forms of MUSCULAR DYSTROPHY. At the present time
there is no treatment, cure, or prevention for
progeria.
See also CELL STRUCTURE AND FUNCTION; GENETIC
DISORDERS.
recombinant DNA A biotechnology technique
that replaces DNAto alter a cell’s function. There
are several methods for performing recombina-
tion, though all ultimately involve extracting the
native DNA from a carrier (called a vector) and
replacing it with the desired DNA. Modified BACTE-
RIA and viruses are common vectors—bacteria
because they replicate rapidly, and viruses because
they can deliver modified DNA into the nucleus of
cells within the body (GENE THERAPY).
One of the most significant uses of recombinant
DNA technology is the production of substances
such as human INSULINsupplementation to treat
DIABETES, which recombinant technology can syn-
thesize in vast quantities in the laboratory to meet
strict quality and consistency standards. Because
such recombinant products are biochemically
indistinguishable from their endogenous (natu-
rally produced in the body) counterparts, they are
an exact replacement, and the body accepts them
as though they were endogenous. Recombinant
DNA technology produces other hormones, too,
such as HUMAN GROWTH HORMONE(HGH) SUPPLEMENT.
See also CELL STRUCTURE AND FUNCTION; CLONING;
HORMONE; VIRUS.
replication error A mistake that occurs when
DNAsequences duplicate before cell division. Repli-
cation errors are accountable for CHROMOSOMAL
DISORDERSsuch as AUTOSOMAL TRISOMY, in which a
GAMETE(also called a sex cell or germ cell) receives
two copies of a CHROMOSOMEinstead of the normal
single copy. Gametes, which are haploid cells,
each carry one-half the complement of chromo-
somes so when they unite to form the ZYGOTEthat
will become a new human being, the zygote con-
tains the full complement of genetic material.
With a replication error such as an autosomal tri-
somy, the zygote receives an extra chromosome—
two copies from one gamete and one copy from
the other gamete. The result is a chromosomal dis-
order such as DOWN SYNDROME. Replication errors
also may have harmless consequences when they
occur in DNA sequences that do not encode struc-
tural or regulatory sequences of gene activity.
See also MUTATION; VARIATION.RNA The abbreviation for ribonucleic acid. RNA
is a single-strand molecule consisting of ribose; a
sugar; and nucleotides made up of the nitrogen
bases adenine, uracil, guanine, and cytosine. RNA
exists in a number of forms, all of which serve as
biochemical messengers that carry the instructions
of DNAto the ribosomes, structures in the cell’s
cytoplasm. Ribosomes synthesize (manufacture)
the proteins the genes encode. RNA also may
function as the carrier of GENETIC CODEwithin the
mitochondria.
For further discussion of RNA 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; GENE;
NUCLEOTIDE.RNA 139