoding regions, whose functions may include providing
chromosomal structural integrity and regulating where, when, and in
what quantity proteins are made. The human genome is estimated to
contain 30,000 to 40,000 genes.
Although genes get a lot of
attention, it’s the proteins that perform most life
functions and even make up the majority of cellular structures.
Proteins are large, complex molecules made up of smaller subunits
called amino acids. Chemical properties that distinguish the 20
different amino acids cause the protein chains to fold up into
specific three-dimensional structures that define their particular
functions in the cell.
The constellation of all proteins
in a cell is called its proteome. Unlike the relatively
unchanging genome, the dynamic proteome changes from minute to
minute in response to tens of thousands of intra- and extracellular
environmental signals. A protein’s chemistry and behavior are
specified by the gene sequence and by the number and identities of
other proteins made in the same cell at the same time and with
which it associates and reacts. Studies to explore protein
structure and activities, known as proteomics, will be the focus of
much research for decades to come and will help elucidate the
molecular basis of health and disease.
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