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6. Mutations.pdf

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Biology (Sci)
BIOL 202
Tamara Western

Mutations: While mutations are considered in the media to be gigantic monster producing phenomenon resulting from extreme nuclear radiation and the likes, the reality of mutation is discrete phenotypic changes, like in the color of an ant These changes are sometimes involved in color, or slight morphological changes, but we have yet to create a Godzilla-like mutant organism Genetic variation has two main causes: mutation and recombination Mutation causes the rise of new alleles, which are new variants of genes; they slightly differ from the wild-type in their aspects and resulting phenotype Recombination is when alleles are shuffled differently during meiosis, which can cause the passing out of maternal of paternal chromosomes The balance between mutation and selection allows for adaptation to occur, and is in fact the basis of evolution Mutations are changes in a DNA sequence which can lead to an altered coding region or a gene regulatory sequence This usually results in an altered gene product and therefore an altered phenotype o This could mean an altered protein product, but is could also cause an altered functional RNAs; in this course, we will focus in proteins Note that the wild-type variant is arbitrarily defined; for example, the Arabidopsis plant was originally discovered in Columbia, and therefore the Columbian plant is the wild type, while everything else is a mutant version o Therefore, the determine of the wild type merely depends on context The word mutant is an adjective, so you cant say something is a mutant Some mutations can be passed on to the next generation while others cannot; it depends on the type of cells that are mutated Somatic mutations are on body tissue cells, which means that they will NOT be heritable o An example is UV exposure causing brown patches on the skin; these skin cells will replicate and the path will become bigger, but the persons children will not be born brown Germline mutations fall within the cells that make gametes, and therefore have a chance of being passed on to the next generation o Since we are diploid organisms, usually only one chromosome will acquire the mutation, and therefore only half of the gametes will have the mutant allele o Therefore, only about half of the progeny will acquire the mutation, while the others will be wild-type There are some general types of mutations which we will later break down into more specific groups: Mutations can either be spontaneous or induced o Spontaneous mutations are thought to happen in the background, through things like DNA replication errors, breakage of DNA through oxidative stress, and background UV that we cant avoid The majority of these are fixed by cell repair mechanisms o Induced mutations happen upon exposure to mutation-inducing agents, or mutagens These can either be chemical or through radiation The scale, or size, of the mutation is also important: o Point mutations only change one or a few nucleotides through addition/deletion We will focus on these o Insertion mutations is when large chunks of DNA are inserted into a sequence through, for example, transposable elements o Chromosomal mutations involve loss or gain of large bits of an entire chromosome In the past, we used to wonder is spontaneous mutations even occur; it was thought that mutation only occurred to adapt to environmental changes The Luria-Delbruck test for spontaneous mutation was able to show that mutations do in fact happen randomly o 20 samples of bacteria were placed in a happy medium and allowed to grow o All 20 samples were infested with bacteriophage, and the experiment was to see how many bacterial colonies developed a resistance If mutation wasnt random, then similar resistance patterns should have been seen in all colonies, and the mutations should have only happened after the addition of phage If mutation is random, then the mutations could happen at any time (before or after addition of phage), and the amount of resistance between samples should vary a lot o Indeed, after the experiment was taken out, random mutation was observed Some samples developed the resistance early on, meaning that a large number of bacteria survived, while other colonies didnt develop any resistance at all It has also been seen that the rate of spontaneous mutation increases with the complexity of the organismo This is thought to happen simply because more complex organisms have more DNA, i.e. more to go wrong There are various types of spontaneous mutations: Replication errors involve tautomeric shifts in bases allow mispairing between them; electrons shuffle around randomly, which could cause temporary chemical shifts in the bases o This could fool a DNA polymerase into adding the wrong base randomly, but since the shift is very short, the wrong base is usually picked up by proofreading enzymes Depurination causes the DNA double helix to literally lose a purine base, and if left with a hole known as a apurinic site (AP site) o Some human cells can lose as many as 10,000 purines per day, but there are efficient repair mechanisms that repair these very quickly o This can also be induced by some reactive oxygen species, in which the mutation rate increases dramatically Deamination occurs upon the removal of an amine group from a base
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