Gene Mutations
 A mutation is a change in the nucleotide sequence of a
gene.
 Although the initial change may not occur in the coding
strand of the gene, after replication daughter DNA molecules
with mutations in coding strand will appear in the next
population of organisms.
Types of mutations
1-Base-substitution mutations
 Single base changes (point mutations) may be transitions or
transversions.
 In transition mutation, pyrimidine base is changed to another
pyrimidine or purine base is changed to another purine base
 In transversions mutations, purine base is changed into either
of two pyrimidines and vice versa.
 When the gene is transcribed into mRNA, it will include the
base mutation which may have one of several effects when
translated into proteins.
A- There may be no detectable effect because of the degeneracy
of the code. This would be more likely if the changed base in
the mRNA was to fall on the third nucleotide of a codon.
Because of wobble, the translation of a codon is least sensitive
to a change at the third position.
B- A missense effect will occur when a different amino acid is
incorporated at the corresponding site in the protein molecule.
This mistaken amino acid or missense, depending on its
location in the specific protein, might be acceptable, partially
acceptable or unacceptable to the function of the protein.
 If an acceptable missense effect occurs, the resulting protein
molecule may not be distinguishable from the normal one.
 A partially acceptable missense will result in molecule with
partial but abnormal function.
 If unacceptable missense effect occurs, then the protein
molecule will not be capable of functioning in its assigned
role.
C-A nonsense codon may appear that would then result in the
premature termination of amino acid incorporation into peptide
chain and the production of only a fragment of the intended pr
protein molecule.
 The probability is high that a prematurely terminated protein
molecule or peptide fragment would not function in its assigned
role.
2- Frame shift mutations
 Frame shift mutations result from the deletion or insertion of
nucleotides in the gene and thus generate altered nucleotides
sequences of mRNA molecules.
 The deletion of a single nucleotides from the coding strand of a
gene results in an altered reading frame in the mRNA. This
alteration results in an error in the translation of mRNA after the
single nucleotide deletion.
 This may also result in the appearance of a nonsense codon
and thus production of a polypeptide both not correct and
premature.
 If 3 nucleotides or multiple of 3 were deleted from a gene the
corresponding messenger when translated would provide a
protein from which was missing the corresponding number of
amino acids.
 If, however, deletion of one or 2 nucleotides occur just prior to
within the normal termination codon (nonsense), the reading of
the normal termination signal is disturbed.
 Insertions of one or two or nonmultiples of 3 nucleotides into a
gene in an mRNA in which the reading frame is distorted upon
translation, and the same effects that occur with the deletions
are reflected in the mRNA translation.
 This may cause faulty amino acid sequences distal to insertion
and the generation of a nonsense codon at or distal to the
insertion or perhaps reading through the normal termination
codon.
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