It is so intolerant to change because reassigning a codon to a different amino acid type or stop codon affects thousands of proteins that use that codon simultaneously. The probably that none of those mutations are deleterious is extremely small.
Genetic code changes are more common in the mitochondrial code. First of all the mitochondrial genome is much smaller, ~16kb for vertebrates. Moreover, in cases I have looked at the change in codon use seems to happen when first there is a case of extreme bias against using a codon. When a codon is (almost) not used at all it can be re-purposed without affecting any proteins. Bart On Tue, Mar 19, 2013 at 2:05 PM, Jacob Keller < j-kell...@fsm.northwestern.edu> wrote: > I don't understand this argument, as it would apply equally to all > features of the theoretical LUCA > >> No it won't. Different features would have different tolerance levels to >> modifications. > > > Yes, this "tolerance" is the second (hidden or implicit) principle I > referred to. So you'd have to explain why the codon convention is so > intolerant/invariant relative to the other features--it seems to me that > either it is at an optimum or there is some big barrier holding it in > place. And you'd have to explain this without invoking interchange of DNA, > viruses, etc, as we're talking about a LUCA here, right? And you'll have to > make sure that whatever reason you invoke cannot be applied to other > features of this LUCA which are indeed seen to be variable. > > JPK > > > ******************************************* > > Jacob Pearson Keller, PhD > > Looger Lab/HHMI Janelia Farms Research Campus > > 19700 Helix Dr, Ashburn, VA 20147 > > email: kell...@janelia.hhmi.org > > ******************************************* > -- Bart Hazes Associate Professor Dept. of Medical Microbiology & Immunology University of Alberta
