Lecture 4 Flashcards
When we are replicating DNA, we have 2 chances to create an unfavored tautomer. describe these.
During the initial replication fork
During mismatch repair
(these are the only 2 times that DNA exists as a single strand and are therefore able to tautomerize)
There is only 1 opportunity to create the syn-anti isomerization.
During the DNA replication fork formation
this is the only time transitions can occur
State the 2 possible mispairings between bases that can occur and cause transition mutations from this single base pair. A-T
A-C*
A*-C
(* = unfavored tautomer form of the base)
For transition mutations define it, and state was is required for a bond to form.
one regular nucleotide, that bonds incorrectly with an unfavored tautomer
still requires there to be 1 pyrimidine and 1 purine
(this creates non watson and crick base pairs)
In the absence of proofreading, what is more frequent and therefore more likely to occur, Transversions, or transitions?
Transitions (they are more prevalent IN THE ABSENCE OF PROOFREADING than Transversions)
Explain why Transversions are more likely to occur if an organism is capable of proofreading DNA during the DNA replication process. Compare this to transitions.
Transversions the get a 2nd chance to occur at the single strands that occur during DNA proofreading
Transitions can only occur during the replication fork (they only have 1 chance to occur, while transversions have 2 chances to occur)
State the 2 possible mispairings between bases that can occur and cause transversion mutations from this single base pair. A-T
A*-Asyn
A*-Gsyn
(* = unfavored tautomer form of the base)
State the 2 categories of substitution mutations that we have covered.
- Altered hydrogen bond potential
ex. Topal Fresco alternate forms of base pairing - Error-prone repair of replication roadblocks
ex. self-depurinating stem loops (bc an apurinic site will pause the replication machinery)
State the 6 different mechanisms by which induced mutations occur
- Base Analogs
- Alkylating Agents
- Intercalating Agents
- Adduct-forming Agents
- UV light
- Ionizing Radiation
Give an example of a Base analog induced mutation.
Thymine being altered into Bromouracil (by adding a Bromine) and then being able to pair with the incorrect base (depending on if it’s in it’s enol or keto form)
Basically BrdU can pair with A or G
Give an example of a Alkylating Agent induced mutation.
Mustard Gas (EMS) can modify Guanine with an ethyl group so that G can not pair with C.
Instead it becomes ethylguanine and binds incorrectly with T
Give an example of a Intercalating Agent, Adduct-forming Agent, or UV light induced mutation.
these 3 are lumped together because they can all occur from the “roadblocks” that Psoralens can create in DNA.
Psoralens are intercalating agents that, upon exposure to UV light, form adducts (bonds) with neighboring pyrimidine residues (roadblocks).
When DNA polymerase encounters the “roadblock” it must stop and do it’s best to fill in the messed up areas.
Give an example of a UV light exposure induced mutation. (Ignore Psoralens for this question)
Irradiation with UV light forms T-T (Thymine-Thymine) dimers
Give an example of an ionizing radiation induced mutation.
Radiation rays cause electrons to be ejected from an atom, leaving free radicals that cause Induced Mutations
True or False:
Ionizing radiation exposure is the reason mutagens occur. explain.
True? (sort of)
The mutagenic properties of ionizing radiation are a secondary effect caused by the free radicals that are created from exposure to ionizing radiation.
