different types of mutations

1. Ikram Ullah
M.Phil MLSc

2. Types of Mutations
Point Mutations
 A point mutation is a change in a single DNA base.
 It is a transition if a purine replaces a purine (A to
G or G to A) or a pyrimidine replaces a pyrimidine
(C to T or T to C).
 It is a transversion if a purine replaces a pyrimidine
or vice versa (A or G to T or C).

4. Missense and Nonsense Mutations
 A point mutation that changes a codon that normally
specifies a particular amino acid into one that codes
for a different amino acid is called a missense
mutation
 The point mutation that causes sickle cell disease is a
missense mutation.

5.  The DNA sequence CTC encodes the mRNA codon
GAG, which specifies glutamic acid.
 In sickle cell disease, the mutation changes the DNA
sequence to CAC, which encodes GUG in the mRNA,
which specifies valine.
 This mutation changes the protein’s shape, which
alters its function.

7.  A point mutation that changes a codon specifying an
amino acid into a “stop” codon—UAA, UAG, or UGA in
mRNA—is a nonsense mutation
 A premature stop codon shortens the protein product,
which can greatly influence the phenotype
 For example, in factor XI deficiency a blood clotting
disorder, a GAA codon specifying glutamic acid is
changed to UAA, signifying “stop.”

10. Splice Site Mutations
 A point mutation can greatly affect a gene’s product if
it alters a site where introns are normally removed
from the mRNA. This is called a splice site mutation
 It can affect the phenotype if an intron is translated
into amino acids, or if an exon is skipped instead of
being translated, shortening the protein

11.  A missense mutation can cause “exon skipping” that
need not alter the amino acid sequence, but removes a
few amino acids.
 The mutation creates an intron splicing site where
there should not be one
 An entire exon is “skipped” when the mRNA is
translated into protein, as if it were an intron.
 An exon-skipping mutation is a deletion at the mRNA
level, but a missense mutation at the DNA level

14. Thank You!