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So I was thinking that if each cell has P(X) of becoming cancerous, then the chance of cancer is 1-((1-P(X))^n) where n is the number of cells in the organism.

Since larger organisms have more cells than smaller organisms (I'm guessing that larger organisms simply have more cells than smaller organisms as you can't have larger cells since there is an optimum surface area to volume ratio), does this mean then that larger creatures are more likely to have cancer cause they have more cells?

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I would say that the question is more complicated since it really depends on the number of generations of cells. For instance muscle cells rarely get cancel but they also rarely divide. –  bobthejoe Mar 21 '12 at 0:19
    
You also have to take into account that a lot of larger organisms have a large genome with a low percentage actually encoding exons. –  GWW Mar 21 '12 at 2:56
    
@GWW: Is there actually a correlation between body size and fraction of coding DNA? –  Mechanical snail Sep 15 '12 at 7:57

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As mentioned in the comments, this question is quite complicated. If the chance of a single cell from different organisms getting cancer was the same, then you would be correct, but this is not the case.

Different organisms have evolved to live different lengths of time. This is rather obvious when you think about it: mice have a maximum lifespan of ~3 years, humans ~80years (these figures are of course massive generalizations - but it doesn't really matter). Yet 'old' mice (> 2 years) still get age-related diseases such as cardiovascular disease, and cancer. It should be fairly clear that the risk of a 2 year old mouse getting (age-related) cancer is much higher than a 2 year old human.

The explanation for this isn't exactly concrete, but it goes something like this; mice invest a lot more resources in growing very fast and reaching a reproductive age very early, whereas humans develop much more slowly, and invest a higher proportion of their resources in maintaining tissue function, thereby reaching reproductive maturity much later than the mouse. The Hayflick limit for a mouse is about 10, whereas for humans this is closer to 60.

Thus the rate of aging in humans is much lower than that of the mouse, and it comes back to evolution: mortality in the wild is very high for the mouse, and thus mice that invest heavily in reaching early reproductive maturity are selected for - and those that invest in anti-cancer mechanisms take longer to reach reproductive maturity, and thus have less chance of actually doing it!

So to answer your question - smaller organisms tend to have higher risks of getting cancer because their lifespans are comparatively shorter, and thus they 'age' at a faster rate and invest less resources in maintaining function (e.g. genome integrity).

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