CASE FILES: BIOCHEMISTRY
A nsw ers
C. A CGG repeat expansion of 230 base pairs is just at the limit of
the number of repeats required to have the full mutation. Expansions
from 7 to 60 are seen in normal patients and those exceeding 230 are
positive for fragile X syndrome. Because some of the DNA was
methylated, this could help to silence the expression of FMRP and
cause some of the phenotype seen in the patient. Therefore, this sce-
nario would be considered borderline fragile X.
B. A nonsense mutation causes a premature stop codon on a single
nucleotide substitution. The original sequence, TAT, coding for a
tyrosine, must have been mutated to either TAA or TAG, leaving a
stop codon in place of the tyrosine.
A. Deletion of G in the third codon gives the sequence:
TTT ACC GTT TAT CTA GGG ATG ^
TTT ACC TTT ATC TAG GGA TG
This causes a frameshift and a premature stop codon to be generated.
Therefore, the final protein product would look like the one in question.
CTA GGG ATG
B IO C H E M IS T R Y PE A R L S
Mutations are alterations in the DNA base sequence that do not get
There are various types of mutations including point mutations,
deletions, and insertions.
Point mutations arise when one base pair is substituted by another,
and they are the most common types of mutations.
Point mutations can be further subdivided into three categories:
silent, missense, and nonsense.
Many fragile X individuals have an increase in the number of CGG
repeats, which may expand to over 230 repeats.