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Table of Contents
Why Do Genetics
More Basic Concepts
Mutagenesis in vitro
Effects of Mutations
Two Factor Crosses
Other Mapping Methods
Problem Set 1
Problem Set 2
Effects of mutations
V. EFFECTS OF MUTATIONS
V A. POTENTIAL EFFECTS OF MUTATIONS
Mutations in untranslated regions:
Transcribed: Mutations that occur in transcribed but untranslated regions might still affect the translation system by affecting the recognition signal for binding of ribosomes. They might conceivably affect mRNA stability, attenuation, and, where the gene product is an RNA, mutations might cause a loss of product function or cause improper processing or modification of the product.
Untranscribed regions: Mutations in regions that are neither transcribed nor translated might affect either transcriptional start or stop signals and thus the regulation of the region in question. It is also possible they might affect "structural" regions of the DNA, affecting gene expression indirectly.
Large deletions, inversions, and duplications: Such mutations can span both translated and transcribed regions and as such they can have a variety of effects. For example, they can generate transcript fusions or gene fusions; they can generate strong polarity; and deletions and inversions typically eliminate the products of the affected genes. Large inversions also have the subtle effect of changing gene position with respect to the origin of replication, thus changing the average copy number per cell. Duplications can have phenotypes when the copy number of that particular region is critical.
Insertions, insertion sequences, and transposons: Mutations involving such mechanisms will destroy whatever function was encoded by the affected region. They typically cause polarity due to their encoding of RNA termination signals and can occasionally provide new promoters reading into the flanking regions.
V B. NUMEROLOGY
numerology: 1. a system of occultism built around numbers...; 3. divination by numbers.
divination: ... 3. a successful guess; a clever conjecture [from Webster's Unabridged Dictionary].
One of the central themes of this course is that the frequency at which events occur makes predictions as to their type. To put it another way, phenotypes which appear frequently must occur by either (l) any of a large number of infrequent genome alterations or (2) a particular, very frequent genome alteration. The frequency of events determines the mode of analysis necessary in order to determine them. Ignoring hot spots, the following list gives an approximate idea of the sorts of frequencies with which various sorts of events occur:
Obviously, the above table is a bit deceptive. For example, it has already been stated that some insertion sequences revert at a reasonable frequency and some scarcely revert at all. On the other hand, if an event is occurring at 10-2 frequency, you may safely assume that it does not involve the occurrence of some very particular base substitution mutation since such events are simply too rare. It must involve a loss of a plasmid or something to do with duplications since these are the major events in bacteria which can occur at that frequency. Similarly, if a phenotype is occurring at very low frequency, then you may assume it is not arising by some reasonably common event like the loss of a plasmid or the knockout of a gene, but rather by some very specific base substitution or base change of some sort. The frequency of an event reflects both the likelihood of the mutational mechanism as well as the target size for events which cause the desired phenotype.
[See sample problems 10 and 11]
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