HMB265H1 Lecture Notes - Lecture 3: Single-Nucleotide Polymorphism, Allele Frequency, Dna Replication

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Lecture 3: Molecular Basis of Genetic Polymorphisms & Their Detection
Mutations are the source of allelic variation
Mutation is the process whereby genes change from one allelic form to another. The creation of entirely new
alleles can occur
o Creates variation
Genes mutate randomly, at any time and in any cell of an organism
Can arise spontaneously during normal replication, or can be induced by a mutagen
Only mutations in germ line cells can be transmitted to progeny. Somatic mutations can not be transmitted
Inherited mutations appear as alleles in populations of individuals
Allele frequency is the percentage of the total number of gene copies represented by one allele
o Wild-type: allele whose frequency is ≥ 1%
o Mutant allele: allele whose frequency is < 1%
o Monomorphic: gene with only one wild-type allele
o Polymorphic: gene with more than one wild-type allele
Detectable change (difference) in a given locus/gene
Forward mutation: changes wild-type allele to a different allele
Reverse mutation: causes novel mutation to revert back to wild-type (reversion)
Mutations affecting phenotype occur very rarely
Different genes mutate at different rates
o Mutation rate varies from 1 in 1000 to 1 in 1,000,000,000 per gamete per generation
Rate of forward mutation is almost always higher than rate of reverse mutation
Mutation can occur during normal DNA replication
Mutation rate can increase after exposure to mutagen
o UV light, chemicals
Classification of Mutations
Wild type: is the starting sequence
Substitution: base is replaced by one of the other three bases
Deletion: block of one or more DNA pairs is lost
Insertion: block of one or more DNA pairs is added
Inversion: rotation of piece of DNA
Reciprocal translocation: parts of nonhomologous chromosomes change places
Chromosomal rearrangements: affect many genes at one time
SNP’s are single nucleotide polymorphisms
o They are alleles where only a single base is changed
Gene Expression and Alleles
Allelic differences at the DNA level can influence mRNA expression and/or protein function, and thus the
phenotype
Important domain of an enzyme is the active site
o If there is a mutation in the mRNA code than a different protein could be made and result in a disease due
to the non-functional protein
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