Encyclopedia of Health and Medicine

nucleotide 135
See also CELL STRUCTURE AND FUNCTION; CHROMO-
SOMAL DISORDERS; EDWARDS SYNDROME; INHERITANCE
PATTERNS; MUTATION; PATAU’S SYNDROME; REPLICATION
ERROR.
mutation Permanent alterations in the ALLELE
pairings, or genes, on the chromosomes that pass
on to new cells and ultimately to offspring. Mutations
are the process through which genetic
change takes place. Some mutations are beneficial,
some are neutral, and some are harmful.
Mutations occur as changes in the GENE’s
NUCLEOTIDE sequences. These changes may take the
form of
• point mutations, also called base mutations,
which are analogous to changing one letter in a
word and occur when one nucleotide substitutes
for another
• deletion mutations, which are analogous to
removing a word from a sentence and occur
when the gene drops a nucleotide sequence
• insertion mutations, which are analogous to
adding a word to a sentence and occur when
the gene adds a nucleotide sequence
• increased repeat mutations, which occur when
a normally repeated nucleotide repeats extra
times
A germline mutation affects a GAMETE (ovum or
spermatozoon) or ZYGOTE and is present from CON-
CEPTION, passing to the child. WILMS’S TUMOR and
HEMOPHILIA are examples of germline mutations
that cause disease. A monogenic MUTATION affects a
single gene. Duchenne’s MUSCULAR DYSTROPHY and
SICKLE CELL DISEASE are among the conditions that
occur as a result of monogenic mutations. Polygenic
mutations involve multiple alleles of numerous
genes, often across chromosomes. Polygenic
mutations often do not clearly result in GENETIC
DISORDERS though establish GENETIC PREDISPOSITION.
Conditions such as CARDIOVASCULAR DISEASE (CVD),
DIABETES, and some types of cancer occur as a
result of polygenic mutations in combination with
lifestyle (environmental) factors.
See also AUTOSOMAL TRISOMY; CHROMOSOMAL DIS-
ORDERS; LIFESTYLE AND HEALTH.
nucleotide A structural component of DNA and
RNA. A DNA nucleotide contains deoxyribose and
a nitrogen base of adenine, guanine, thymine, or
cytosine, which form pairs called base pairs. An
RNA nucleotide contains ribose and a nitrogen
base of paired adenine, guanine, uracil, or cytosine.
Each DNA or RNA molecule contains thousands
of nucleotides. The order in which the
nucleotides appear is the base sequence and conveys
the GENETIC CODE for the proteins the DNA or
RNA molecule encodes. Base sequences, arranged
in triplets (trinucleotides), make up GENES.
See also CELL STRUCTURE AND FUNCTION; CHROMO-
SOME; GENOME.