Spontaneous and Induced Mutations

Trinucelotide repeat expansion (TRNE's)

a spontaneous mutation; unusual where there is a repeated sequence of 3 nucleotides that readily increases in number from one generation to the next occurs form replication slippage - example of anticipation: progressively worsening form one generation to the next TRNE's contain at least one C and G. this causes a hair pin to form; after the hairpin forms, polymerase moves down strand and may slip off template bc of loop. it will then back up a bit and hop back on to resume replication, which will cause it to repeat most of the hair pin region that it already copied. this is why it increases from each generation

mutagen

an agent that causes changes in the structure of DNA

induced mutations

caused by environmental agents- chemical or physical; enter cells and change dna structure

spontaneous mutation

change in dna structure that results from natural biological or chemical processes

mutations caused by spontaneous lesions

depuration, deamination, oxidative damage, trinucleotide repeat expansion

types of induced mutations

incorporation of base analogs, specific mispairing, intercalating molecules, base damage (UV and ionizing radiation)

incorporation of base analogs

induced mutation - 5 bromosecule ? 5 BU- analog of thymine it can be incorporated into dna instead, and if so, it will pair with adenine. at a relatively high rate, it could have a tautomeric shift, causing it to base pair with guanine . this changes original AT base pair to G5Bu this is an example of a tradition mutation bc it is has T that should bond with A (a purine) now transitioning to bonding with G (also a purine) - this mutation is often used in chemotherapy to kill cancer cells - happen IN dna

uv light

induced mutation causing base damage - non ionizing radiation, penetrates the surface of the organism, then cause formation of thymine dimers that don't base pair properly the lesions that uv light causes makes pyrimidine residues in the same strand unite, which causes a replication block dna pol will not slide across resulting in a mutation

ionizing radiation

induced mutation causing base damage - dna is sensitive to short wave length and high energy that can alter its structure ex. are x rays and gamma rays - this radiation goes deep into tissues , producing chemically reactive free radicals these can cause base deletions, oxidized bases, single stranded nicks, or cross linking

specific mispairing

induced mutation that does not occur in DNA - instead of incorporating in DNA, it just alters the bases such that specific base mispairing is made example of this is EMS (ethyl methanesulfonate) which when exposed to cell, adds an alkyl group to various positions on all 4 bases because of this, it is called an alkylating agent that disrupts appropriate base pairing

intercalating molecules

induced mutation; occurs in the dna slip themselves in between stacked nitrogen bases and core DNA helix causing insertions or deletions of specific base pairs - fit between the nucleic acid base pairs

tautomers

isomers of bases that differ in position of atoms and bonds b/w atoms

what is ames test and how does it work

it is a test that uses strains of bacterium, salmonella, typhimurium, to determine if a substance is a mutagen - bacteria can't synthesize amino acid histidine which they need without a specific enzyme. the test starts with sample a strain of bacterium that has a point mutation causing it to not have the enzyme to produce histidine. because of this, the bacterium will not be able to grow unless histidine is added - the other sample also has this strain, , but with a possible mutagen added. we want to test if this is for sure a mutagen, which would result in the colonies in this sample to grow faster (the mutagen contains a mutation to produce histodine) -there is also the chance that a random reversion or second mutation could occur on first sample, which would allow histidine production. after some time, check the two samples to see which has more growing colonies. if sample with mutagen has more, then it is definitely a mutagen- it mutates to add histidine faster then the spontaneous method did. mutagen= adding chemical causes mutation

spontaneous mutations are..

random; natural selection results in differential reproductive success of organisms that are better adapted to their environments

causes of mutations

recombination (deletions, translocation, inversions) and transposable elements (insert into sequence of gene)

replication slippage

spontaneous - DNA replication error loops in the single stranded regions stabilized by slipped mispairing of repeated sequences - the repeated sequences cause a hair pin loop to form Trnes?

tautomeric shift

spontaneous dna replication error DS dna tends to hold base pairs in most stable form; when they unwind, a shift may occur. must occur immediately prior to replication involves isomers of bases (tatoumers) causing mismatch base pairing - the keto form is the most stable, but if there is a base pair mismatch, the enol or imino form can occur

oxidative damage

spontaneous lesion mutation aerobic organisms use O2 as terminal receptor for ETC; reactive oxygen species (ROS) are products of O2 metabolism in all aerobics...over time if they accumulate they can cause damage to cellular molecules. cells use enzymes to prevent this build up, but oxidative stress occurs when there is an imbalance of ROS proteins and the ability to break them down (enzymes); the over accumulation of ROS results in oxidative DNA damage which can cause damage in DNA structure - guanine is particularly vulnerable

deamination

spontaneous lesion mutation removal of amino group from molecule ex. removal of amino group from cytosine produces uracil. U binds with A, so if C is changed to U, that c that should have originally binded with G will now be a U that binds with A. the new strand will incorporate A into the daughter strand rather G. if repair system doesn't take place a mutation could occur this changes G-C base pairs to A-T

depuration

spontaneous mutation lesion chemical change removal or purine (A or G) from DNA happens because covalent bond between deoxyribose and purine is unstable; when the purine is released, this creates an apurinic site. any of the 4 nucleotides can be added to an apurinic site, so if the repair systems fails to identify the site, mutation can occur form mismatch base pairing

mutations caused by errors in DNA replication

tautomeric shift, replication slippage

Base damage

two examples, uv light and ionizing radiation