ch7 Flashcards
True or false: gene mutations may be spontaneous or induced
True
Are gene mutations always silent?
No, they can be detrimental
What is the rate of mutations
10^-7 to 10^-11 per base pair per round of replication. This is very low which is a good thing
What are the major types of mutation
- Point mutations
- Frame shift mutations
- Conditional mutations
- Suppressor mutations
How many base pairs are involved with point mutations
1 or a few (look at page 1 ch.7 notes if this doesnt make sense
Explain frame shift mutations
Insertion or deletion of base pairs resulting in alternate codon usage
True or false: frame shift mutations often result in intercalating agents
true
Define intercalating agents
planar, aromatic molecules that can insert between the base pairs of DNA
How do intercalating agents result in frame shift mutations
They push them apart slightly and cause more then one nucleotide to be inserted during replication
Can very large insertions of deletions only a result of intercalating agents
no, very large insertions or deletions may also result from chromosomal breakage and repair
Explain conditional mutations
mutants whose altered phenotype is only expressed under certain conditions
- most commonly used with temperature sensitive phenotypes
- usually in genes which would result in lethal effects if they were always inactive
Explain how temperature sensitive phenotypes exhibit conditional mutations
- The cells grow at low temperatures without a noticeable different phenotype
- when moved to high temperature, the mutant phenotype is expressed
Explain suppressor mutations
A mutation which has two mutations, one of which compensates for the presence of the other to generate a normal phenotype overall
-Commonly occurs in multi-enzyme complexes
True or false: some chemical mutagens have very specific results
True
Explain base analogs as a type of chemical mutagen
- i.e. 5 bromouracil can be inserted into DNA in the place of thymine
- This results in the occasional change of AT->GC base pairs as 5-bromouracil can base pair with A and G
Explain monofunctional alkylating agents as a type of chemical mutagen
Ethyl methane sulphonate methylates G then the methylates G can pair with T
Explain bifunctional alkylating agents as a type of chemical mutagen
- Nitrogen mustards, mitomycin nitrosoguanidine cross links the DNA strands together
- This cross link must be excised before transcription or replication can occur
- faulty excision leads to insertions and deletions
How does Radiation act as a mutagen
- Bases of DNA strongly absorb UV light
- Lethal effect of UV light is due to the induction of the formation of pyrimidine dimers
- These dimers block replication and transcription
What must be done when dimers form and block replication
- repairs must be made
- This area is excised starting 8 nucleotides up on the 5’ side and 4 nucleotides down on the 3’ side
- DNA polymerase I carries out the repair synthesis
- Nicks in the DNA backbone are sealed by DNA ligase
True or False: If the level of damage caused by radiation causing mutations is reasonably low the repair is accurate and complete
True (DNA polymerase I proof reads as it makes the new strand
What types of radiation are included in the ionizing radiation that causes mutations
x-rays and gamma rays
Why does ionizing radiation cause damage
By ionizing water and other cellular components and generating free radicals
True or False: Ionizing radiation can penetrate glass and tissues
True
True or False: ionizing radiation is not used as a plant and animal mutagen
False
True or False: ionizing radiation can react with and inactivate macromolecules
True
- DNA inactivation can lead to permanent effects due to gene dosage
- Can cause breaks in DNA backbone
- This breakage can lead to loss of pieces
Explain SOS responses in regards to DNA damage
-If there is a high degree of damage to DNA, then repair mechanisms can lead to mutations
What does high levels of DNA damage cause (SOS response)
The activation of many different repair systems, some of which are template independant
What are most genes involved in the SOS response repressed by
the presence of the LexA protein
What happens to LexA once repairs are complete in the SOS response
RecA protease function ceases and the LexA binds to the promoter regions of the repair genes to shit down the response
