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 chromo-
somes in its nucleus (except erythrocytes, which
do not have nuclei). DNA has a characteristic dou-
ble-helix structure that resembles a gently twisting
ladder. The supporting rails of this structure are
deoxyribose, a sugar-phosphate, and the cross-
bands 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 bil-
lion 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 HUMANGENOME
PROJECT concluded their mapping of the human
GENOME, which included determining the entire
biochemical sequence of human DNA. Chromo-
somes are structures of DNA, and genes are seg-
ments of chromosomes (also made up of DNA).
For further discussion of DNA within the con-
text 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.

Down syndrome An AUTOSOMAL TRISOMYdisorder
that results from a REPLICATION ERRORduring cell
division in which a GAMETE(sex cell) ends up with
two copies of CHROMOSOME21 instead of the nor-
mal single copy (as haploid cells, gametes contain
one half the complement of chromosomes). At

CONCEPTIONthe 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 chromo-

some, 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 pro-

duces 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 CONCEPTIONmore than offsets the increase

in age-related risk. Down syndrome is the most

commonly occurring of the autosomal trisomy dis-

orders.

Symptoms and Diagnostic Path

Children born with Down syndrome often have

characteristic facial features, which include

• flat, upwardly slanting eyes with extra fatty tis-
  sue in the lids


• rounded face with a small NOSEand MOUTH


• small ears


• broad, short neck


• short stature with noticeably small hands and
  short fingers

Other findings of Down syndrome include con-

genital anomalies affecting the HEART, intestines,

and other organs. About half of infants born with

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