G

gamete A spermatozoon (SPERMcell) or an ovum
(egg cell). A gamete, also called a germ cell or sex
cell, is a haploid cell; it contains half the comple-
ment of chromosomes and genetic material neces-
sary to encode (result in creating) an individual.
When two gametes merge they produce a single
diploid cell, the ZYGOTE, which then contains the
full complement of chromosomes needed for life.
For further discussion of gametes 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; CONCEPTION; OVULATION.

gene A segment of coding DNA (DNA that
instructs the structure and function of cells
throughout the body) composed of a specific
sequence of nucleotides. The gene is the basic unit
of inheritance that directs every facet of the body’s
appearance and functions. Genes align along chro-
mosomes in pairs. Each CHROMOSOME(AUTOSOME)
contains thousands of genes, except the sex chro-
mosomes which contain only a few hundred
genes.
Each gene has a specific location on the chro-
mosome, called its locus, and encodes a specific
function (either a protein or RNAtranscription).
The HUMANGENOMEPROJECTidentified 19,599 con-
firmed genes and 2,188 probable genes at its con-
clusion in April 2003. GENETIC DISORDERS occur
when there are disruptions of the ALLELEpairings
or there is damage to the gene or the chromosome
at or near the gene’s locus.
Each gene has a specific task, which it carries
out through a process called encoding. The gene
instructs the cells to synthesize (produce) a spe-
cific protein. Ribosomes, specialized structures

within each cell, synthesize the proteins. The pro-

tein then carries the gene’s message to its target

and initiates the appropriate sequence of biochem-

ical events to implement the message.

For further discussion of genes within the con-

text of the structures and functions of genetics,

please see the overview section “Genetics and

Molecular Medicine.”

See also CHROMOSOMAL DISORDERS; GENOME;

INHERITANCE PATTERNS; MUTATION; NUCLEOTIDE; SEX

CHROMOSOME.

gene therapy Treatment methods, most of

which remain experimental, that attempt to

manipulate genetic structure or gene encoding.

GENEtherapy targets either germline (GAMETE) or

somatic cells, using vectors to deliver genes within

cells. Germline gene therapy aims to prevent a

genetic disorder from passing to new genera-

tions, while somatic gene therapy targets genetic

disorders that already exist in individuals. Most

often the goal of gene therapy is to replace a

defective gene with a healthy, functional gene.

The vectors typically used are inactivated viruses

into which scientists insert the replacement gene.

The VIRUSenters the target cell and delivers the

gene.

Applications of gene therapy have not been as

successful as researchers have hoped was possible,

however, and at present the US Food and Drug

Administration (FDA) has not approved any gene

therapy methods for use in the United States. The

effects of gene therapy appear time-limited, and

viral vectors often initiate immune responses.

Researchers continue to investigate safe and effec-

tive mechanisms to therapeutically manipulate

genes with the goal of treating or curing GENETIC

DISORDERS.

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