Why is a deficiency of any given dNTP lethal? And why is an excess of any given dNTP mutagenic?

Question

aaronq · Answer

if an organism can't synthesize all four dNTPs then it's cells can't replicate DNA correctly, and the cells can't further divide. 

I'm not sure about the second part, but a possible explanation could be that the presence of a certain dNTP in a higher concentration than other dNTPs causes it to be incorporated erroneously simply due to probabilistic effects. For example, if you had a bag with 4 different objects, but you had multiples of 1 of them. You have a higher probability of pulling out that 1 type of object with a higher frequency.

anonymous · Answer

@aaronq can you elaborate on the first sentence?

aaronq · Answer

Cells, in order to divide, need to go through mitosis. Prior to mitosis they go through several stages, one of them is the Synthesis (S) phase where the nuclear DNA is duplicated. If that cells doesn't have all 4 dNTPs, it wont be able to duplicate it's DNA and it'll undergo apoptosis (at one point or another).