• Question: Mutations occur when a mistake slips through the proofreading, an event which only happens about once every several million base pairs. Is this true?!!!!

    Asked by Ayse to Tristan, Sophie, Ravinder, Matt, Barbara on 7 Mar 2015.
    • Photo: Barbara Shih

      Barbara Shih answered on 7 Mar 2015:

      I don’t know the exact rate for escaped errors; it says at the end of this video http://highered.mheducation.com/sites/9834092339/student_view0/chapter14/proofreading_function_of_dna_polymerase.html. Considering there are over 3 billion base pairs in human genome, and stem cells divide 10^5-10^13 times in one’s life time, and however many stem cells there are in a human body, there are plenty of chances for escaped errors. Fortunately, for a cell to become cancerous/problematic, mutations need to occur at several important genes, and the mutated cells would also need to evade detection from the immune system.

    • Photo: Sophie Robinson

      Sophie Robinson answered on 10 Mar 2015:

      Although mistakes can happen during DNA replication, they are extremely rare. This is due to the DNA proofreading ability of the enzyme that copies DNA, called DNA polymerase. Whilst copying DNA, if an incorrect base is inserted, DNA polymerase can reverse and remove the incorrect base. If this didn’t happen, mistakes would occur approximately one in every million bases, which is quite a high error rate. However, because of this proofreading ability this rate is reduced to one in every hundred million. But as Barbara said, for cancer to occur, mutations need to happen at several important genes in combination, therefore the chance of a cell becoming cancerous when it divides is less than one in a hundred million.

    • Photo: Matthew Moore

      Matthew Moore answered on 13 Mar 2015:

      Sophie is right it is very rare! Unless a mutation occurs or has been inherited in a gene which encodes a DNA repair protein! Mutations can also be accumulated due to environmental factors such as particular chemicals or UV rays!

      Mitochondrial DNA on the other hand accumulates more mutations as it doesn’t have repair mechanisms.

      Polymerases (the enzymes which replicate DNA) and other proteins really are very very good at avoiding mistakes!

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