Spontaneous and Induced Mutations Flashcards
What was the aim of Luria and Delburk’s experiment?
To see if mutations were spontaneous or a result of environmental pressure
What should we expect if the stimulus triggers a mutation in the DNA of the bacteria?
Depending on the time at which we cause the stimulus/ phage we should expect a fixed number of bacteria that will be resistant
What did Luria and Delburk’s experiment demonstrate?
Mutations are not induced by the environmental stimulus but rather by occuring randomly in the population of the bacteria
What is the spontaneous mutation rate in eukaryotes?
10^-4–> 10^-6 per gene per generation
What happens to most spontaneous errors?
They are corrected by proof-reading activity of DNA polymerase
What can DNA replication errors be?
- Point mutations: small insertions or deletions
- Base-pair substitution mutations can result from “wobble” pairing
What is “wobble” pairing?
Pairing within bases which is not the standard Watson-Crick base pairing
Give examples of “wobble” pairing
- Base pairing between normal pyrimidines and rare forms of purines [A G]
- Base pairing between rare forms of pyrimidines [C T] and normal forms of purines
What do we get in the 2nd generation progeny as a result of mismatch caused by wobble base pairing?
Production of one mutant and 3 wt
When is it deletion?
If it is on the template strand
When is it insertion?
If it occurs on the newly-synthesised strand
Give examples fo spontaneous chemical changes
Depurination
Deamination
Depurination
Removal of the purine from DNA by breaking the bond with its deoxyribose in the backbone
Deamination
Removal of the amino group from a base [cytoine to uracil]
How can depurination and deamination be repaired?
Base excision repair system
Induced Mutations
Mutations induced in our DNA because of exposure to chemicals called mutagens
How can we induce mutations?
- Radiation
- Ultraviolet: strongly absorbed by purines and pyrimidines forming abnormal chemical bonds called thymine dimers
Classes of chemical mutagens
- Base analogs
- Base modifying agents
- Intercalating agents
Base analogs
Incorporates a base in an alternative state in front of a base analog, which will lead to a base change
Describe 5-bromouracil as an example fo a base analog
5BU has a bromine residue instead of a methyl group of thymine so it resembles thymine and incorporates itself in DNA during replication.
In its rare state 5BU pairs only with guanine [3 hydrogen bonds] resulting in a TA-> CG transition mutation
Base modifying agents
Agents that can induce mutations at any stage of the cell cycle
3 types of base modifying agents
- Deaminating agents remove amino groups [nitrous acid]
- Hydroxylating agents
- Alkylating agents that add alkyl groups to bases
Intercalating agents
Used in the lab to label DNA/ RNA during electrophoresis as they intercalate between bases
Discuss the process used by intercalating discs
- At replication, a template that contains an intercalated agent will cause invertion of a random extra base
- The base-pair addition is complete after another round of replication during which the intercalating disc is lost
- If an intercalating agent inserts into new DNA in place of a normal base, the next round of replication will result in a deletion mutation
- Result in frameshift mutations
What is the Ames test?
Screens for potential carcinogens
How is the Ames test carried out?
We start from a strain that is histidine -. We take the strain and mix it with liver extract from a rat and place the bacteria into the chemical and we create 2 plates: non-permissive and permissive one
in the permissive plate, we will have normal growth of the bacteria while in the non-permissive we won’t have bacterial growth
We might have a situation where the bacteria grows in teh medium where it wasn’t supposed to–> MUTATION
Nutritional mutations
Auxotrophic mutants are easily detectable for microorganisms that normally can grow on minimal medium
Replica plating
Cells are first grown on supplemented medium and then the colonies are transferred to minimal medium as well as to control a plate of supplemented medium. Colonies that grow on the supplemented but not on media are selected for further study
Conditional mutations
Mutant that you observe the phenotype when they are in a specific condition i.e. heat sensitivity
