MCB problem 6 quiz

1)

a) A normal cell with no genetic defects was exposed to large amounts of a DNA-damaging chemical (chemical X) for 5 minutes. After a few hours, the cell died. Based on what you had learnt today, account for the death of the cell. (5 marks)


b) Some environmental pollutants (e.g. cigarette smoke) can cause adducts to form on DNA. These are organic molecules that bind to DNA, causing a bulge in the molecule. Suggest how these adducts can interfere with cell replication. (5 marks)






a) The reason why the cell will die is because the DNA has been denatured. In today's lesson, we learned that there can be many possible ways of DNA denaturing. Firstly, it could be due to a foreign molecule attaching to the bases, and not allowing the DNA polymerase or helicase to pass through the DNA for transcription or for replication. Secondly, it could be that there is a cross link, which causes pyrimidine dimers to occur. These cross link can be caused by the UV rays emitting photons to the DNA. When this occurs, the sugar-phosphate backbone will also get distorted too. In this case, it is able to repair when the photolyase are able to remove the pyrimidine dimers. Thirdly, it will cause the sugar-phosphate backbone to break. In this case, the DNA ligase IV is able to join the sugar-phosphate backbone back. Forthly, it is the Base mismatch. The DNA glycosylase can recognize the mismatch base site and remove its base. Then, the endonuclease removes neighboring nucleotides, and the gap is filled by DNA polymerase I and DNA ligase.
The death of the cells could be due to the fact that the cell do not have enough immunity. Like for example maybe it does not have enough of the repairing mechanism. Like for example, in the problem statement, we could see that the one with a lack of DNA ligase IV cannot survive very long. Therefore, it does not have the ability to do faster repairing. Also, the damage rate is more than the repair rate, therefore, the rate of survival will keep dropping.
b) In cell replication, helicase will firstly go through the DNA, and it will split the double strand into a single strand, and therefore it will start reading a single strand coding, and therefore it will then replicate mRNA. In order to replicate mRNA, the helicase needs to go though the DNA, and with the molecule obstructing the pathway, the helicase might not be able to split the double strand.
Also, in the replication process, where the DNA will replicate from a single strand to a double strand by the DNA polymerase III. The DNA Polymerase III will not be able to read the complete sequence of the DNA, and with the molecule obstructing the reading, it will not be able to go on to complete the full strand coding, therefore the DNA sequence will not be complete.