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Lecture 7

Lecture 7 - Understanding Mutations
Lecture 7 - Understanding Mutations

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Western University
Biology 2581B
Jim Karagiannis

LECTURE 6: UNDERSTANDING MUTATIONS Key Concepts 1. Types of mutations 2. Spontaneous vs. induced mutations 3. Forms of DNArepair What is a Mutation? • Mutations are heritable changes in DNAbase sequences • The change can be inherited • Mutation phenotype o Altered appearance o Altered growth conditions o Altered behaviour o Altered molecules • Something one can track • Amutation that changes a wild-type allele of a gene to a different allele is called a forward mutation o Resulting novel mutant allele can be either recessive or dominant to the original wild type • Mutations can also cause a novel mutant allele to revert back to wild- type in a process known as reverse mutation, or reversion What is a Mutant? • An organism which experiences a change in DNAsequence • Who is the mutant and who is the wild type? o What is considered to be the norm = WT o What is more frequent = WT o Who was first = WT o Who displays a negative effect = M Is a mutation a good or a bad thing? • It depends • Most mutations do not have any effect • Can cause neutral changes • Very few are beneficial • Some cause problems • Some are lethal • But this could depend on the conditions you use to observe Types of Mutations • Small changes – based on nucleotides • Base pair substitutions – when a base at a certain position in one strand of the DNA molecule is replaced by one of the other three bases; after DNAreplication, a new base pair will appear in the daughter double helix o Transitions – purine to purine  2 – GC toAT,AT to GC o Transversions – purine to pyrimidine, pyrimidine to purine  4 – GC to TA, GC to CG,AT to CG,AT to TA • Insertions – addition of one or more nucleotide pairs • Deletions – elimination of one or more nucleotide pairs • Inversions – 180 degree rotations of a segment of a DNAmolecule • Translocation – parts of two non-homologous chromosomes change places • Large changes affecting chromosomes or parts of chromosomes – chromosome rearrangements o Duplication o Deletion o Insertion o Inversion o Translocation o Reciprocal translocation o Genome duplication Mutations are: • Rare events • Spontaneous mutations occur at a very low rate • 2-12 x 10 per gene per gamete • Gene dependent • Can happen all the time • Random • Have many different reasons o Errors during replication o Radicals o UV damage • Although estimates of mutation rates are extremely rough, they nonetheless support three general conclusions: o Mutations affecting phenotypes occur very rarely o Different genes mutate at different rates o The rate of forward mutation (disruption of gene function) is almost always higher than the rate of reversion Fluctuation Test Results • Results of the fluctuation test showed that most plates supported zero to a few resistant colonies, but a few harbored hundreds of resistant colonies • From this observation of substantial fluctuation in the number of resistant colonies in different petri plates, it was concluded that bacterial resistance arises from mutations that exist before exposure to bacteriophage. • After exposure, however, the bactericide in the petri plate becomes a selective agent that kills of non-resistant cells, allowing only the preexisting resistant ones to survive Using Replica Plating • Another technique, known as replica plating, was used to demonstrate more directly that the mutations conferring bacterial resistance occur before the cells encounter the bactericide that selects for their resistance • Pressing a master plate onto a velvet surface transfers some cells from each bacterial colony onto the velvet. Pressing a replica plate onto the velvet then transfers some cells from each colony onto the replica plate. Investigators track which colonies on the master plate are able to grow on the replica plate (only penicillin-resistant ones) • Colonies on a master plate without penicillin are sequentially transferred to three replica plates with penicillin. Resistant colonies grow in the same positions on all three replicas, showing that some colonies on the master plate had multiple
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