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We use no. of differences in Amino Acid sequences of some homologous proteins from two species or use DNA finger printing to guess when did the two lines separated. Using Poisson distribution and exponential rate law we can give approx time when two lines got separated if we know the constant for that protein or DNA. But as we are examining a specefic prot or a specefic DNA sequence[ DNA sequence complimentary to the probe we use], we should comment just about the evolution of that specefic prot. or DNA. We can , still , get a constriant like -as the prot lines/DNA lines got separate T years ago, the organism-lines must have separated Tyrs ago or Before.
For correct judgement of time of separation we have to compare many prots and DNAs of the two organisms.A rigorousssest thing!!!

Am I correct??? Please help...

One more thing-
What makes some sequce conserved??? What is the mechanism behind it???

We must use conserved sequences of DNA or Proteins as if the rate is faster then nearly correct no. of mutations is not obtained due to overlapping of mutations.I think if we divide the whole DNA into longer units this error may be reduced .But still we can't use fastly changing proteins eg. fibrinogen in human. So, this reduses our resolution.



 For getting the separation pt using AA sequences of homologous prots.
we use Poisson distribution. There in the eqn, we get a constant 1/2 in the expression of the rate constant [ ''the fraction of changed amino acids per unit time''].Why is it there??? is it a constant then how could we determine it??? By comparing C-14 / U-235 dating of available fossils and their protein/DNA sequences of those fossilized organisms???

We can use this to find out how prot families may be linked...same way we can use this for judging the evolution DNA sequences. Is it being done???


Shrei