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Lecture 8

Lecture 8 - Allele Classification

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Biology 2581B
Jim Karagiannis

LECTURE 7: ALLELE CLASSIFICATION Key Concepts 1. Classification of mutations a. Based on the effect on translation b. Based on the effect on function Mutations affect Translation • Silent mutation o Amino acids are encoded by 1 to 6 codons o Silent mutations change a codon into a mutant codon that specifies exactly the same amino acid o Wobble position is altered (last nucleotide in a codon) o Do not alter the protein’s primary sequence • Missense mutation o Exchange one amino acid against another o Severity of effect: depends what substitution and where o Is substituted amino acid has chemical properties similar to the one it replaces, then it may have little or no effect on protein function o Missense mutations that cause substitution of an amino acid with very different properties are likely to have more noticeable consequences • Nonsense mutation o Mutations cause a premature stop codon (UAAUAG UGA) o Results in a truncated protein o Severity of effect: depends on where • Frameshift mutation o Insertions and deletions of nucleotides within the coding sequence can cause frameshifts o Results in wrong protein sequences o Severity of effect: depends on where o Adding three nucleotides would result in an entire codon, thus a frameshift is not caused. However, adding one or two nucleotides causes a frameshift mutation • However, not all mutations affect sequences translated into proteins • Mutations in non-coding regions can affect gene expression o Increased protein levels o Decreased protein levels o No protein o Mis-expressed proteins – wrong time or wrong space MutationsAffect Function • Loss of function o Null or amorphic o Such mutations either prevent synthesis of protein or promote synthesis of protein incapable of carrying out any function o Hypomorphic – loss-of-function mutation that produces either much less of a protein or a protein with very weak but detectable function o Incomplete dominance – some combinations of alleles generate phenotypes that vary continuously with the amount of functional gene product, giving rise to incomplete dominance o Dominant loss of function o Dominant negative loss of function • Gain of function o Hypermorphorphic o Neopmorphic Loss of Function • Most are recessive • That means, you have one copy that is working and one that is not working • Null or amorphic mutation o No protein synthesis or protein is so deformed that it cannot function • Hypomorphic mutation o Reduced protein synthesis or protein is somewhat deformed that it has very weak function o Eyeless mutation in Drosophila o Different alleles have different degrees of impact  No eyes, small eyes
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