Mutation & DNA Repair Flashcards
(11 cards)
Yeast mutants that require adenine
A mutation in yeast genes that are necessary for cells to make their own adenine results in giving the cell a requirement for adenine in its growth medium
Some of these mutations are useful because in addition to requiring adenine, their colonies develop a pink or red colour
What are cell cycle checkpoints?
A surveillance system that interrupts the progression of the cell cycle when damage to the genome or spindle is detected, or when the cell has failed to complete an earlier stage in the cycle
Makes a decision as to whether or not to continue the process
What is the DNA damage checkpoint?
Responsible for detecting the status of newly replicated DNA
Arrests the cell cycle at G2 if the cell has damaged DNA, preventing entry into M phase
What gene is involved in the DNA damage checkpoint?
RAD9 = involved in repair of damaged DNA
Normal cells arrest at G2 phase when DNA damage is detected- this arrest is the consequence of the activity of the RAD9 protein (among others)
Cells that have a deletion or mutation in RAD9 (rad9 mutants) fail to arrest at G2 phase even when they have sustained significant damage to their DNA
Consequence of UV radiation
Does not break DNA chain outright, it is selectively absorbed by the aromatic rings of the purine and pyrimidine bases = forms pyrimidine dimers which disrupt the local structure of the DNA double helix and prevent normal DNA replication
So its energy is more concentrated and is as damaging as ionising radiation
Repair systems for UV damage
One of the most efficient repair systems in yeast is photoreactivation —> when UV damaged cells are exposed to sunlight that has the UV wavelengths filtered out, a specific enzyme uses the energy from the visible parts of the solar spectrum to reverse the reaction that produces pyrimidine dimers. If the dimers are repaired before the DNA tries to replicate them, they have no effect on the cell
Another process is excision repair —> involves a sequence of events in which the damaged portion on one of the strands of the DNA double helix is removed by enzymes and then, the gap is accurately resynthesised with the remaining strand acting as the template.
Normal yeast have 4 mechanisms for DNA repair
Describe photoreactivation
Requires light
Photoreactivation is catalysed by a single protein called photolyase which uses the energy in a photon of light to chemically break apart a pyrimidine dimers in DNA
Describe excision repair
- Region of DNA containing the dimer or other damage is physically cut out and then replaced by new DNA synthesis
- It can work in damage created by agents other than UV and on lesions other than pyrimidine dimers
- In E.Coli, it requires 6 proteins: 3 are involved in finding the damaged region of the DNA and cutting the DNA strand around the lesion, one participates in removing the damaged bit, DNA polymerase replaces the portion which was removed and a final enzyme is DNA ligaments which glues the new and old portions back together
- In yeast, at least 12 proteins may participate in excision repair
What is error-prone repair?
Where mistakes are made when a cell tries to repair a lesion in its DNA
In the case of pyrimidine dimers, mistakes may happen when two dimers are near each other on opposite strands of the DNA and if excision repair tries to happen, it wont know how to copy the dimer when it attempts to carry out the repair DNA synthesis because the dimer is not a normal part of the DNA —> might make a mistake rather than not repair the gap in the DNA
Sometimes, an error prone process is the only way to repair DNA damage —> most mutations arising after UV treatment of cells are the result of error-prone repair of the DNA lesions
What is recombinational repair?
When pyrimidine dimers block DNA replication in a eukaryotic chromosome, a polymerase can start replication at other places downstream. The results of replicating a DNA molecule or chromosome containing a dimer is a gap in one of the DNA where the dimer blocked a portion from being copied
A gap in DNA means that one strand is missing information as the strand must be replicated before it divides. The most frequent way that cells fill the gap is by genetic recombination with another DNA molecule or chromosome containing the same or similar information
The recombinational repair system is a 4th process involved in repair of UV damage to DNA and the genes which make the proteins functioning in this system have been identified because mutations in them block recombination
Lethal effects of radiation
- reproductive death (cell replicates damages DNA so cannot divide)
- metabolic death (high radiation dose kills the cell outright)
- increased risk of cancer
- sublethal chromosome changes which stimulates genetic recombination (in single-celled organisms)
