Transcription of Chapter 8 Hidden Markov Chains - math.rutgers.edu
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Chapter 8 Hidden Markov Definition and basic theoryLetX1,X2,X3,..be the nucleotide bases observed along a randomly chosenDNA strand. The Markov chain models introduced so far for modelling{Xn}have made the assumption ofhomogeneity, that is, that the transitionprobabilitiesaij=P(Xn+1=j xn=i),are the same for all sitesn. (Sincenis generally thought of as time in Markovchain theory, this property is often calledtime-homogeneity.) As differentparts of a DNA strand may have different functions or chemical properties,the assumption of homogeneity may be very inaccurate. A case in point is thephenomenon ofCpG-islands. In some regions of DNA, the two base sequence5 GC 3 appears with markedly greater frequency than in other culprit is a process called DNA methylation, whereby methyl groups (-CH3) attach themselves to nucleotide bases. The methyl group binds to baseCin the pair and promotes its decay, or rather, transformation, toT. Thisprocess operates commonly over most of the DNA sequence.
2 CHAPTER 8. HIDDEN MARKOV CHAINS the succession of bases inside CpG islands alone and a separate Markov chain to model the bases outside CpGislands.
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