Lecture 14 Flashcards
What is a mutation?
A mutation is a change that occurs in a DNA sequence.
For a child to inherit a mutation, where in the body must that mutation occur? What risks are associated with a mutation in another part of the body?
A mutation must occur on the germ cells for it to be passed on to a child. Mutations that occur in other parts of the body only affect the individual they occur in.
What are some causes of DNA damage?
Some causes of DNA damage are radiation, sunlight and chemicals.
What is a biological process that could have cause a mismatch DNA damage?
An error during replication may result in mismatch DNA damage (that is not corrected by polymerase - in E. coli only)
What is the importance of the fact that the new strand of DNA is not methylated for a short time in E coli when repairing this type of mismatch?
It allows for any errors on the newly synthesised to be corrected as the methylated strand is identified as the template strand. If both strands were methylated at the same time, the template strand would have not been identified –> don’t know which base to fix.
What is depurination?
Depurination is where the nucleic bases (specifically purines) are hydrolytically cleaved off and is replaced with an -OH group (as purines are good leaving groups). This results in the formation of a apurinic site. (Base excision repair)
What is deamination?
Deamination is the removal of an amine group with the use of a water molecule. (Base excision repair)
What is the purpose of a uracil glycosylase and why is it important?
Uracil glycosylases prevent mutagenesis by eliminating uracil from DNA molecules by cleaving the N-glycosylic bond and initiating the base-excision repair (BER) pathway
There are four steps in base excision repair. Name the protein that catalyses each step, and what happens next.
Firstly, Uracil glycosylase removes the base by cleaving the N-glycosylic bond. AP endonuclease comes in and cuts open the phosphate backbone where the base is removed. DNA polymerase I and adds in correct short strand of DNA (5’ to 3’). Finally, DNA ligase patches up the backbone.
What is a thymine dimer? Why is that a problem for the DNA?
Thymine dimers are kinks in the DNA molecule where adjacent thymine bases form covalent bonds to each other as a result of UV light damage.
What is the usual source of a thymine dimer?
The usual source of thymine dimers is through UV light damage.
There are four steps in nucleotide excision repair. What protein catalyses each step and what happens?
Firstly, excinuclease cut the backbone around the affected area. DNA helicase comes in and unwinds the affect area. DNA polymerase I adds new strand of DNA to the strand (5’ to 3’). Finally, DNA ligase patches the backbone back into place.
Why can’t thymine dimers be removed by base pair excision?
The kink (covalent bonds) in the DNA caused by the dimer prevents DNA polymerase I from removing the affected nucleotides.
What is sunburnt? True/ False: Tans are healthy
Sunburn is a radiation burn caused by exposure to the UV produced by the sun.
Tans are unhealthy (FALSE)
Why is a double stranded DNA break potentially dangerous? What can cause a double stranded DNA break?
It is potentially dangerous as it may lead to the potential break down of the chromosome by protective mechanism. The double-stranded breaks are caused by ionising radiation, oxidising agents and errors in DNA replication
