|[Previous] | [Next]|
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
III B. CHEMICAL MUTAGENS
Chemical mutagens are defined as those compounds that increase the frequency of some types of mutations. They vary in their potency since this term reflects their ability to enter the cell, their reactivity with DNA, their general toxicity, and the likelihood that the type of chemical change they introduce into the DNA will be corrected by a repair system (section IIIC). The concerns in the use of mutagens are discussed in sections IIIF and G. Most of the following mutagens are used in vivo treatments, but some of them can also be used in vitro.
While the section below provides models for the molecular basis of many of these mutagens, it is exceedingly difficult to examine the actual mode of mutagenesis in vivo, because you are necessarily examining very rare events. Even if you can verify that a particular modified base can be formed in vitro by reaction with a mutagen, and that the modified base can give rise to a stable mutation in vivo, it cannot be assumed that this is the pathway for mutagenesis for the majority of the observed in vivo events.
III B1. BASE ANALOG MUTAGENS
Base analog mutagens are chemicals that look like normal bases and as such fool the DNA replication system. Their essential property is that they base-pair with two different bases thus making mutations because of their lack of consistency in base-pairing. To be mutagens they must be incorporated into the DNA and therefore they need be present during active DNA synthesis. An example is 5-bromo-deoxyuridine (5BU), which can exist in two tautomeric forms: typically it exists in a keto form (T mimic) that pairs with A, but it can also exist in an enol form (C mimic) that pairs with G.
Each of these chemicals will continue to mutagenize with time because of their constant likelihood of mispairing. By the same argument, it requires subsequent rounds of replication for any mutation to be generated since this requires "mispairing" during replication. Further, it takes another round of replication before the mutation is stabilized, that is, before both strands of DNA have the "mutant information". Until that occurs, the mismatch repair system can still recognize and remove the inappropriate base. This is termed "mutation fixation" and explains why these mutagens must be present during active DNA replication.
III B2. ALKYLATORS
These chemicals react directly with certain bases and thus do not require active DNA synthesis in order to act but still do require DNA synthesis in order to be "fixed". They are very commonly used because they are powerful mutagens in nearly every biological system. Examples of alkylators include ethyl methane sulfonate (EMS), methyl methane sulfonate (MMS), diethylsulfate (DES), and nitrosoguanidine (NTG, NG, MNNG) as shown in figure 8. These mutagens tend to prefer G-rich regions, reacting to form a variety of modified G residues, the result often being depurination. Some of these modified G residues have the property of inducing error-prone repair (see sec. III C) although mispairing of the altered base might also be possible. This stimulation of error-prone repair allows all sorts of mutation types to occur as a result of these mutagens, though base substitutions are by far the most frequent. It also appears that alkylated bases can mispair during replication. The relative contribution of all of these mechanisms to actual mutagenesis is unclear.
III B3. OTHER CHEMICAL MUTAGENS
Nitrous acid is another chemical mutagen that causes oxidative deamination of particular bases. It converts adenine to hypoxanthine (which now pairs with C), cytosine to uracil (which now pairs with A) and finally guanine to xanthine (which still continues to pair with C). Unlike the above mutagens, nitrous acid alters a base directly to a "miscoding" form and thus does not require subsequent DNA synthesis for its effect.
Yet another class of chemical mutagens, the so-called "ICR" compounds, induce frameshift mutations and these will be treated in the section on frameshift mutations. Some are depicted in figure 9 and these require DNA synthesis in order to cause mutations. They apparently mutagenize by "intercalating" between adjacent bases, perhaps making synthesis/repair systems think there is another base at that position.
III B4. UV MUTAGENESIS
Exposure of bacterial cells to UV light generates primarily cyclobutane dimers and py(6-4)pyo photo products at adjacent pyrimidine bases. While the former seem necessary for induction of SOS repair (see below), the latter seem to be the actual sites at which the mutations eventually occur.
|This page was last built with Frontier and Web Warrior on a Macintosh on Thu, Sep 21, 2000 at 1:00:39 PM.|