CLINICAL CASES
125
Table 13-1c
NONSENSE MUTATION
Normal
GGC
CTG
ACA
GGG
C
AA
AAA
CTG
Gly
Leu
Thr
Gly
Gln
Lys
Leu
A nonsense mutation found in
Duchenne muscular dystrophy
GGC
Gly
CTG
Leu
ACA
Thr
GGG
Gly
T
AA
STOP
AAA
CTG
Another type of mutation is a deletion in which one or more DNA bases
have been removed. However, an insertion is a mutation wherein one or more
DNA bases have been added. These two types of mutations can cause a shift
in the open reading frame, called frameshift mutations, of a gene if the inser-
tion or deletion is a multiple of one or two bases. Typically, a frameshift
mutation will result in disease either as result of altering the protein prod-
uct, leaving it nonfunctional, or generation of a premature stop codon,
which confers a truncated form of the altered protein (Table 13-2). Both cases
present major problems because the altered protein will not likely be able to
carry out the normal duties required of it. Since codons are read as multiples
of three, an insertion or deletion of three or a multiple of three may or may not
be as deleterious to the gene as multiples of one or two. In this situation, the
resulting protein could include extra amino acids in the case of insertions or
lose amino acids in the case of deletions. It is possible that this could hinder
the protein from normally functioning.
Table 13-2
FRAMESHIFT MUTATION
FRAMESHIFT RESULTING IN ALTERED PROTEIN
CCC
GCA
TA
T
CAT
TTT
ACC
Pro
Ala
Tyr
His
Phe
Thr
i
Deletion of
TA
CCC
GCA
TCA
TTT
TAC
Pro
Ala
Ser
Phe
Tyr
FRAMESHIFT CAUSING PREMATURE STOP
CCC
GCA
TA
T
C
AT
TTT
ACC
Pro
Ala
Tyr
His
Phe
Thr
i
Deletion of
TC
CCC
GCA
TAA
Pro
Ala
STOP
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