CLINICAL CASES
123
D efinitions
Point mutation: The substitution of a single nucleotide in the genetic
material of an organism. These include silent, missense, and nonsense
mutations.
Silent mutation: A single nucleotide is exchanged by another, but this
alteration does not change the amino acid for which the codon codes.
The final protein product remains unchanged.
Missense mutation: A single nucleotide is exchanged by another, and this
alteration does have an effect on the coding amino acid. The final pro-
tein product is also modified. The modification may or may not be dele-
terious to the final protein, depending on the function of the amino acid.
Nonsense mutation: A single nucleotide is exchanged by another, which
produces a new stop codon at this position. This premature stop codon
generally results in a truncated form of the protein and most often leaves
it as an inactive form.
Deletion: One or more nucleotides are removed from the genetic sequence.
If the deletions are multiples of one or two, a frameshift will be the
result, which will likely damage the final protein product. A deletion of
three or a multiple of three does not shift the reading frame, rather it
would merely remove a codon(s). The final protein product would lose
amino acid(s), which may or may not leave it inoperative.
Insertion: One or more nucleotides are added to the genetic sequence.
These are the opposite of deletions.
Trinucleotide repeat expansion: Amplification from one generation to the
next of three nucleotide repeats in the coding or noncoding regions of
DNA. The mechanism may arise from DNA complimentary strand slip-
page. This is associated with fragile X syndrome and myotonic dystrophy.
DNA methylation: Process by which methyl groups are added to DNA
bases (most often cytosine). Methylation functions to regulate gene
expression because heavily methylated genes are not expressed. Also,
bacteria use methylated DNA as a defense mechanism. Every organism
has different patterns of methylated DNA, and bacteria take advantage
of this by destroying foreign DNA via nucleases, enzymes that cut DNA
at specific sites.
D ISC U SSIO N
Replication often produces changes in the chemical makeup of DNA. Many
of these changes are easily repaired; however, those alterations in the DNA
base sequence that do not get repaired are referred to as mutations. There
are various types of mutations including point mutations, deletions, and
insertions.
Point mutations arise when one base pair is substituted for another, and
they are the most common types of mutations. Point mutations can be further
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