Encyclopedia of Health and Medicine

D
DNA The abbreviation for deoxyribonucleic
acid. DNA is the molecule of heredity; its
sequences form the body’s GENETIC CODE. Each cell
in the body contains DNA within the chromosomes
in its nucleus (except erythrocytes, which
do not have nuclei). DNA has a characteristic double-helix
structure that resembles a gently twisting
ladder. The supporting rails of this structure are
deoxyribose, a sugar-phosphate, and the crossbands
are nitrogen bases: adenine (A), thymine
(T), guanine (G), and cytosine (C). These bases
pair in precise, predictable patterns arranged in
nearly endless combinations, more than three billion
in all.
British scientists James Watson and Francis
Crick unraveled the double-helix structure of
DNA in 1953, identifying its two spiraling,
sugar–phosphate (deoxyribose) supports and
cross-bands of paired nucleic acids. Just 50 years
later researchers involved with the HUMAN GENOME
PROJECT concluded their mapping of the human
GENOME, which included determining the entire
biochemical sequence of human DNA. Chromosomes
are structures of DNA, and genes are segments
of chromosomes (also made up of DNA).
For further discussion of DNA within the context
of the structures and functions of genetics,
please see the overview section “Genetics and
Molecular Medicine.”
See also CELL STRUCTURE AND FUNCTION; CHROMO-
SOME; ERYTHROCYTE; GENOTYPE; RNA.
CONCEPTION the ZYGOTE thus ends up with three
instead of the normal two copies of chromosome
21, which ultimately produces multiple congenital
anomalies. When all cells carry the extra chromosome,
the resulting anomalies occasionally may be
so severe that the disorder is lethal before birth.
Sometimes Down syndrome occurs as a mosaic
disorder in which some but not all cells contain
the extra chromosome 21, which typically produces
milder symptoms.
Down syndrome occurs in about 1 in 1,200 live
births in the United States and about 350,000
Americans currently live with Down syndrome,
many independently. Though the risk for Down
syndrome increases dramatically with maternal
age, most infants who have Down syndrome are
born to younger mothers because the increased
rate of CONCEPTION more than offsets the increase
in age-related risk. Down syndrome is the most
commonly occurring of the autosomal trisomy disorders.
Symptoms and Diagnostic Path
Children born with Down syndrome often have
characteristic facial features, which include
• flat, upwardly slanting eyes with extra fatty tissue
in the lids
• rounded face with a small NOSE and MOUTH
• small ears
• broad, short neck
Down syndrome An AUTOSOMAL TRISOMY disorder • short stature with noticeably small hands and
that results from a REPLICATION ERROR during cell short fingers
division in which a GAMETE (sex cell) ends up with
two copies of CHROMOSOME 21 instead of the normal
single copy (as haploid cells, gametes contain
one half the complement of chromosomes). At
Other findings of Down syndrome include congenital
anomalies affecting the HEART, intestines,
and other organs. About half of infants born with
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