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Lecture 8

Lecture 8: "Allele Classification"

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Biology 2581B
Jim Karagiannis

Genetics Lecture No. 8: Allele Classification th Monday February 4 , 2013 Mutations Affecting Translation: -Recall that mutations happen on the DNA level, but then can affect the RNA sequence and final protein product. Given a wildtype sequence, base substitution changes can alter the codon on an RNA level, but at the protein level, no effect on the protein’s function or polypeptide sequence is observed. This is because the redundancy in the 4 possible codons encoding for alanine differ only in the third nucleotide (wobble position). If a change occurs here, then a silent mutation results. Note that methionine is specific (has only one possible codon), making silent mutations impossible. -If a base substitution produces missense mutation (an amino acid that is similar in nature to the originally predicted one), little or no effect on the protein product results. However a missense mutation in amino acid codons at crucial positions can have a large effect on protein function. Nonsense mutations introduce a stop codon (UAA, UAG, UGA) where there was none and result in a truncated protein (shortened protein). The closer the stop codon is to the start codon, the greater the severity of the effect on the final protein’s function. -In an insertion mutation, all other codons after the inserted nucleotide are completely different (shifts the reading frame). These mutations can be caused by one or two nucleotides, but in the insertion of three nucleotides, a codon is essentially inserted. This action doesn’t cause a frameshift, but introduces a new codon into the amino acid sequence. Similar to a nonsense mutation, position of inserted base(s) (one or two) determines the severity of the mutation’s effect. Similar to a missense mutation, the insertion of three nucleotides at crucial positions can have a large effect on protein function. Mutations Affecting Function (Null & Amorphic Loss Of Function Mutations): -Not all mutations affect the sequences that are translated into proteins. Mutations in non-coding regions can affect gene expression: increased protein levels, decreased protein levels, no protein, or misexpressed protein (wrong timing or wrong tissue). In recessive loss of function mutations, a very prevalent class of mutations, the wildtype allele of heterozygotes is still sufficient to carry out the needs of the organism. In null loss of function mutations, no synthesis of the protein encoded by the mutated gene occurs (e.g. knock-out gene). In amorphic loss of function mutations, the mutation still allows for the synthesis of transcript and later protein, but the conformation of the protein product is too deformed to function at all. The wild type allele is sufficient for producing functional proteins in heterozygotes carrying alleles with null or amorphic mutations. An example of a null mutation this is in the agamous mutation of Arabidopsis thaliana, where a single base-pair mutation knocks out the gamous gene (for creating gametes) and results in a totally different phenotype. Hypomorphic Loss Of Function Mutations & Incomplete Dominance: -In hypomorphic loss of function mutations, there is either reduced protei
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