Types of Mutations
Point mutations are changes to a single base in the DNA code and may involve either base substitutions, insertions or deletions
Base Substitutions
A base substitution mutation may create either a silent, missense or nonsense mutation depending on how the change impacts the polypeptide sequence
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Silent mutations occur when the DNA change does not alter the amino acid sequence of the polypeptide
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This is possible because the genetic code is degenerate and certain codons may code for the same amino acid
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Missense mutations occur when the DNA change alters a single amino acid in the polypeptide chain
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Sickle cell anaemia is a disease caused by a single base substitution mutation (GAG → GTG ; Glu → Val)
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Nonsense mutations occur when the DNA change creates a premature STOP codon which truncates the polypeptide
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Cystic fibrosis is a disease which may result from a nonsense mutation (this may not be the only cause though)
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Frameshift Mutations
Frameshift mutations involve either the addition (insertion) or removal (deletion) of a single base of DNA, changing the reading frame
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This change will affect every codon beyond the point of mutation and thus may dramatically change the amino acid sequence
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Hence, frameshift mutations typically have a significant impact on cellular activity, as there is a high likelihood the polypeptide will cease to function
Block Mutations
Block mutations are changes to segments of a chromosome, resulting in large scale changes in the DNA of an organism
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Block mutations are commonly caused by transposons (mobile genetic elements that can change positions within the genome)
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There are many different types of block mutations that can exist, including large scale insertions (duplications), deletions or translocations