Mutation and its types
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This document summarizes different types of mutations that can occur at the DNA level, including point mutations, missense mutations, and nonsense mutations. It provides examples of each type of mutation. A point mutation is a change in a single DNA base, and can be a transition or transversion. A missense mutation changes a codon coding for one amino acid into a codon for a different amino acid, like the mutation causing sickle cell disease. A nonsense mutation changes a codon into a stop codon, shortening the resulting protein. A splice site mutation can affect intron removal from mRNA, altering the protein product.
Mutation and its types
- 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
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